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12 LLVM Programmer's Manual
16 <li><a href="#introduction">Introduction</a></li>
17 <li><a href="#general">General Information</a>
19 <li><a href="#stl">The C++ Standard Template Library</a></li>
21 <li>The <tt>-time-passes</tt> option</li>
22 <li>How to use the LLVM Makefile system</li>
23 <li>How to write a regression test</li>
28 <li><a href="#apis">Important and useful LLVM APIs</a>
30 <li><a href="#isa">The <tt>isa<></tt>, <tt>cast<></tt>
31 and <tt>dyn_cast<></tt> templates</a> </li>
32 <li><a href="#string_apis">Passing strings (the <tt>StringRef</tt>
33 and <tt>Twine</tt> classes)</a>
35 <li><a href="#StringRef">The <tt>StringRef</tt> class</a> </li>
36 <li><a href="#Twine">The <tt>Twine</tt> class</a> </li>
39 <li><a href="#DEBUG">The <tt>DEBUG()</tt> macro and <tt>-debug</tt>
42 <li><a href="#DEBUG_TYPE">Fine grained debug info with <tt>DEBUG_TYPE</tt>
43 and the <tt>-debug-only</tt> option</a> </li>
46 <li><a href="#Statistic">The <tt>Statistic</tt> class & <tt>-stats</tt>
49 <li>The <tt>InstVisitor</tt> template
50 <li>The general graph API
52 <li><a href="#ViewGraph">Viewing graphs while debugging code</a></li>
55 <li><a href="#datastructure">Picking the Right Data Structure for a Task</a>
57 <li><a href="#ds_sequential">Sequential Containers (std::vector, std::list, etc)</a>
59 <li><a href="#dss_arrayref">llvm/ADT/ArrayRef.h</a></li>
60 <li><a href="#dss_fixedarrays">Fixed Size Arrays</a></li>
61 <li><a href="#dss_heaparrays">Heap Allocated Arrays</a></li>
62 <li><a href="#dss_tinyptrvector">"llvm/ADT/TinyPtrVector.h"</a></li>
63 <li><a href="#dss_smallvector">"llvm/ADT/SmallVector.h"</a></li>
64 <li><a href="#dss_vector"><vector></a></li>
65 <li><a href="#dss_deque"><deque></a></li>
66 <li><a href="#dss_list"><list></a></li>
67 <li><a href="#dss_ilist">llvm/ADT/ilist.h</a></li>
68 <li><a href="#dss_packedvector">llvm/ADT/PackedVector.h</a></li>
69 <li><a href="#dss_other">Other Sequential Container Options</a></li>
71 <li><a href="#ds_string">String-like containers</a>
73 <li><a href="#dss_stringref">llvm/ADT/StringRef.h</a></li>
74 <li><a href="#dss_twine">llvm/ADT/Twine.h</a></li>
75 <li><a href="#dss_smallstring">llvm/ADT/SmallString.h</a></li>
76 <li><a href="#dss_stdstring">std::string</a></li>
78 <li><a href="#ds_set">Set-Like Containers (std::set, SmallSet, SetVector, etc)</a>
80 <li><a href="#dss_sortedvectorset">A sorted 'vector'</a></li>
81 <li><a href="#dss_smallset">"llvm/ADT/SmallSet.h"</a></li>
82 <li><a href="#dss_smallptrset">"llvm/ADT/SmallPtrSet.h"</a></li>
83 <li><a href="#dss_denseset">"llvm/ADT/DenseSet.h"</a></li>
84 <li><a href="#dss_sparseset">"llvm/ADT/SparseSet.h"</a></li>
85 <li><a href="#dss_FoldingSet">"llvm/ADT/FoldingSet.h"</a></li>
86 <li><a href="#dss_set"><set></a></li>
87 <li><a href="#dss_setvector">"llvm/ADT/SetVector.h"</a></li>
88 <li><a href="#dss_uniquevector">"llvm/ADT/UniqueVector.h"</a></li>
89 <li><a href="#dss_immutableset">"llvm/ADT/ImmutableSet.h"</a></li>
90 <li><a href="#dss_otherset">Other Set-Like Container Options</a></li>
92 <li><a href="#ds_map">Map-Like Containers (std::map, DenseMap, etc)</a>
94 <li><a href="#dss_sortedvectormap">A sorted 'vector'</a></li>
95 <li><a href="#dss_stringmap">"llvm/ADT/StringMap.h"</a></li>
96 <li><a href="#dss_indexedmap">"llvm/ADT/IndexedMap.h"</a></li>
97 <li><a href="#dss_densemap">"llvm/ADT/DenseMap.h"</a></li>
98 <li><a href="#dss_valuemap">"llvm/ADT/ValueMap.h"</a></li>
99 <li><a href="#dss_intervalmap">"llvm/ADT/IntervalMap.h"</a></li>
100 <li><a href="#dss_map"><map></a></li>
101 <li><a href="#dss_inteqclasses">"llvm/ADT/IntEqClasses.h"</a></li>
102 <li><a href="#dss_immutablemap">"llvm/ADT/ImmutableMap.h"</a></li>
103 <li><a href="#dss_othermap">Other Map-Like Container Options</a></li>
105 <li><a href="#ds_bit">BitVector-like containers</a>
107 <li><a href="#dss_bitvector">A dense bitvector</a></li>
108 <li><a href="#dss_smallbitvector">A "small" dense bitvector</a></li>
109 <li><a href="#dss_sparsebitvector">A sparse bitvector</a></li>
113 <li><a href="#common">Helpful Hints for Common Operations</a>
115 <li><a href="#inspection">Basic Inspection and Traversal Routines</a>
117 <li><a href="#iterate_function">Iterating over the <tt>BasicBlock</tt>s
118 in a <tt>Function</tt></a> </li>
119 <li><a href="#iterate_basicblock">Iterating over the <tt>Instruction</tt>s
120 in a <tt>BasicBlock</tt></a> </li>
121 <li><a href="#iterate_institer">Iterating over the <tt>Instruction</tt>s
122 in a <tt>Function</tt></a> </li>
123 <li><a href="#iterate_convert">Turning an iterator into a
124 class pointer</a> </li>
125 <li><a href="#iterate_complex">Finding call sites: a more
126 complex example</a> </li>
127 <li><a href="#calls_and_invokes">Treating calls and invokes
128 the same way</a> </li>
129 <li><a href="#iterate_chains">Iterating over def-use &
130 use-def chains</a> </li>
131 <li><a href="#iterate_preds">Iterating over predecessors &
132 successors of blocks</a></li>
135 <li><a href="#simplechanges">Making simple changes</a>
137 <li><a href="#schanges_creating">Creating and inserting new
138 <tt>Instruction</tt>s</a> </li>
139 <li><a href="#schanges_deleting">Deleting <tt>Instruction</tt>s</a> </li>
140 <li><a href="#schanges_replacing">Replacing an <tt>Instruction</tt>
141 with another <tt>Value</tt></a> </li>
142 <li><a href="#schanges_deletingGV">Deleting <tt>GlobalVariable</tt>s</a> </li>
145 <li><a href="#create_types">How to Create Types</a></li>
147 <li>Working with the Control Flow Graph
149 <li>Accessing predecessors and successors of a <tt>BasicBlock</tt>
157 <li><a href="#threading">Threads and LLVM</a>
159 <li><a href="#startmultithreaded">Entering and Exiting Multithreaded Mode
161 <li><a href="#shutdown">Ending execution with <tt>llvm_shutdown()</tt></a></li>
162 <li><a href="#managedstatic">Lazy initialization with <tt>ManagedStatic</tt></a></li>
163 <li><a href="#llvmcontext">Achieving Isolation with <tt>LLVMContext</tt></a></li>
164 <li><a href="#jitthreading">Threads and the JIT</a></li>
168 <li><a href="#advanced">Advanced Topics</a>
171 <li><a href="#SymbolTable">The <tt>ValueSymbolTable</tt> class</a></li>
172 <li><a href="#UserLayout">The <tt>User</tt> and owned <tt>Use</tt> classes' memory layout</a></li>
175 <li><a href="#coreclasses">The Core LLVM Class Hierarchy Reference</a>
177 <li><a href="#Type">The <tt>Type</tt> class</a> </li>
178 <li><a href="#Module">The <tt>Module</tt> class</a></li>
179 <li><a href="#Value">The <tt>Value</tt> class</a>
181 <li><a href="#User">The <tt>User</tt> class</a>
183 <li><a href="#Instruction">The <tt>Instruction</tt> class</a></li>
184 <li><a href="#Constant">The <tt>Constant</tt> class</a>
186 <li><a href="#GlobalValue">The <tt>GlobalValue</tt> class</a>
188 <li><a href="#Function">The <tt>Function</tt> class</a></li>
189 <li><a href="#GlobalVariable">The <tt>GlobalVariable</tt> class</a></li>
196 <li><a href="#BasicBlock">The <tt>BasicBlock</tt> class</a></li>
197 <li><a href="#Argument">The <tt>Argument</tt> class</a></li>
204 <div class="doc_author">
205 <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a>,
206 <a href="mailto:dhurjati@cs.uiuc.edu">Dinakar Dhurjati</a>,
207 <a href="mailto:ggreif@gmail.com">Gabor Greif</a>,
208 <a href="mailto:jstanley@cs.uiuc.edu">Joel Stanley</a>,
209 <a href="mailto:rspencer@x10sys.com">Reid Spencer</a> and
210 <a href="mailto:owen@apple.com">Owen Anderson</a></p>
213 <!-- *********************************************************************** -->
215 <a name="introduction">Introduction </a>
217 <!-- *********************************************************************** -->
221 <p>This document is meant to highlight some of the important classes and
222 interfaces available in the LLVM source-base. This manual is not
223 intended to explain what LLVM is, how it works, and what LLVM code looks
224 like. It assumes that you know the basics of LLVM and are interested
225 in writing transformations or otherwise analyzing or manipulating the
228 <p>This document should get you oriented so that you can find your
229 way in the continuously growing source code that makes up the LLVM
230 infrastructure. Note that this manual is not intended to serve as a
231 replacement for reading the source code, so if you think there should be
232 a method in one of these classes to do something, but it's not listed,
233 check the source. Links to the <a href="/doxygen/">doxygen</a> sources
234 are provided to make this as easy as possible.</p>
236 <p>The first section of this document describes general information that is
237 useful to know when working in the LLVM infrastructure, and the second describes
238 the Core LLVM classes. In the future this manual will be extended with
239 information describing how to use extension libraries, such as dominator
240 information, CFG traversal routines, and useful utilities like the <tt><a
241 href="/doxygen/InstVisitor_8h-source.html">InstVisitor</a></tt> template.</p>
245 <!-- *********************************************************************** -->
247 <a name="general">General Information</a>
249 <!-- *********************************************************************** -->
253 <p>This section contains general information that is useful if you are working
254 in the LLVM source-base, but that isn't specific to any particular API.</p>
256 <!-- ======================================================================= -->
258 <a name="stl">The C++ Standard Template Library</a>
263 <p>LLVM makes heavy use of the C++ Standard Template Library (STL),
264 perhaps much more than you are used to, or have seen before. Because of
265 this, you might want to do a little background reading in the
266 techniques used and capabilities of the library. There are many good
267 pages that discuss the STL, and several books on the subject that you
268 can get, so it will not be discussed in this document.</p>
270 <p>Here are some useful links:</p>
274 <li><a href="http://www.dinkumware.com/manuals/#Standard C++ Library">Dinkumware
275 C++ Library reference</a> - an excellent reference for the STL and other parts
276 of the standard C++ library.</li>
278 <li><a href="http://www.tempest-sw.com/cpp/">C++ In a Nutshell</a> - This is an
279 O'Reilly book in the making. It has a decent Standard Library
280 Reference that rivals Dinkumware's, and is unfortunately no longer free since the
281 book has been published.</li>
283 <li><a href="http://www.parashift.com/c++-faq-lite/">C++ Frequently Asked
286 <li><a href="http://www.sgi.com/tech/stl/">SGI's STL Programmer's Guide</a> -
288 href="http://www.sgi.com/tech/stl/stl_introduction.html">Introduction to the
291 <li><a href="http://www.research.att.com/%7Ebs/C++.html">Bjarne Stroustrup's C++
294 <li><a href="http://64.78.49.204/">
295 Bruce Eckel's Thinking in C++, 2nd ed. Volume 2 Revision 4.0 (even better, get
300 <p>You are also encouraged to take a look at the <a
301 href="CodingStandards.html">LLVM Coding Standards</a> guide which focuses on how
302 to write maintainable code more than where to put your curly braces.</p>
306 <!-- ======================================================================= -->
308 <a name="stl">Other useful references</a>
314 <li><a href="http://www.fortran-2000.com/ArnaudRecipes/sharedlib.html">Using
315 static and shared libraries across platforms</a></li>
322 <!-- *********************************************************************** -->
324 <a name="apis">Important and useful LLVM APIs</a>
326 <!-- *********************************************************************** -->
330 <p>Here we highlight some LLVM APIs that are generally useful and good to
331 know about when writing transformations.</p>
333 <!-- ======================================================================= -->
335 <a name="isa">The <tt>isa<></tt>, <tt>cast<></tt> and
336 <tt>dyn_cast<></tt> templates</a>
341 <p>The LLVM source-base makes extensive use of a custom form of RTTI.
342 These templates have many similarities to the C++ <tt>dynamic_cast<></tt>
343 operator, but they don't have some drawbacks (primarily stemming from
344 the fact that <tt>dynamic_cast<></tt> only works on classes that
345 have a v-table). Because they are used so often, you must know what they
346 do and how they work. All of these templates are defined in the <a
347 href="/doxygen/Casting_8h-source.html"><tt>llvm/Support/Casting.h</tt></a>
348 file (note that you very rarely have to include this file directly).</p>
351 <dt><tt>isa<></tt>: </dt>
353 <dd><p>The <tt>isa<></tt> operator works exactly like the Java
354 "<tt>instanceof</tt>" operator. It returns true or false depending on whether
355 a reference or pointer points to an instance of the specified class. This can
356 be very useful for constraint checking of various sorts (example below).</p>
359 <dt><tt>cast<></tt>: </dt>
361 <dd><p>The <tt>cast<></tt> operator is a "checked cast" operation. It
362 converts a pointer or reference from a base class to a derived class, causing
363 an assertion failure if it is not really an instance of the right type. This
364 should be used in cases where you have some information that makes you believe
365 that something is of the right type. An example of the <tt>isa<></tt>
366 and <tt>cast<></tt> template is:</p>
368 <div class="doc_code">
370 static bool isLoopInvariant(const <a href="#Value">Value</a> *V, const Loop *L) {
371 if (isa<<a href="#Constant">Constant</a>>(V) || isa<<a href="#Argument">Argument</a>>(V) || isa<<a href="#GlobalValue">GlobalValue</a>>(V))
374 // <i>Otherwise, it must be an instruction...</i>
375 return !L->contains(cast<<a href="#Instruction">Instruction</a>>(V)->getParent());
380 <p>Note that you should <b>not</b> use an <tt>isa<></tt> test followed
381 by a <tt>cast<></tt>, for that use the <tt>dyn_cast<></tt>
386 <dt><tt>dyn_cast<></tt>:</dt>
388 <dd><p>The <tt>dyn_cast<></tt> operator is a "checking cast" operation.
389 It checks to see if the operand is of the specified type, and if so, returns a
390 pointer to it (this operator does not work with references). If the operand is
391 not of the correct type, a null pointer is returned. Thus, this works very
392 much like the <tt>dynamic_cast<></tt> operator in C++, and should be
393 used in the same circumstances. Typically, the <tt>dyn_cast<></tt>
394 operator is used in an <tt>if</tt> statement or some other flow control
395 statement like this:</p>
397 <div class="doc_code">
399 if (<a href="#AllocationInst">AllocationInst</a> *AI = dyn_cast<<a href="#AllocationInst">AllocationInst</a>>(Val)) {
405 <p>This form of the <tt>if</tt> statement effectively combines together a call
406 to <tt>isa<></tt> and a call to <tt>cast<></tt> into one
407 statement, which is very convenient.</p>
409 <p>Note that the <tt>dyn_cast<></tt> operator, like C++'s
410 <tt>dynamic_cast<></tt> or Java's <tt>instanceof</tt> operator, can be
411 abused. In particular, you should not use big chained <tt>if/then/else</tt>
412 blocks to check for lots of different variants of classes. If you find
413 yourself wanting to do this, it is much cleaner and more efficient to use the
414 <tt>InstVisitor</tt> class to dispatch over the instruction type directly.</p>
418 <dt><tt>cast_or_null<></tt>: </dt>
420 <dd><p>The <tt>cast_or_null<></tt> operator works just like the
421 <tt>cast<></tt> operator, except that it allows for a null pointer as an
422 argument (which it then propagates). This can sometimes be useful, allowing
423 you to combine several null checks into one.</p></dd>
425 <dt><tt>dyn_cast_or_null<></tt>: </dt>
427 <dd><p>The <tt>dyn_cast_or_null<></tt> operator works just like the
428 <tt>dyn_cast<></tt> operator, except that it allows for a null pointer
429 as an argument (which it then propagates). This can sometimes be useful,
430 allowing you to combine several null checks into one.</p></dd>
434 <p>These five templates can be used with any classes, whether they have a
435 v-table or not. To add support for these templates, you simply need to add
436 <tt>classof</tt> static methods to the class you are interested casting
437 to. Describing this is currently outside the scope of this document, but there
438 are lots of examples in the LLVM source base.</p>
443 <!-- ======================================================================= -->
445 <a name="string_apis">Passing strings (the <tt>StringRef</tt>
446 and <tt>Twine</tt> classes)</a>
451 <p>Although LLVM generally does not do much string manipulation, we do have
452 several important APIs which take strings. Two important examples are the
453 Value class -- which has names for instructions, functions, etc. -- and the
454 StringMap class which is used extensively in LLVM and Clang.</p>
456 <p>These are generic classes, and they need to be able to accept strings which
457 may have embedded null characters. Therefore, they cannot simply take
458 a <tt>const char *</tt>, and taking a <tt>const std::string&</tt> requires
459 clients to perform a heap allocation which is usually unnecessary. Instead,
460 many LLVM APIs use a <tt>StringRef</tt> or a <tt>const Twine&</tt> for
461 passing strings efficiently.</p>
463 <!-- _______________________________________________________________________ -->
465 <a name="StringRef">The <tt>StringRef</tt> class</a>
470 <p>The <tt>StringRef</tt> data type represents a reference to a constant string
471 (a character array and a length) and supports the common operations available
472 on <tt>std:string</tt>, but does not require heap allocation.</p>
474 <p>It can be implicitly constructed using a C style null-terminated string,
475 an <tt>std::string</tt>, or explicitly with a character pointer and length.
476 For example, the <tt>StringRef</tt> find function is declared as:</p>
478 <pre class="doc_code">
479 iterator find(StringRef Key);
482 <p>and clients can call it using any one of:</p>
484 <pre class="doc_code">
485 Map.find("foo"); <i>// Lookup "foo"</i>
486 Map.find(std::string("bar")); <i>// Lookup "bar"</i>
487 Map.find(StringRef("\0baz", 4)); <i>// Lookup "\0baz"</i>
490 <p>Similarly, APIs which need to return a string may return a <tt>StringRef</tt>
491 instance, which can be used directly or converted to an <tt>std::string</tt>
492 using the <tt>str</tt> member function. See
493 "<tt><a href="/doxygen/classllvm_1_1StringRef_8h-source.html">llvm/ADT/StringRef.h</a></tt>"
494 for more information.</p>
496 <p>You should rarely use the <tt>StringRef</tt> class directly, because it contains
497 pointers to external memory it is not generally safe to store an instance of the
498 class (unless you know that the external storage will not be freed). StringRef is
499 small and pervasive enough in LLVM that it should always be passed by value.</p>
503 <!-- _______________________________________________________________________ -->
505 <a name="Twine">The <tt>Twine</tt> class</a>
510 <p>The <tt><a href="/doxygen/classllvm_1_1Twine.html">Twine</a></tt> class is an
511 efficient way for APIs to accept concatenated strings. For example, a common
512 LLVM paradigm is to name one instruction based on
513 the name of another instruction with a suffix, for example:</p>
515 <div class="doc_code">
517 New = CmpInst::Create(<i>...</i>, SO->getName() + ".cmp");
521 <p>The <tt>Twine</tt> class is effectively a lightweight
522 <a href="http://en.wikipedia.org/wiki/Rope_(computer_science)">rope</a>
523 which points to temporary (stack allocated) objects. Twines can be implicitly
524 constructed as the result of the plus operator applied to strings (i.e., a C
525 strings, an <tt>std::string</tt>, or a <tt>StringRef</tt>). The twine delays
526 the actual concatenation of strings until it is actually required, at which
527 point it can be efficiently rendered directly into a character array. This
528 avoids unnecessary heap allocation involved in constructing the temporary
529 results of string concatenation. See
530 "<tt><a href="/doxygen/Twine_8h_source.html">llvm/ADT/Twine.h</a></tt>"
531 and <a href="#dss_twine">here</a> for more information.</p>
533 <p>As with a <tt>StringRef</tt>, <tt>Twine</tt> objects point to external memory
534 and should almost never be stored or mentioned directly. They are intended
535 solely for use when defining a function which should be able to efficiently
536 accept concatenated strings.</p>
542 <!-- ======================================================================= -->
544 <a name="DEBUG">The <tt>DEBUG()</tt> macro and <tt>-debug</tt> option</a>
549 <p>Often when working on your pass you will put a bunch of debugging printouts
550 and other code into your pass. After you get it working, you want to remove
551 it, but you may need it again in the future (to work out new bugs that you run
554 <p> Naturally, because of this, you don't want to delete the debug printouts,
555 but you don't want them to always be noisy. A standard compromise is to comment
556 them out, allowing you to enable them if you need them in the future.</p>
558 <p>The "<tt><a href="/doxygen/Debug_8h-source.html">llvm/Support/Debug.h</a></tt>"
559 file provides a macro named <tt>DEBUG()</tt> that is a much nicer solution to
560 this problem. Basically, you can put arbitrary code into the argument of the
561 <tt>DEBUG</tt> macro, and it is only executed if '<tt>opt</tt>' (or any other
562 tool) is run with the '<tt>-debug</tt>' command line argument:</p>
564 <div class="doc_code">
566 DEBUG(errs() << "I am here!\n");
570 <p>Then you can run your pass like this:</p>
572 <div class="doc_code">
574 $ opt < a.bc > /dev/null -mypass
575 <i><no output></i>
576 $ opt < a.bc > /dev/null -mypass -debug
581 <p>Using the <tt>DEBUG()</tt> macro instead of a home-brewed solution allows you
582 to not have to create "yet another" command line option for the debug output for
583 your pass. Note that <tt>DEBUG()</tt> macros are disabled for optimized builds,
584 so they do not cause a performance impact at all (for the same reason, they
585 should also not contain side-effects!).</p>
587 <p>One additional nice thing about the <tt>DEBUG()</tt> macro is that you can
588 enable or disable it directly in gdb. Just use "<tt>set DebugFlag=0</tt>" or
589 "<tt>set DebugFlag=1</tt>" from the gdb if the program is running. If the
590 program hasn't been started yet, you can always just run it with
593 <!-- _______________________________________________________________________ -->
595 <a name="DEBUG_TYPE">Fine grained debug info with <tt>DEBUG_TYPE</tt> and
596 the <tt>-debug-only</tt> option</a>
601 <p>Sometimes you may find yourself in a situation where enabling <tt>-debug</tt>
602 just turns on <b>too much</b> information (such as when working on the code
603 generator). If you want to enable debug information with more fine-grained
604 control, you define the <tt>DEBUG_TYPE</tt> macro and the <tt>-debug</tt> only
605 option as follows:</p>
607 <div class="doc_code">
610 DEBUG(errs() << "No debug type\n");
611 #define DEBUG_TYPE "foo"
612 DEBUG(errs() << "'foo' debug type\n");
614 #define DEBUG_TYPE "bar"
615 DEBUG(errs() << "'bar' debug type\n"));
617 #define DEBUG_TYPE ""
618 DEBUG(errs() << "No debug type (2)\n");
622 <p>Then you can run your pass like this:</p>
624 <div class="doc_code">
626 $ opt < a.bc > /dev/null -mypass
627 <i><no output></i>
628 $ opt < a.bc > /dev/null -mypass -debug
633 $ opt < a.bc > /dev/null -mypass -debug-only=foo
635 $ opt < a.bc > /dev/null -mypass -debug-only=bar
640 <p>Of course, in practice, you should only set <tt>DEBUG_TYPE</tt> at the top of
641 a file, to specify the debug type for the entire module (if you do this before
642 you <tt>#include "llvm/Support/Debug.h"</tt>, you don't have to insert the ugly
643 <tt>#undef</tt>'s). Also, you should use names more meaningful than "foo" and
644 "bar", because there is no system in place to ensure that names do not
645 conflict. If two different modules use the same string, they will all be turned
646 on when the name is specified. This allows, for example, all debug information
647 for instruction scheduling to be enabled with <tt>-debug-type=InstrSched</tt>,
648 even if the source lives in multiple files.</p>
650 <p>The <tt>DEBUG_WITH_TYPE</tt> macro is also available for situations where you
651 would like to set <tt>DEBUG_TYPE</tt>, but only for one specific <tt>DEBUG</tt>
652 statement. It takes an additional first parameter, which is the type to use. For
653 example, the preceding example could be written as:</p>
656 <div class="doc_code">
658 DEBUG_WITH_TYPE("", errs() << "No debug type\n");
659 DEBUG_WITH_TYPE("foo", errs() << "'foo' debug type\n");
660 DEBUG_WITH_TYPE("bar", errs() << "'bar' debug type\n"));
661 DEBUG_WITH_TYPE("", errs() << "No debug type (2)\n");
669 <!-- ======================================================================= -->
671 <a name="Statistic">The <tt>Statistic</tt> class & <tt>-stats</tt>
678 href="/doxygen/Statistic_8h-source.html">llvm/ADT/Statistic.h</a></tt>" file
679 provides a class named <tt>Statistic</tt> that is used as a unified way to
680 keep track of what the LLVM compiler is doing and how effective various
681 optimizations are. It is useful to see what optimizations are contributing to
682 making a particular program run faster.</p>
684 <p>Often you may run your pass on some big program, and you're interested to see
685 how many times it makes a certain transformation. Although you can do this with
686 hand inspection, or some ad-hoc method, this is a real pain and not very useful
687 for big programs. Using the <tt>Statistic</tt> class makes it very easy to
688 keep track of this information, and the calculated information is presented in a
689 uniform manner with the rest of the passes being executed.</p>
691 <p>There are many examples of <tt>Statistic</tt> uses, but the basics of using
692 it are as follows:</p>
695 <li><p>Define your statistic like this:</p>
697 <div class="doc_code">
699 #define <a href="#DEBUG_TYPE">DEBUG_TYPE</a> "mypassname" <i>// This goes before any #includes.</i>
700 STATISTIC(NumXForms, "The # of times I did stuff");
704 <p>The <tt>STATISTIC</tt> macro defines a static variable, whose name is
705 specified by the first argument. The pass name is taken from the DEBUG_TYPE
706 macro, and the description is taken from the second argument. The variable
707 defined ("NumXForms" in this case) acts like an unsigned integer.</p></li>
709 <li><p>Whenever you make a transformation, bump the counter:</p>
711 <div class="doc_code">
713 ++NumXForms; // <i>I did stuff!</i>
720 <p>That's all you have to do. To get '<tt>opt</tt>' to print out the
721 statistics gathered, use the '<tt>-stats</tt>' option:</p>
723 <div class="doc_code">
725 $ opt -stats -mypassname < program.bc > /dev/null
726 <i>... statistics output ...</i>
730 <p> When running <tt>opt</tt> on a C file from the SPEC benchmark
731 suite, it gives a report that looks like this:</p>
733 <div class="doc_code">
735 7646 bitcodewriter - Number of normal instructions
736 725 bitcodewriter - Number of oversized instructions
737 129996 bitcodewriter - Number of bitcode bytes written
738 2817 raise - Number of insts DCEd or constprop'd
739 3213 raise - Number of cast-of-self removed
740 5046 raise - Number of expression trees converted
741 75 raise - Number of other getelementptr's formed
742 138 raise - Number of load/store peepholes
743 42 deadtypeelim - Number of unused typenames removed from symtab
744 392 funcresolve - Number of varargs functions resolved
745 27 globaldce - Number of global variables removed
746 2 adce - Number of basic blocks removed
747 134 cee - Number of branches revectored
748 49 cee - Number of setcc instruction eliminated
749 532 gcse - Number of loads removed
750 2919 gcse - Number of instructions removed
751 86 indvars - Number of canonical indvars added
752 87 indvars - Number of aux indvars removed
753 25 instcombine - Number of dead inst eliminate
754 434 instcombine - Number of insts combined
755 248 licm - Number of load insts hoisted
756 1298 licm - Number of insts hoisted to a loop pre-header
757 3 licm - Number of insts hoisted to multiple loop preds (bad, no loop pre-header)
758 75 mem2reg - Number of alloca's promoted
759 1444 cfgsimplify - Number of blocks simplified
763 <p>Obviously, with so many optimizations, having a unified framework for this
764 stuff is very nice. Making your pass fit well into the framework makes it more
765 maintainable and useful.</p>
769 <!-- ======================================================================= -->
771 <a name="ViewGraph">Viewing graphs while debugging code</a>
776 <p>Several of the important data structures in LLVM are graphs: for example
777 CFGs made out of LLVM <a href="#BasicBlock">BasicBlock</a>s, CFGs made out of
778 LLVM <a href="CodeGenerator.html#machinebasicblock">MachineBasicBlock</a>s, and
779 <a href="CodeGenerator.html#selectiondag_intro">Instruction Selection
780 DAGs</a>. In many cases, while debugging various parts of the compiler, it is
781 nice to instantly visualize these graphs.</p>
783 <p>LLVM provides several callbacks that are available in a debug build to do
784 exactly that. If you call the <tt>Function::viewCFG()</tt> method, for example,
785 the current LLVM tool will pop up a window containing the CFG for the function
786 where each basic block is a node in the graph, and each node contains the
787 instructions in the block. Similarly, there also exists
788 <tt>Function::viewCFGOnly()</tt> (does not include the instructions), the
789 <tt>MachineFunction::viewCFG()</tt> and <tt>MachineFunction::viewCFGOnly()</tt>,
790 and the <tt>SelectionDAG::viewGraph()</tt> methods. Within GDB, for example,
791 you can usually use something like <tt>call DAG.viewGraph()</tt> to pop
792 up a window. Alternatively, you can sprinkle calls to these functions in your
793 code in places you want to debug.</p>
795 <p>Getting this to work requires a small amount of configuration. On Unix
796 systems with X11, install the <a href="http://www.graphviz.org">graphviz</a>
797 toolkit, and make sure 'dot' and 'gv' are in your path. If you are running on
798 Mac OS/X, download and install the Mac OS/X <a
799 href="http://www.pixelglow.com/graphviz/">Graphviz program</a>, and add
800 <tt>/Applications/Graphviz.app/Contents/MacOS/</tt> (or wherever you install
801 it) to your path. Once in your system and path are set up, rerun the LLVM
802 configure script and rebuild LLVM to enable this functionality.</p>
804 <p><tt>SelectionDAG</tt> has been extended to make it easier to locate
805 <i>interesting</i> nodes in large complex graphs. From gdb, if you
806 <tt>call DAG.setGraphColor(<i>node</i>, "<i>color</i>")</tt>, then the
807 next <tt>call DAG.viewGraph()</tt> would highlight the node in the
808 specified color (choices of colors can be found at <a
809 href="http://www.graphviz.org/doc/info/colors.html">colors</a>.) More
810 complex node attributes can be provided with <tt>call
811 DAG.setGraphAttrs(<i>node</i>, "<i>attributes</i>")</tt> (choices can be
812 found at <a href="http://www.graphviz.org/doc/info/attrs.html">Graph
813 Attributes</a>.) If you want to restart and clear all the current graph
814 attributes, then you can <tt>call DAG.clearGraphAttrs()</tt>. </p>
816 <p>Note that graph visualization features are compiled out of Release builds
817 to reduce file size. This means that you need a Debug+Asserts or
818 Release+Asserts build to use these features.</p>
824 <!-- *********************************************************************** -->
826 <a name="datastructure">Picking the Right Data Structure for a Task</a>
828 <!-- *********************************************************************** -->
832 <p>LLVM has a plethora of data structures in the <tt>llvm/ADT/</tt> directory,
833 and we commonly use STL data structures. This section describes the trade-offs
834 you should consider when you pick one.</p>
837 The first step is a choose your own adventure: do you want a sequential
838 container, a set-like container, or a map-like container? The most important
839 thing when choosing a container is the algorithmic properties of how you plan to
840 access the container. Based on that, you should use:</p>
843 <li>a <a href="#ds_map">map-like</a> container if you need efficient look-up
844 of an value based on another value. Map-like containers also support
845 efficient queries for containment (whether a key is in the map). Map-like
846 containers generally do not support efficient reverse mapping (values to
847 keys). If you need that, use two maps. Some map-like containers also
848 support efficient iteration through the keys in sorted order. Map-like
849 containers are the most expensive sort, only use them if you need one of
850 these capabilities.</li>
852 <li>a <a href="#ds_set">set-like</a> container if you need to put a bunch of
853 stuff into a container that automatically eliminates duplicates. Some
854 set-like containers support efficient iteration through the elements in
855 sorted order. Set-like containers are more expensive than sequential
859 <li>a <a href="#ds_sequential">sequential</a> container provides
860 the most efficient way to add elements and keeps track of the order they are
861 added to the collection. They permit duplicates and support efficient
862 iteration, but do not support efficient look-up based on a key.
865 <li>a <a href="#ds_string">string</a> container is a specialized sequential
866 container or reference structure that is used for character or byte
869 <li>a <a href="#ds_bit">bit</a> container provides an efficient way to store and
870 perform set operations on sets of numeric id's, while automatically
871 eliminating duplicates. Bit containers require a maximum of 1 bit for each
872 identifier you want to store.
877 Once the proper category of container is determined, you can fine tune the
878 memory use, constant factors, and cache behaviors of access by intelligently
879 picking a member of the category. Note that constant factors and cache behavior
880 can be a big deal. If you have a vector that usually only contains a few
881 elements (but could contain many), for example, it's much better to use
882 <a href="#dss_smallvector">SmallVector</a> than <a href="#dss_vector">vector</a>
883 . Doing so avoids (relatively) expensive malloc/free calls, which dwarf the
884 cost of adding the elements to the container. </p>
886 <!-- ======================================================================= -->
888 <a name="ds_sequential">Sequential Containers (std::vector, std::list, etc)</a>
892 There are a variety of sequential containers available for you, based on your
893 needs. Pick the first in this section that will do what you want.
895 <!-- _______________________________________________________________________ -->
897 <a name="dss_arrayref">llvm/ADT/ArrayRef.h</a>
901 <p>The llvm::ArrayRef class is the preferred class to use in an interface that
902 accepts a sequential list of elements in memory and just reads from them. By
903 taking an ArrayRef, the API can be passed a fixed size array, an std::vector,
904 an llvm::SmallVector and anything else that is contiguous in memory.
910 <!-- _______________________________________________________________________ -->
912 <a name="dss_fixedarrays">Fixed Size Arrays</a>
916 <p>Fixed size arrays are very simple and very fast. They are good if you know
917 exactly how many elements you have, or you have a (low) upper bound on how many
921 <!-- _______________________________________________________________________ -->
923 <a name="dss_heaparrays">Heap Allocated Arrays</a>
927 <p>Heap allocated arrays (new[] + delete[]) are also simple. They are good if
928 the number of elements is variable, if you know how many elements you will need
929 before the array is allocated, and if the array is usually large (if not,
930 consider a <a href="#dss_smallvector">SmallVector</a>). The cost of a heap
931 allocated array is the cost of the new/delete (aka malloc/free). Also note that
932 if you are allocating an array of a type with a constructor, the constructor and
933 destructors will be run for every element in the array (re-sizable vectors only
934 construct those elements actually used).</p>
937 <!-- _______________________________________________________________________ -->
939 <a name="dss_tinyptrvector">"llvm/ADT/TinyPtrVector.h"</a>
944 <p><tt>TinyPtrVector<Type></tt> is a highly specialized collection class
945 that is optimized to avoid allocation in the case when a vector has zero or one
946 elements. It has two major restrictions: 1) it can only hold values of pointer
947 type, and 2) it cannot hold a null pointer.</p>
949 <p>Since this container is highly specialized, it is rarely used.</p>
953 <!-- _______________________________________________________________________ -->
955 <a name="dss_smallvector">"llvm/ADT/SmallVector.h"</a>
959 <p><tt>SmallVector<Type, N></tt> is a simple class that looks and smells
960 just like <tt>vector<Type></tt>:
961 it supports efficient iteration, lays out elements in memory order (so you can
962 do pointer arithmetic between elements), supports efficient push_back/pop_back
963 operations, supports efficient random access to its elements, etc.</p>
965 <p>The advantage of SmallVector is that it allocates space for
966 some number of elements (N) <b>in the object itself</b>. Because of this, if
967 the SmallVector is dynamically smaller than N, no malloc is performed. This can
968 be a big win in cases where the malloc/free call is far more expensive than the
969 code that fiddles around with the elements.</p>
971 <p>This is good for vectors that are "usually small" (e.g. the number of
972 predecessors/successors of a block is usually less than 8). On the other hand,
973 this makes the size of the SmallVector itself large, so you don't want to
974 allocate lots of them (doing so will waste a lot of space). As such,
975 SmallVectors are most useful when on the stack.</p>
977 <p>SmallVector also provides a nice portable and efficient replacement for
982 <!-- _______________________________________________________________________ -->
984 <a name="dss_vector"><vector></a>
989 std::vector is well loved and respected. It is useful when SmallVector isn't:
990 when the size of the vector is often large (thus the small optimization will
991 rarely be a benefit) or if you will be allocating many instances of the vector
992 itself (which would waste space for elements that aren't in the container).
993 vector is also useful when interfacing with code that expects vectors :).
996 <p>One worthwhile note about std::vector: avoid code like this:</p>
998 <div class="doc_code">
1001 std::vector<foo> V;
1007 <p>Instead, write this as:</p>
1009 <div class="doc_code">
1011 std::vector<foo> V;
1019 <p>Doing so will save (at least) one heap allocation and free per iteration of
1024 <!-- _______________________________________________________________________ -->
1026 <a name="dss_deque"><deque></a>
1030 <p>std::deque is, in some senses, a generalized version of std::vector. Like
1031 std::vector, it provides constant time random access and other similar
1032 properties, but it also provides efficient access to the front of the list. It
1033 does not guarantee continuity of elements within memory.</p>
1035 <p>In exchange for this extra flexibility, std::deque has significantly higher
1036 constant factor costs than std::vector. If possible, use std::vector or
1037 something cheaper.</p>
1040 <!-- _______________________________________________________________________ -->
1042 <a name="dss_list"><list></a>
1046 <p>std::list is an extremely inefficient class that is rarely useful.
1047 It performs a heap allocation for every element inserted into it, thus having an
1048 extremely high constant factor, particularly for small data types. std::list
1049 also only supports bidirectional iteration, not random access iteration.</p>
1051 <p>In exchange for this high cost, std::list supports efficient access to both
1052 ends of the list (like std::deque, but unlike std::vector or SmallVector). In
1053 addition, the iterator invalidation characteristics of std::list are stronger
1054 than that of a vector class: inserting or removing an element into the list does
1055 not invalidate iterator or pointers to other elements in the list.</p>
1058 <!-- _______________________________________________________________________ -->
1060 <a name="dss_ilist">llvm/ADT/ilist.h</a>
1064 <p><tt>ilist<T></tt> implements an 'intrusive' doubly-linked list. It is
1065 intrusive, because it requires the element to store and provide access to the
1066 prev/next pointers for the list.</p>
1068 <p><tt>ilist</tt> has the same drawbacks as <tt>std::list</tt>, and additionally
1069 requires an <tt>ilist_traits</tt> implementation for the element type, but it
1070 provides some novel characteristics. In particular, it can efficiently store
1071 polymorphic objects, the traits class is informed when an element is inserted or
1072 removed from the list, and <tt>ilist</tt>s are guaranteed to support a
1073 constant-time splice operation.</p>
1075 <p>These properties are exactly what we want for things like
1076 <tt>Instruction</tt>s and basic blocks, which is why these are implemented with
1077 <tt>ilist</tt>s.</p>
1079 Related classes of interest are explained in the following subsections:
1081 <li><a href="#dss_ilist_traits">ilist_traits</a></li>
1082 <li><a href="#dss_iplist">iplist</a></li>
1083 <li><a href="#dss_ilist_node">llvm/ADT/ilist_node.h</a></li>
1084 <li><a href="#dss_ilist_sentinel">Sentinels</a></li>
1088 <!-- _______________________________________________________________________ -->
1090 <a name="dss_packedvector">llvm/ADT/PackedVector.h</a>
1095 Useful for storing a vector of values using only a few number of bits for each
1096 value. Apart from the standard operations of a vector-like container, it can
1097 also perform an 'or' set operation.
1102 <div class="doc_code">
1106 FirstCondition = 0x1,
1107 SecondCondition = 0x2,
1112 PackedVector<State, 2> Vec1;
1113 Vec1.push_back(FirstCondition);
1115 PackedVector<State, 2> Vec2;
1116 Vec2.push_back(SecondCondition);
1119 return Vec1[0]; // returns 'Both'.
1126 <!-- _______________________________________________________________________ -->
1128 <a name="dss_ilist_traits">ilist_traits</a>
1132 <p><tt>ilist_traits<T></tt> is <tt>ilist<T></tt>'s customization
1133 mechanism. <tt>iplist<T></tt> (and consequently <tt>ilist<T></tt>)
1134 publicly derive from this traits class.</p>
1137 <!-- _______________________________________________________________________ -->
1139 <a name="dss_iplist">iplist</a>
1143 <p><tt>iplist<T></tt> is <tt>ilist<T></tt>'s base and as such
1144 supports a slightly narrower interface. Notably, inserters from
1145 <tt>T&</tt> are absent.</p>
1147 <p><tt>ilist_traits<T></tt> is a public base of this class and can be
1148 used for a wide variety of customizations.</p>
1151 <!-- _______________________________________________________________________ -->
1153 <a name="dss_ilist_node">llvm/ADT/ilist_node.h</a>
1157 <p><tt>ilist_node<T></tt> implements a the forward and backward links
1158 that are expected by the <tt>ilist<T></tt> (and analogous containers)
1159 in the default manner.</p>
1161 <p><tt>ilist_node<T></tt>s are meant to be embedded in the node type
1162 <tt>T</tt>, usually <tt>T</tt> publicly derives from
1163 <tt>ilist_node<T></tt>.</p>
1166 <!-- _______________________________________________________________________ -->
1168 <a name="dss_ilist_sentinel">Sentinels</a>
1172 <p><tt>ilist</tt>s have another specialty that must be considered. To be a good
1173 citizen in the C++ ecosystem, it needs to support the standard container
1174 operations, such as <tt>begin</tt> and <tt>end</tt> iterators, etc. Also, the
1175 <tt>operator--</tt> must work correctly on the <tt>end</tt> iterator in the
1176 case of non-empty <tt>ilist</tt>s.</p>
1178 <p>The only sensible solution to this problem is to allocate a so-called
1179 <i>sentinel</i> along with the intrusive list, which serves as the <tt>end</tt>
1180 iterator, providing the back-link to the last element. However conforming to the
1181 C++ convention it is illegal to <tt>operator++</tt> beyond the sentinel and it
1182 also must not be dereferenced.</p>
1184 <p>These constraints allow for some implementation freedom to the <tt>ilist</tt>
1185 how to allocate and store the sentinel. The corresponding policy is dictated
1186 by <tt>ilist_traits<T></tt>. By default a <tt>T</tt> gets heap-allocated
1187 whenever the need for a sentinel arises.</p>
1189 <p>While the default policy is sufficient in most cases, it may break down when
1190 <tt>T</tt> does not provide a default constructor. Also, in the case of many
1191 instances of <tt>ilist</tt>s, the memory overhead of the associated sentinels
1192 is wasted. To alleviate the situation with numerous and voluminous
1193 <tt>T</tt>-sentinels, sometimes a trick is employed, leading to <i>ghostly
1196 <p>Ghostly sentinels are obtained by specially-crafted <tt>ilist_traits<T></tt>
1197 which superpose the sentinel with the <tt>ilist</tt> instance in memory. Pointer
1198 arithmetic is used to obtain the sentinel, which is relative to the
1199 <tt>ilist</tt>'s <tt>this</tt> pointer. The <tt>ilist</tt> is augmented by an
1200 extra pointer, which serves as the back-link of the sentinel. This is the only
1201 field in the ghostly sentinel which can be legally accessed.</p>
1204 <!-- _______________________________________________________________________ -->
1206 <a name="dss_other">Other Sequential Container options</a>
1210 <p>Other STL containers are available, such as std::string.</p>
1212 <p>There are also various STL adapter classes such as std::queue,
1213 std::priority_queue, std::stack, etc. These provide simplified access to an
1214 underlying container but don't affect the cost of the container itself.</p>
1219 <!-- ======================================================================= -->
1221 <a name="ds_string">String-like containers</a>
1227 There are a variety of ways to pass around and use strings in C and C++, and
1228 LLVM adds a few new options to choose from. Pick the first option on this list
1229 that will do what you need, they are ordered according to their relative cost.
1232 Note that is is generally preferred to <em>not</em> pass strings around as
1233 "<tt>const char*</tt>"'s. These have a number of problems, including the fact
1234 that they cannot represent embedded nul ("\0") characters, and do not have a
1235 length available efficiently. The general replacement for '<tt>const
1236 char*</tt>' is StringRef.
1239 <p>For more information on choosing string containers for APIs, please see
1240 <a href="#string_apis">Passing strings</a>.</p>
1243 <!-- _______________________________________________________________________ -->
1245 <a name="dss_stringref">llvm/ADT/StringRef.h</a>
1250 The StringRef class is a simple value class that contains a pointer to a
1251 character and a length, and is quite related to the <a
1252 href="#dss_arrayref">ArrayRef</a> class (but specialized for arrays of
1253 characters). Because StringRef carries a length with it, it safely handles
1254 strings with embedded nul characters in it, getting the length does not require
1255 a strlen call, and it even has very convenient APIs for slicing and dicing the
1256 character range that it represents.
1260 StringRef is ideal for passing simple strings around that are known to be live,
1261 either because they are C string literals, std::string, a C array, or a
1262 SmallVector. Each of these cases has an efficient implicit conversion to
1263 StringRef, which doesn't result in a dynamic strlen being executed.
1266 <p>StringRef has a few major limitations which make more powerful string
1267 containers useful:</p>
1270 <li>You cannot directly convert a StringRef to a 'const char*' because there is
1271 no way to add a trailing nul (unlike the .c_str() method on various stronger
1275 <li>StringRef doesn't own or keep alive the underlying string bytes.
1276 As such it can easily lead to dangling pointers, and is not suitable for
1277 embedding in datastructures in most cases (instead, use an std::string or
1278 something like that).</li>
1280 <li>For the same reason, StringRef cannot be used as the return value of a
1281 method if the method "computes" the result string. Instead, use
1284 <li>StringRef's do not allow you to mutate the pointed-to string bytes and it
1285 doesn't allow you to insert or remove bytes from the range. For editing
1286 operations like this, it interoperates with the <a
1287 href="#dss_twine">Twine</a> class.</li>
1290 <p>Because of its strengths and limitations, it is very common for a function to
1291 take a StringRef and for a method on an object to return a StringRef that
1292 points into some string that it owns.</p>
1296 <!-- _______________________________________________________________________ -->
1298 <a name="dss_twine">llvm/ADT/Twine.h</a>
1303 The Twine class is used as an intermediary datatype for APIs that want to take
1304 a string that can be constructed inline with a series of concatenations.
1305 Twine works by forming recursive instances of the Twine datatype (a simple
1306 value object) on the stack as temporary objects, linking them together into a
1307 tree which is then linearized when the Twine is consumed. Twine is only safe
1308 to use as the argument to a function, and should always be a const reference,
1313 void foo(const Twine &T);
1317 foo(X + "." + Twine(i));
1320 <p>This example forms a string like "blarg.42" by concatenating the values
1321 together, and does not form intermediate strings containing "blarg" or
1325 <p>Because Twine is constructed with temporary objects on the stack, and
1326 because these instances are destroyed at the end of the current statement,
1327 it is an inherently dangerous API. For example, this simple variant contains
1328 undefined behavior and will probably crash:</p>
1331 void foo(const Twine &T);
1335 const Twine &Tmp = X + "." + Twine(i);
1339 <p>... because the temporaries are destroyed before the call. That said,
1340 Twine's are much more efficient than intermediate std::string temporaries, and
1341 they work really well with StringRef. Just be aware of their limitations.</p>
1346 <!-- _______________________________________________________________________ -->
1348 <a name="dss_smallstring">llvm/ADT/SmallString.h</a>
1353 <p>SmallString is a subclass of <a href="#dss_smallvector">SmallVector</a> that
1354 adds some convenience APIs like += that takes StringRef's. SmallString avoids
1355 allocating memory in the case when the preallocated space is enough to hold its
1356 data, and it calls back to general heap allocation when required. Since it owns
1357 its data, it is very safe to use and supports full mutation of the string.</p>
1359 <p>Like SmallVector's, the big downside to SmallString is their sizeof. While
1360 they are optimized for small strings, they themselves are not particularly
1361 small. This means that they work great for temporary scratch buffers on the
1362 stack, but should not generally be put into the heap: it is very rare to
1363 see a SmallString as the member of a frequently-allocated heap data structure
1364 or returned by-value.
1369 <!-- _______________________________________________________________________ -->
1371 <a name="dss_stdstring">std::string</a>
1376 <p>The standard C++ std::string class is a very general class that (like
1377 SmallString) owns its underlying data. sizeof(std::string) is very reasonable
1378 so it can be embedded into heap data structures and returned by-value.
1379 On the other hand, std::string is highly inefficient for inline editing (e.g.
1380 concatenating a bunch of stuff together) and because it is provided by the
1381 standard library, its performance characteristics depend a lot of the host
1382 standard library (e.g. libc++ and MSVC provide a highly optimized string
1383 class, GCC contains a really slow implementation).
1386 <p>The major disadvantage of std::string is that almost every operation that
1387 makes them larger can allocate memory, which is slow. As such, it is better
1388 to use SmallVector or Twine as a scratch buffer, but then use std::string to
1389 persist the result.</p>
1394 <!-- end of strings -->
1398 <!-- ======================================================================= -->
1400 <a name="ds_set">Set-Like Containers (std::set, SmallSet, SetVector, etc)</a>
1405 <p>Set-like containers are useful when you need to canonicalize multiple values
1406 into a single representation. There are several different choices for how to do
1407 this, providing various trade-offs.</p>
1409 <!-- _______________________________________________________________________ -->
1411 <a name="dss_sortedvectorset">A sorted 'vector'</a>
1416 <p>If you intend to insert a lot of elements, then do a lot of queries, a
1417 great approach is to use a vector (or other sequential container) with
1418 std::sort+std::unique to remove duplicates. This approach works really well if
1419 your usage pattern has these two distinct phases (insert then query), and can be
1420 coupled with a good choice of <a href="#ds_sequential">sequential container</a>.
1424 This combination provides the several nice properties: the result data is
1425 contiguous in memory (good for cache locality), has few allocations, is easy to
1426 address (iterators in the final vector are just indices or pointers), and can be
1427 efficiently queried with a standard binary or radix search.</p>
1431 <!-- _______________________________________________________________________ -->
1433 <a name="dss_smallset">"llvm/ADT/SmallSet.h"</a>
1438 <p>If you have a set-like data structure that is usually small and whose elements
1439 are reasonably small, a <tt>SmallSet<Type, N></tt> is a good choice. This set
1440 has space for N elements in place (thus, if the set is dynamically smaller than
1441 N, no malloc traffic is required) and accesses them with a simple linear search.
1442 When the set grows beyond 'N' elements, it allocates a more expensive representation that
1443 guarantees efficient access (for most types, it falls back to std::set, but for
1444 pointers it uses something far better, <a
1445 href="#dss_smallptrset">SmallPtrSet</a>).</p>
1447 <p>The magic of this class is that it handles small sets extremely efficiently,
1448 but gracefully handles extremely large sets without loss of efficiency. The
1449 drawback is that the interface is quite small: it supports insertion, queries
1450 and erasing, but does not support iteration.</p>
1454 <!-- _______________________________________________________________________ -->
1456 <a name="dss_smallptrset">"llvm/ADT/SmallPtrSet.h"</a>
1461 <p>SmallPtrSet has all the advantages of <tt>SmallSet</tt> (and a <tt>SmallSet</tt> of pointers is
1462 transparently implemented with a <tt>SmallPtrSet</tt>), but also supports iterators. If
1463 more than 'N' insertions are performed, a single quadratically
1464 probed hash table is allocated and grows as needed, providing extremely
1465 efficient access (constant time insertion/deleting/queries with low constant
1466 factors) and is very stingy with malloc traffic.</p>
1468 <p>Note that, unlike <tt>std::set</tt>, the iterators of <tt>SmallPtrSet</tt> are invalidated
1469 whenever an insertion occurs. Also, the values visited by the iterators are not
1470 visited in sorted order.</p>
1474 <!-- _______________________________________________________________________ -->
1476 <a name="dss_denseset">"llvm/ADT/DenseSet.h"</a>
1482 DenseSet is a simple quadratically probed hash table. It excels at supporting
1483 small values: it uses a single allocation to hold all of the pairs that
1484 are currently inserted in the set. DenseSet is a great way to unique small
1485 values that are not simple pointers (use <a
1486 href="#dss_smallptrset">SmallPtrSet</a> for pointers). Note that DenseSet has
1487 the same requirements for the value type that <a
1488 href="#dss_densemap">DenseMap</a> has.
1493 <!-- _______________________________________________________________________ -->
1495 <a name="dss_sparseset">"llvm/ADT/SparseSet.h"</a>
1500 <p>SparseSet holds a small number of objects identified by unsigned keys of
1501 moderate size. It uses a lot of memory, but provides operations that are
1502 almost as fast as a vector. Typical keys are physical registers, virtual
1503 registers, or numbered basic blocks.</p>
1505 <p>SparseSet is useful for algorithms that need very fast clear/find/insert/erase
1506 and fast iteration over small sets. It is not intended for building composite
1507 data structures.</p>
1511 <!-- _______________________________________________________________________ -->
1513 <a name="dss_FoldingSet">"llvm/ADT/FoldingSet.h"</a>
1519 FoldingSet is an aggregate class that is really good at uniquing
1520 expensive-to-create or polymorphic objects. It is a combination of a chained
1521 hash table with intrusive links (uniqued objects are required to inherit from
1522 FoldingSetNode) that uses <a href="#dss_smallvector">SmallVector</a> as part of
1525 <p>Consider a case where you want to implement a "getOrCreateFoo" method for
1526 a complex object (for example, a node in the code generator). The client has a
1527 description of *what* it wants to generate (it knows the opcode and all the
1528 operands), but we don't want to 'new' a node, then try inserting it into a set
1529 only to find out it already exists, at which point we would have to delete it
1530 and return the node that already exists.
1533 <p>To support this style of client, FoldingSet perform a query with a
1534 FoldingSetNodeID (which wraps SmallVector) that can be used to describe the
1535 element that we want to query for. The query either returns the element
1536 matching the ID or it returns an opaque ID that indicates where insertion should
1537 take place. Construction of the ID usually does not require heap traffic.</p>
1539 <p>Because FoldingSet uses intrusive links, it can support polymorphic objects
1540 in the set (for example, you can have SDNode instances mixed with LoadSDNodes).
1541 Because the elements are individually allocated, pointers to the elements are
1542 stable: inserting or removing elements does not invalidate any pointers to other
1548 <!-- _______________________________________________________________________ -->
1550 <a name="dss_set"><set></a>
1555 <p><tt>std::set</tt> is a reasonable all-around set class, which is decent at
1556 many things but great at nothing. std::set allocates memory for each element
1557 inserted (thus it is very malloc intensive) and typically stores three pointers
1558 per element in the set (thus adding a large amount of per-element space
1559 overhead). It offers guaranteed log(n) performance, which is not particularly
1560 fast from a complexity standpoint (particularly if the elements of the set are
1561 expensive to compare, like strings), and has extremely high constant factors for
1562 lookup, insertion and removal.</p>
1564 <p>The advantages of std::set are that its iterators are stable (deleting or
1565 inserting an element from the set does not affect iterators or pointers to other
1566 elements) and that iteration over the set is guaranteed to be in sorted order.
1567 If the elements in the set are large, then the relative overhead of the pointers
1568 and malloc traffic is not a big deal, but if the elements of the set are small,
1569 std::set is almost never a good choice.</p>
1573 <!-- _______________________________________________________________________ -->
1575 <a name="dss_setvector">"llvm/ADT/SetVector.h"</a>
1579 <p>LLVM's SetVector<Type> is an adapter class that combines your choice of
1580 a set-like container along with a <a href="#ds_sequential">Sequential
1581 Container</a>. The important property
1582 that this provides is efficient insertion with uniquing (duplicate elements are
1583 ignored) with iteration support. It implements this by inserting elements into
1584 both a set-like container and the sequential container, using the set-like
1585 container for uniquing and the sequential container for iteration.
1588 <p>The difference between SetVector and other sets is that the order of
1589 iteration is guaranteed to match the order of insertion into the SetVector.
1590 This property is really important for things like sets of pointers. Because
1591 pointer values are non-deterministic (e.g. vary across runs of the program on
1592 different machines), iterating over the pointers in the set will
1593 not be in a well-defined order.</p>
1596 The drawback of SetVector is that it requires twice as much space as a normal
1597 set and has the sum of constant factors from the set-like container and the
1598 sequential container that it uses. Use it *only* if you need to iterate over
1599 the elements in a deterministic order. SetVector is also expensive to delete
1600 elements out of (linear time), unless you use it's "pop_back" method, which is
1604 <p><tt>SetVector</tt> is an adapter class that defaults to
1605 using <tt>std::vector</tt> and a size 16 <tt>SmallSet</tt> for the underlying
1606 containers, so it is quite expensive. However,
1607 <tt>"llvm/ADT/SetVector.h"</tt> also provides a <tt>SmallSetVector</tt>
1608 class, which defaults to using a <tt>SmallVector</tt> and <tt>SmallSet</tt>
1609 of a specified size. If you use this, and if your sets are dynamically
1610 smaller than <tt>N</tt>, you will save a lot of heap traffic.</p>
1614 <!-- _______________________________________________________________________ -->
1616 <a name="dss_uniquevector">"llvm/ADT/UniqueVector.h"</a>
1622 UniqueVector is similar to <a href="#dss_setvector">SetVector</a>, but it
1623 retains a unique ID for each element inserted into the set. It internally
1624 contains a map and a vector, and it assigns a unique ID for each value inserted
1627 <p>UniqueVector is very expensive: its cost is the sum of the cost of
1628 maintaining both the map and vector, it has high complexity, high constant
1629 factors, and produces a lot of malloc traffic. It should be avoided.</p>
1633 <!-- _______________________________________________________________________ -->
1635 <a name="dss_immutableset">"llvm/ADT/ImmutableSet.h"</a>
1641 ImmutableSet is an immutable (functional) set implementation based on an AVL
1643 Adding or removing elements is done through a Factory object and results in the
1644 creation of a new ImmutableSet object.
1645 If an ImmutableSet already exists with the given contents, then the existing one
1646 is returned; equality is compared with a FoldingSetNodeID.
1647 The time and space complexity of add or remove operations is logarithmic in the
1648 size of the original set.
1651 There is no method for returning an element of the set, you can only check for
1657 <!-- _______________________________________________________________________ -->
1659 <a name="dss_otherset">Other Set-Like Container Options</a>
1665 The STL provides several other options, such as std::multiset and the various
1666 "hash_set" like containers (whether from C++ TR1 or from the SGI library). We
1667 never use hash_set and unordered_set because they are generally very expensive
1668 (each insertion requires a malloc) and very non-portable.
1671 <p>std::multiset is useful if you're not interested in elimination of
1672 duplicates, but has all the drawbacks of std::set. A sorted vector (where you
1673 don't delete duplicate entries) or some other approach is almost always
1680 <!-- ======================================================================= -->
1682 <a name="ds_map">Map-Like Containers (std::map, DenseMap, etc)</a>
1686 Map-like containers are useful when you want to associate data to a key. As
1687 usual, there are a lot of different ways to do this. :)
1689 <!-- _______________________________________________________________________ -->
1691 <a name="dss_sortedvectormap">A sorted 'vector'</a>
1697 If your usage pattern follows a strict insert-then-query approach, you can
1698 trivially use the same approach as <a href="#dss_sortedvectorset">sorted vectors
1699 for set-like containers</a>. The only difference is that your query function
1700 (which uses std::lower_bound to get efficient log(n) lookup) should only compare
1701 the key, not both the key and value. This yields the same advantages as sorted
1706 <!-- _______________________________________________________________________ -->
1708 <a name="dss_stringmap">"llvm/ADT/StringMap.h"</a>
1714 Strings are commonly used as keys in maps, and they are difficult to support
1715 efficiently: they are variable length, inefficient to hash and compare when
1716 long, expensive to copy, etc. StringMap is a specialized container designed to
1717 cope with these issues. It supports mapping an arbitrary range of bytes to an
1718 arbitrary other object.</p>
1720 <p>The StringMap implementation uses a quadratically-probed hash table, where
1721 the buckets store a pointer to the heap allocated entries (and some other
1722 stuff). The entries in the map must be heap allocated because the strings are
1723 variable length. The string data (key) and the element object (value) are
1724 stored in the same allocation with the string data immediately after the element
1725 object. This container guarantees the "<tt>(char*)(&Value+1)</tt>" points
1726 to the key string for a value.</p>
1728 <p>The StringMap is very fast for several reasons: quadratic probing is very
1729 cache efficient for lookups, the hash value of strings in buckets is not
1730 recomputed when looking up an element, StringMap rarely has to touch the
1731 memory for unrelated objects when looking up a value (even when hash collisions
1732 happen), hash table growth does not recompute the hash values for strings
1733 already in the table, and each pair in the map is store in a single allocation
1734 (the string data is stored in the same allocation as the Value of a pair).</p>
1736 <p>StringMap also provides query methods that take byte ranges, so it only ever
1737 copies a string if a value is inserted into the table.</p>
1739 <p>StringMap iteratation order, however, is not guaranteed to be deterministic,
1740 so any uses which require that should instead use a std::map.</p>
1743 <!-- _______________________________________________________________________ -->
1745 <a name="dss_indexedmap">"llvm/ADT/IndexedMap.h"</a>
1750 IndexedMap is a specialized container for mapping small dense integers (or
1751 values that can be mapped to small dense integers) to some other type. It is
1752 internally implemented as a vector with a mapping function that maps the keys to
1753 the dense integer range.
1757 This is useful for cases like virtual registers in the LLVM code generator: they
1758 have a dense mapping that is offset by a compile-time constant (the first
1759 virtual register ID).</p>
1763 <!-- _______________________________________________________________________ -->
1765 <a name="dss_densemap">"llvm/ADT/DenseMap.h"</a>
1771 DenseMap is a simple quadratically probed hash table. It excels at supporting
1772 small keys and values: it uses a single allocation to hold all of the pairs that
1773 are currently inserted in the map. DenseMap is a great way to map pointers to
1774 pointers, or map other small types to each other.
1778 There are several aspects of DenseMap that you should be aware of, however. The
1779 iterators in a DenseMap are invalidated whenever an insertion occurs, unlike
1780 map. Also, because DenseMap allocates space for a large number of key/value
1781 pairs (it starts with 64 by default), it will waste a lot of space if your keys
1782 or values are large. Finally, you must implement a partial specialization of
1783 DenseMapInfo for the key that you want, if it isn't already supported. This
1784 is required to tell DenseMap about two special marker values (which can never be
1785 inserted into the map) that it needs internally.</p>
1788 DenseMap's find_as() method supports lookup operations using an alternate key
1789 type. This is useful in cases where the normal key type is expensive to
1790 construct, but cheap to compare against. The DenseMapInfo is responsible for
1791 defining the appropriate comparison and hashing methods for each alternate
1797 <!-- _______________________________________________________________________ -->
1799 <a name="dss_valuemap">"llvm/ADT/ValueMap.h"</a>
1805 ValueMap is a wrapper around a <a href="#dss_densemap">DenseMap</a> mapping
1806 Value*s (or subclasses) to another type. When a Value is deleted or RAUW'ed,
1807 ValueMap will update itself so the new version of the key is mapped to the same
1808 value, just as if the key were a WeakVH. You can configure exactly how this
1809 happens, and what else happens on these two events, by passing
1810 a <code>Config</code> parameter to the ValueMap template.</p>
1814 <!-- _______________________________________________________________________ -->
1816 <a name="dss_intervalmap">"llvm/ADT/IntervalMap.h"</a>
1821 <p> IntervalMap is a compact map for small keys and values. It maps key
1822 intervals instead of single keys, and it will automatically coalesce adjacent
1823 intervals. When then map only contains a few intervals, they are stored in the
1824 map object itself to avoid allocations.</p>
1826 <p> The IntervalMap iterators are quite big, so they should not be passed around
1827 as STL iterators. The heavyweight iterators allow a smaller data structure.</p>
1831 <!-- _______________________________________________________________________ -->
1833 <a name="dss_map"><map></a>
1839 std::map has similar characteristics to <a href="#dss_set">std::set</a>: it uses
1840 a single allocation per pair inserted into the map, it offers log(n) lookup with
1841 an extremely large constant factor, imposes a space penalty of 3 pointers per
1842 pair in the map, etc.</p>
1844 <p>std::map is most useful when your keys or values are very large, if you need
1845 to iterate over the collection in sorted order, or if you need stable iterators
1846 into the map (i.e. they don't get invalidated if an insertion or deletion of
1847 another element takes place).</p>
1851 <!-- _______________________________________________________________________ -->
1853 <a name="dss_inteqclasses">"llvm/ADT/IntEqClasses.h"</a>
1858 <p>IntEqClasses provides a compact representation of equivalence classes of
1859 small integers. Initially, each integer in the range 0..n-1 has its own
1860 equivalence class. Classes can be joined by passing two class representatives to
1861 the join(a, b) method. Two integers are in the same class when findLeader()
1862 returns the same representative.</p>
1864 <p>Once all equivalence classes are formed, the map can be compressed so each
1865 integer 0..n-1 maps to an equivalence class number in the range 0..m-1, where m
1866 is the total number of equivalence classes. The map must be uncompressed before
1867 it can be edited again.</p>
1871 <!-- _______________________________________________________________________ -->
1873 <a name="dss_immutablemap">"llvm/ADT/ImmutableMap.h"</a>
1879 ImmutableMap is an immutable (functional) map implementation based on an AVL
1881 Adding or removing elements is done through a Factory object and results in the
1882 creation of a new ImmutableMap object.
1883 If an ImmutableMap already exists with the given key set, then the existing one
1884 is returned; equality is compared with a FoldingSetNodeID.
1885 The time and space complexity of add or remove operations is logarithmic in the
1886 size of the original map.
1890 <!-- _______________________________________________________________________ -->
1892 <a name="dss_othermap">Other Map-Like Container Options</a>
1898 The STL provides several other options, such as std::multimap and the various
1899 "hash_map" like containers (whether from C++ TR1 or from the SGI library). We
1900 never use hash_set and unordered_set because they are generally very expensive
1901 (each insertion requires a malloc) and very non-portable.</p>
1903 <p>std::multimap is useful if you want to map a key to multiple values, but has
1904 all the drawbacks of std::map. A sorted vector or some other approach is almost
1911 <!-- ======================================================================= -->
1913 <a name="ds_bit">Bit storage containers (BitVector, SparseBitVector)</a>
1917 <p>Unlike the other containers, there are only two bit storage containers, and
1918 choosing when to use each is relatively straightforward.</p>
1920 <p>One additional option is
1921 <tt>std::vector<bool></tt>: we discourage its use for two reasons 1) the
1922 implementation in many common compilers (e.g. commonly available versions of
1923 GCC) is extremely inefficient and 2) the C++ standards committee is likely to
1924 deprecate this container and/or change it significantly somehow. In any case,
1925 please don't use it.</p>
1927 <!-- _______________________________________________________________________ -->
1929 <a name="dss_bitvector">BitVector</a>
1933 <p> The BitVector container provides a dynamic size set of bits for manipulation.
1934 It supports individual bit setting/testing, as well as set operations. The set
1935 operations take time O(size of bitvector), but operations are performed one word
1936 at a time, instead of one bit at a time. This makes the BitVector very fast for
1937 set operations compared to other containers. Use the BitVector when you expect
1938 the number of set bits to be high (IE a dense set).
1942 <!-- _______________________________________________________________________ -->
1944 <a name="dss_smallbitvector">SmallBitVector</a>
1948 <p> The SmallBitVector container provides the same interface as BitVector, but
1949 it is optimized for the case where only a small number of bits, less than
1950 25 or so, are needed. It also transparently supports larger bit counts, but
1951 slightly less efficiently than a plain BitVector, so SmallBitVector should
1952 only be used when larger counts are rare.
1956 At this time, SmallBitVector does not support set operations (and, or, xor),
1957 and its operator[] does not provide an assignable lvalue.
1961 <!-- _______________________________________________________________________ -->
1963 <a name="dss_sparsebitvector">SparseBitVector</a>
1967 <p> The SparseBitVector container is much like BitVector, with one major
1968 difference: Only the bits that are set, are stored. This makes the
1969 SparseBitVector much more space efficient than BitVector when the set is sparse,
1970 as well as making set operations O(number of set bits) instead of O(size of
1971 universe). The downside to the SparseBitVector is that setting and testing of random bits is O(N), and on large SparseBitVectors, this can be slower than BitVector. In our implementation, setting or testing bits in sorted order
1972 (either forwards or reverse) is O(1) worst case. Testing and setting bits within 128 bits (depends on size) of the current bit is also O(1). As a general statement, testing/setting bits in a SparseBitVector is O(distance away from last set bit).
1980 <!-- *********************************************************************** -->
1982 <a name="common">Helpful Hints for Common Operations</a>
1984 <!-- *********************************************************************** -->
1988 <p>This section describes how to perform some very simple transformations of
1989 LLVM code. This is meant to give examples of common idioms used, showing the
1990 practical side of LLVM transformations. <p> Because this is a "how-to" section,
1991 you should also read about the main classes that you will be working with. The
1992 <a href="#coreclasses">Core LLVM Class Hierarchy Reference</a> contains details
1993 and descriptions of the main classes that you should know about.</p>
1995 <!-- NOTE: this section should be heavy on example code -->
1996 <!-- ======================================================================= -->
1998 <a name="inspection">Basic Inspection and Traversal Routines</a>
2003 <p>The LLVM compiler infrastructure have many different data structures that may
2004 be traversed. Following the example of the C++ standard template library, the
2005 techniques used to traverse these various data structures are all basically the
2006 same. For a enumerable sequence of values, the <tt>XXXbegin()</tt> function (or
2007 method) returns an iterator to the start of the sequence, the <tt>XXXend()</tt>
2008 function returns an iterator pointing to one past the last valid element of the
2009 sequence, and there is some <tt>XXXiterator</tt> data type that is common
2010 between the two operations.</p>
2012 <p>Because the pattern for iteration is common across many different aspects of
2013 the program representation, the standard template library algorithms may be used
2014 on them, and it is easier to remember how to iterate. First we show a few common
2015 examples of the data structures that need to be traversed. Other data
2016 structures are traversed in very similar ways.</p>
2018 <!-- _______________________________________________________________________ -->
2020 <a name="iterate_function">Iterating over the </a><a
2021 href="#BasicBlock"><tt>BasicBlock</tt></a>s in a <a
2022 href="#Function"><tt>Function</tt></a>
2027 <p>It's quite common to have a <tt>Function</tt> instance that you'd like to
2028 transform in some way; in particular, you'd like to manipulate its
2029 <tt>BasicBlock</tt>s. To facilitate this, you'll need to iterate over all of
2030 the <tt>BasicBlock</tt>s that constitute the <tt>Function</tt>. The following is
2031 an example that prints the name of a <tt>BasicBlock</tt> and the number of
2032 <tt>Instruction</tt>s it contains:</p>
2034 <div class="doc_code">
2036 // <i>func is a pointer to a Function instance</i>
2037 for (Function::iterator i = func->begin(), e = func->end(); i != e; ++i)
2038 // <i>Print out the name of the basic block if it has one, and then the</i>
2039 // <i>number of instructions that it contains</i>
2040 errs() << "Basic block (name=" << i->getName() << ") has "
2041 << i->size() << " instructions.\n";
2045 <p>Note that i can be used as if it were a pointer for the purposes of
2046 invoking member functions of the <tt>Instruction</tt> class. This is
2047 because the indirection operator is overloaded for the iterator
2048 classes. In the above code, the expression <tt>i->size()</tt> is
2049 exactly equivalent to <tt>(*i).size()</tt> just like you'd expect.</p>
2053 <!-- _______________________________________________________________________ -->
2055 <a name="iterate_basicblock">Iterating over the </a><a
2056 href="#Instruction"><tt>Instruction</tt></a>s in a <a
2057 href="#BasicBlock"><tt>BasicBlock</tt></a>
2062 <p>Just like when dealing with <tt>BasicBlock</tt>s in <tt>Function</tt>s, it's
2063 easy to iterate over the individual instructions that make up
2064 <tt>BasicBlock</tt>s. Here's a code snippet that prints out each instruction in
2065 a <tt>BasicBlock</tt>:</p>
2067 <div class="doc_code">
2069 // <i>blk is a pointer to a BasicBlock instance</i>
2070 for (BasicBlock::iterator i = blk->begin(), e = blk->end(); i != e; ++i)
2071 // <i>The next statement works since operator<<(ostream&,...)</i>
2072 // <i>is overloaded for Instruction&</i>
2073 errs() << *i << "\n";
2077 <p>However, this isn't really the best way to print out the contents of a
2078 <tt>BasicBlock</tt>! Since the ostream operators are overloaded for virtually
2079 anything you'll care about, you could have just invoked the print routine on the
2080 basic block itself: <tt>errs() << *blk << "\n";</tt>.</p>
2084 <!-- _______________________________________________________________________ -->
2086 <a name="iterate_institer">Iterating over the </a><a
2087 href="#Instruction"><tt>Instruction</tt></a>s in a <a
2088 href="#Function"><tt>Function</tt></a>
2093 <p>If you're finding that you commonly iterate over a <tt>Function</tt>'s
2094 <tt>BasicBlock</tt>s and then that <tt>BasicBlock</tt>'s <tt>Instruction</tt>s,
2095 <tt>InstIterator</tt> should be used instead. You'll need to include <a
2096 href="/doxygen/InstIterator_8h-source.html"><tt>llvm/Support/InstIterator.h</tt></a>,
2097 and then instantiate <tt>InstIterator</tt>s explicitly in your code. Here's a
2098 small example that shows how to dump all instructions in a function to the standard error stream:<p>
2100 <div class="doc_code">
2102 #include "<a href="/doxygen/InstIterator_8h-source.html">llvm/Support/InstIterator.h</a>"
2104 // <i>F is a pointer to a Function instance</i>
2105 for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I)
2106 errs() << *I << "\n";
2110 <p>Easy, isn't it? You can also use <tt>InstIterator</tt>s to fill a
2111 work list with its initial contents. For example, if you wanted to
2112 initialize a work list to contain all instructions in a <tt>Function</tt>
2113 F, all you would need to do is something like:</p>
2115 <div class="doc_code">
2117 std::set<Instruction*> worklist;
2118 // or better yet, SmallPtrSet<Instruction*, 64> worklist;
2120 for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I)
2121 worklist.insert(&*I);
2125 <p>The STL set <tt>worklist</tt> would now contain all instructions in the
2126 <tt>Function</tt> pointed to by F.</p>
2130 <!-- _______________________________________________________________________ -->
2132 <a name="iterate_convert">Turning an iterator into a class pointer (and
2138 <p>Sometimes, it'll be useful to grab a reference (or pointer) to a class
2139 instance when all you've got at hand is an iterator. Well, extracting
2140 a reference or a pointer from an iterator is very straight-forward.
2141 Assuming that <tt>i</tt> is a <tt>BasicBlock::iterator</tt> and <tt>j</tt>
2142 is a <tt>BasicBlock::const_iterator</tt>:</p>
2144 <div class="doc_code">
2146 Instruction& inst = *i; // <i>Grab reference to instruction reference</i>
2147 Instruction* pinst = &*i; // <i>Grab pointer to instruction reference</i>
2148 const Instruction& inst = *j;
2152 <p>However, the iterators you'll be working with in the LLVM framework are
2153 special: they will automatically convert to a ptr-to-instance type whenever they
2154 need to. Instead of dereferencing the iterator and then taking the address of
2155 the result, you can simply assign the iterator to the proper pointer type and
2156 you get the dereference and address-of operation as a result of the assignment
2157 (behind the scenes, this is a result of overloading casting mechanisms). Thus
2158 the last line of the last example,</p>
2160 <div class="doc_code">
2162 Instruction *pinst = &*i;
2166 <p>is semantically equivalent to</p>
2168 <div class="doc_code">
2170 Instruction *pinst = i;
2174 <p>It's also possible to turn a class pointer into the corresponding iterator,
2175 and this is a constant time operation (very efficient). The following code
2176 snippet illustrates use of the conversion constructors provided by LLVM
2177 iterators. By using these, you can explicitly grab the iterator of something
2178 without actually obtaining it via iteration over some structure:</p>
2180 <div class="doc_code">
2182 void printNextInstruction(Instruction* inst) {
2183 BasicBlock::iterator it(inst);
2184 ++it; // <i>After this line, it refers to the instruction after *inst</i>
2185 if (it != inst->getParent()->end()) errs() << *it << "\n";
2190 <p>Unfortunately, these implicit conversions come at a cost; they prevent
2191 these iterators from conforming to standard iterator conventions, and thus
2192 from being usable with standard algorithms and containers. For example, they
2193 prevent the following code, where <tt>B</tt> is a <tt>BasicBlock</tt>,
2196 <div class="doc_code">
2198 llvm::SmallVector<llvm::Instruction *, 16>(B->begin(), B->end());
2202 <p>Because of this, these implicit conversions may be removed some day,
2203 and <tt>operator*</tt> changed to return a pointer instead of a reference.</p>
2207 <!--_______________________________________________________________________-->
2209 <a name="iterate_complex">Finding call sites: a slightly more complex
2215 <p>Say that you're writing a FunctionPass and would like to count all the
2216 locations in the entire module (that is, across every <tt>Function</tt>) where a
2217 certain function (i.e., some <tt>Function</tt>*) is already in scope. As you'll
2218 learn later, you may want to use an <tt>InstVisitor</tt> to accomplish this in a
2219 much more straight-forward manner, but this example will allow us to explore how
2220 you'd do it if you didn't have <tt>InstVisitor</tt> around. In pseudo-code, this
2221 is what we want to do:</p>
2223 <div class="doc_code">
2225 initialize callCounter to zero
2226 for each Function f in the Module
2227 for each BasicBlock b in f
2228 for each Instruction i in b
2229 if (i is a CallInst and calls the given function)
2230 increment callCounter
2234 <p>And the actual code is (remember, because we're writing a
2235 <tt>FunctionPass</tt>, our <tt>FunctionPass</tt>-derived class simply has to
2236 override the <tt>runOnFunction</tt> method):</p>
2238 <div class="doc_code">
2240 Function* targetFunc = ...;
2242 class OurFunctionPass : public FunctionPass {
2244 OurFunctionPass(): callCounter(0) { }
2246 virtual runOnFunction(Function& F) {
2247 for (Function::iterator b = F.begin(), be = F.end(); b != be; ++b) {
2248 for (BasicBlock::iterator i = b->begin(), ie = b->end(); i != ie; ++i) {
2249 if (<a href="#CallInst">CallInst</a>* callInst = <a href="#isa">dyn_cast</a><<a
2250 href="#CallInst">CallInst</a>>(&*i)) {
2251 // <i>We know we've encountered a call instruction, so we</i>
2252 // <i>need to determine if it's a call to the</i>
2253 // <i>function pointed to by m_func or not.</i>
2254 if (callInst->getCalledFunction() == targetFunc)
2262 unsigned callCounter;
2269 <!--_______________________________________________________________________-->
2271 <a name="calls_and_invokes">Treating calls and invokes the same way</a>
2276 <p>You may have noticed that the previous example was a bit oversimplified in
2277 that it did not deal with call sites generated by 'invoke' instructions. In
2278 this, and in other situations, you may find that you want to treat
2279 <tt>CallInst</tt>s and <tt>InvokeInst</tt>s the same way, even though their
2280 most-specific common base class is <tt>Instruction</tt>, which includes lots of
2281 less closely-related things. For these cases, LLVM provides a handy wrapper
2283 href="http://llvm.org/doxygen/classllvm_1_1CallSite.html"><tt>CallSite</tt></a>.
2284 It is essentially a wrapper around an <tt>Instruction</tt> pointer, with some
2285 methods that provide functionality common to <tt>CallInst</tt>s and
2286 <tt>InvokeInst</tt>s.</p>
2288 <p>This class has "value semantics": it should be passed by value, not by
2289 reference and it should not be dynamically allocated or deallocated using
2290 <tt>operator new</tt> or <tt>operator delete</tt>. It is efficiently copyable,
2291 assignable and constructable, with costs equivalents to that of a bare pointer.
2292 If you look at its definition, it has only a single pointer member.</p>
2296 <!--_______________________________________________________________________-->
2298 <a name="iterate_chains">Iterating over def-use & use-def chains</a>
2303 <p>Frequently, we might have an instance of the <a
2304 href="/doxygen/classllvm_1_1Value.html">Value Class</a> and we want to
2305 determine which <tt>User</tt>s use the <tt>Value</tt>. The list of all
2306 <tt>User</tt>s of a particular <tt>Value</tt> is called a <i>def-use</i> chain.
2307 For example, let's say we have a <tt>Function*</tt> named <tt>F</tt> to a
2308 particular function <tt>foo</tt>. Finding all of the instructions that
2309 <i>use</i> <tt>foo</tt> is as simple as iterating over the <i>def-use</i> chain
2312 <div class="doc_code">
2316 for (Value::use_iterator i = F->use_begin(), e = F->use_end(); i != e; ++i)
2317 if (Instruction *Inst = dyn_cast<Instruction>(*i)) {
2318 errs() << "F is used in instruction:\n";
2319 errs() << *Inst << "\n";
2324 <p>Note that dereferencing a <tt>Value::use_iterator</tt> is not a very cheap
2325 operation. Instead of performing <tt>*i</tt> above several times, consider
2326 doing it only once in the loop body and reusing its result.</p>
2328 <p>Alternatively, it's common to have an instance of the <a
2329 href="/doxygen/classllvm_1_1User.html">User Class</a> and need to know what
2330 <tt>Value</tt>s are used by it. The list of all <tt>Value</tt>s used by a
2331 <tt>User</tt> is known as a <i>use-def</i> chain. Instances of class
2332 <tt>Instruction</tt> are common <tt>User</tt>s, so we might want to iterate over
2333 all of the values that a particular instruction uses (that is, the operands of
2334 the particular <tt>Instruction</tt>):</p>
2336 <div class="doc_code">
2338 Instruction *pi = ...;
2340 for (User::op_iterator i = pi->op_begin(), e = pi->op_end(); i != e; ++i) {
2347 <p>Declaring objects as <tt>const</tt> is an important tool of enforcing
2348 mutation free algorithms (such as analyses, etc.). For this purpose above
2349 iterators come in constant flavors as <tt>Value::const_use_iterator</tt>
2350 and <tt>Value::const_op_iterator</tt>. They automatically arise when
2351 calling <tt>use/op_begin()</tt> on <tt>const Value*</tt>s or
2352 <tt>const User*</tt>s respectively. Upon dereferencing, they return
2353 <tt>const Use*</tt>s. Otherwise the above patterns remain unchanged.</p>
2357 <!--_______________________________________________________________________-->
2359 <a name="iterate_preds">Iterating over predecessors &
2360 successors of blocks</a>
2365 <p>Iterating over the predecessors and successors of a block is quite easy
2366 with the routines defined in <tt>"llvm/Support/CFG.h"</tt>. Just use code like
2367 this to iterate over all predecessors of BB:</p>
2369 <div class="doc_code">
2371 #include "llvm/Support/CFG.h"
2372 BasicBlock *BB = ...;
2374 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
2375 BasicBlock *Pred = *PI;
2381 <p>Similarly, to iterate over successors use
2382 succ_iterator/succ_begin/succ_end.</p>
2388 <!-- ======================================================================= -->
2390 <a name="simplechanges">Making simple changes</a>
2395 <p>There are some primitive transformation operations present in the LLVM
2396 infrastructure that are worth knowing about. When performing
2397 transformations, it's fairly common to manipulate the contents of basic
2398 blocks. This section describes some of the common methods for doing so
2399 and gives example code.</p>
2401 <!--_______________________________________________________________________-->
2403 <a name="schanges_creating">Creating and inserting new
2404 <tt>Instruction</tt>s</a>
2409 <p><i>Instantiating Instructions</i></p>
2411 <p>Creation of <tt>Instruction</tt>s is straight-forward: simply call the
2412 constructor for the kind of instruction to instantiate and provide the necessary
2413 parameters. For example, an <tt>AllocaInst</tt> only <i>requires</i> a
2414 (const-ptr-to) <tt>Type</tt>. Thus:</p>
2416 <div class="doc_code">
2418 AllocaInst* ai = new AllocaInst(Type::Int32Ty);
2422 <p>will create an <tt>AllocaInst</tt> instance that represents the allocation of
2423 one integer in the current stack frame, at run time. Each <tt>Instruction</tt>
2424 subclass is likely to have varying default parameters which change the semantics
2425 of the instruction, so refer to the <a
2426 href="/doxygen/classllvm_1_1Instruction.html">doxygen documentation for the subclass of
2427 Instruction</a> that you're interested in instantiating.</p>
2429 <p><i>Naming values</i></p>
2431 <p>It is very useful to name the values of instructions when you're able to, as
2432 this facilitates the debugging of your transformations. If you end up looking
2433 at generated LLVM machine code, you definitely want to have logical names
2434 associated with the results of instructions! By supplying a value for the
2435 <tt>Name</tt> (default) parameter of the <tt>Instruction</tt> constructor, you
2436 associate a logical name with the result of the instruction's execution at
2437 run time. For example, say that I'm writing a transformation that dynamically
2438 allocates space for an integer on the stack, and that integer is going to be
2439 used as some kind of index by some other code. To accomplish this, I place an
2440 <tt>AllocaInst</tt> at the first point in the first <tt>BasicBlock</tt> of some
2441 <tt>Function</tt>, and I'm intending to use it within the same
2442 <tt>Function</tt>. I might do:</p>
2444 <div class="doc_code">
2446 AllocaInst* pa = new AllocaInst(Type::Int32Ty, 0, "indexLoc");
2450 <p>where <tt>indexLoc</tt> is now the logical name of the instruction's
2451 execution value, which is a pointer to an integer on the run time stack.</p>
2453 <p><i>Inserting instructions</i></p>
2455 <p>There are essentially two ways to insert an <tt>Instruction</tt>
2456 into an existing sequence of instructions that form a <tt>BasicBlock</tt>:</p>
2459 <li>Insertion into an explicit instruction list
2461 <p>Given a <tt>BasicBlock* pb</tt>, an <tt>Instruction* pi</tt> within that
2462 <tt>BasicBlock</tt>, and a newly-created instruction we wish to insert
2463 before <tt>*pi</tt>, we do the following: </p>
2465 <div class="doc_code">
2467 BasicBlock *pb = ...;
2468 Instruction *pi = ...;
2469 Instruction *newInst = new Instruction(...);
2471 pb->getInstList().insert(pi, newInst); // <i>Inserts newInst before pi in pb</i>
2475 <p>Appending to the end of a <tt>BasicBlock</tt> is so common that
2476 the <tt>Instruction</tt> class and <tt>Instruction</tt>-derived
2477 classes provide constructors which take a pointer to a
2478 <tt>BasicBlock</tt> to be appended to. For example code that
2481 <div class="doc_code">
2483 BasicBlock *pb = ...;
2484 Instruction *newInst = new Instruction(...);
2486 pb->getInstList().push_back(newInst); // <i>Appends newInst to pb</i>
2492 <div class="doc_code">
2494 BasicBlock *pb = ...;
2495 Instruction *newInst = new Instruction(..., pb);
2499 <p>which is much cleaner, especially if you are creating
2500 long instruction streams.</p></li>
2502 <li>Insertion into an implicit instruction list
2504 <p><tt>Instruction</tt> instances that are already in <tt>BasicBlock</tt>s
2505 are implicitly associated with an existing instruction list: the instruction
2506 list of the enclosing basic block. Thus, we could have accomplished the same
2507 thing as the above code without being given a <tt>BasicBlock</tt> by doing:
2510 <div class="doc_code">
2512 Instruction *pi = ...;
2513 Instruction *newInst = new Instruction(...);
2515 pi->getParent()->getInstList().insert(pi, newInst);
2519 <p>In fact, this sequence of steps occurs so frequently that the
2520 <tt>Instruction</tt> class and <tt>Instruction</tt>-derived classes provide
2521 constructors which take (as a default parameter) a pointer to an
2522 <tt>Instruction</tt> which the newly-created <tt>Instruction</tt> should
2523 precede. That is, <tt>Instruction</tt> constructors are capable of
2524 inserting the newly-created instance into the <tt>BasicBlock</tt> of a
2525 provided instruction, immediately before that instruction. Using an
2526 <tt>Instruction</tt> constructor with a <tt>insertBefore</tt> (default)
2527 parameter, the above code becomes:</p>
2529 <div class="doc_code">
2531 Instruction* pi = ...;
2532 Instruction* newInst = new Instruction(..., pi);
2536 <p>which is much cleaner, especially if you're creating a lot of
2537 instructions and adding them to <tt>BasicBlock</tt>s.</p></li>
2542 <!--_______________________________________________________________________-->
2544 <a name="schanges_deleting">Deleting <tt>Instruction</tt>s</a>
2549 <p>Deleting an instruction from an existing sequence of instructions that form a
2550 <a href="#BasicBlock"><tt>BasicBlock</tt></a> is very straight-forward: just
2551 call the instruction's eraseFromParent() method. For example:</p>
2553 <div class="doc_code">
2555 <a href="#Instruction">Instruction</a> *I = .. ;
2556 I->eraseFromParent();
2560 <p>This unlinks the instruction from its containing basic block and deletes
2561 it. If you'd just like to unlink the instruction from its containing basic
2562 block but not delete it, you can use the <tt>removeFromParent()</tt> method.</p>
2566 <!--_______________________________________________________________________-->
2568 <a name="schanges_replacing">Replacing an <tt>Instruction</tt> with another
2574 <h5><i>Replacing individual instructions</i></h5>
2576 <p>Including "<a href="/doxygen/BasicBlockUtils_8h-source.html">llvm/Transforms/Utils/BasicBlockUtils.h</a>"
2577 permits use of two very useful replace functions: <tt>ReplaceInstWithValue</tt>
2578 and <tt>ReplaceInstWithInst</tt>.</p>
2580 <h5><a name="schanges_deleting">Deleting <tt>Instruction</tt>s</a></h5>
2584 <li><tt>ReplaceInstWithValue</tt>
2586 <p>This function replaces all uses of a given instruction with a value,
2587 and then removes the original instruction. The following example
2588 illustrates the replacement of the result of a particular
2589 <tt>AllocaInst</tt> that allocates memory for a single integer with a null
2590 pointer to an integer.</p>
2592 <div class="doc_code">
2594 AllocaInst* instToReplace = ...;
2595 BasicBlock::iterator ii(instToReplace);
2597 ReplaceInstWithValue(instToReplace->getParent()->getInstList(), ii,
2598 Constant::getNullValue(PointerType::getUnqual(Type::Int32Ty)));
2601 <li><tt>ReplaceInstWithInst</tt>
2603 <p>This function replaces a particular instruction with another
2604 instruction, inserting the new instruction into the basic block at the
2605 location where the old instruction was, and replacing any uses of the old
2606 instruction with the new instruction. The following example illustrates
2607 the replacement of one <tt>AllocaInst</tt> with another.</p>
2609 <div class="doc_code">
2611 AllocaInst* instToReplace = ...;
2612 BasicBlock::iterator ii(instToReplace);
2614 ReplaceInstWithInst(instToReplace->getParent()->getInstList(), ii,
2615 new AllocaInst(Type::Int32Ty, 0, "ptrToReplacedInt"));
2621 <h5><i>Replacing multiple uses of <tt>User</tt>s and <tt>Value</tt>s</i></h5>
2623 <p>You can use <tt>Value::replaceAllUsesWith</tt> and
2624 <tt>User::replaceUsesOfWith</tt> to change more than one use at a time. See the
2625 doxygen documentation for the <a href="/doxygen/classllvm_1_1Value.html">Value Class</a>
2626 and <a href="/doxygen/classllvm_1_1User.html">User Class</a>, respectively, for more
2629 <!-- Value::replaceAllUsesWith User::replaceUsesOfWith Point out:
2630 include/llvm/Transforms/Utils/ especially BasicBlockUtils.h with:
2631 ReplaceInstWithValue, ReplaceInstWithInst -->
2635 <!--_______________________________________________________________________-->
2637 <a name="schanges_deletingGV">Deleting <tt>GlobalVariable</tt>s</a>
2642 <p>Deleting a global variable from a module is just as easy as deleting an
2643 Instruction. First, you must have a pointer to the global variable that you wish
2644 to delete. You use this pointer to erase it from its parent, the module.
2647 <div class="doc_code">
2649 <a href="#GlobalVariable">GlobalVariable</a> *GV = .. ;
2651 GV->eraseFromParent();
2659 <!-- ======================================================================= -->
2661 <a name="create_types">How to Create Types</a>
2666 <p>In generating IR, you may need some complex types. If you know these types
2667 statically, you can use <tt>TypeBuilder<...>::get()</tt>, defined
2668 in <tt>llvm/Support/TypeBuilder.h</tt>, to retrieve them. <tt>TypeBuilder</tt>
2669 has two forms depending on whether you're building types for cross-compilation
2670 or native library use. <tt>TypeBuilder<T, true></tt> requires
2671 that <tt>T</tt> be independent of the host environment, meaning that it's built
2673 the <a href="/doxygen/namespacellvm_1_1types.html"><tt>llvm::types</tt></a>
2674 namespace and pointers, functions, arrays, etc. built of
2675 those. <tt>TypeBuilder<T, false></tt> additionally allows native C types
2676 whose size may depend on the host compiler. For example,</p>
2678 <div class="doc_code">
2680 FunctionType *ft = TypeBuilder<types::i<8>(types::i<32>*), true>::get();
2684 <p>is easier to read and write than the equivalent</p>
2686 <div class="doc_code">
2688 std::vector<const Type*> params;
2689 params.push_back(PointerType::getUnqual(Type::Int32Ty));
2690 FunctionType *ft = FunctionType::get(Type::Int8Ty, params, false);
2694 <p>See the <a href="/doxygen/TypeBuilder_8h-source.html#l00001">class
2695 comment</a> for more details.</p>
2701 <!-- *********************************************************************** -->
2703 <a name="threading">Threads and LLVM</a>
2705 <!-- *********************************************************************** -->
2709 This section describes the interaction of the LLVM APIs with multithreading,
2710 both on the part of client applications, and in the JIT, in the hosted
2715 Note that LLVM's support for multithreading is still relatively young. Up
2716 through version 2.5, the execution of threaded hosted applications was
2717 supported, but not threaded client access to the APIs. While this use case is
2718 now supported, clients <em>must</em> adhere to the guidelines specified below to
2719 ensure proper operation in multithreaded mode.
2723 Note that, on Unix-like platforms, LLVM requires the presence of GCC's atomic
2724 intrinsics in order to support threaded operation. If you need a
2725 multhreading-capable LLVM on a platform without a suitably modern system
2726 compiler, consider compiling LLVM and LLVM-GCC in single-threaded mode, and
2727 using the resultant compiler to build a copy of LLVM with multithreading
2731 <!-- ======================================================================= -->
2733 <a name="startmultithreaded">Entering and Exiting Multithreaded Mode</a>
2739 In order to properly protect its internal data structures while avoiding
2740 excessive locking overhead in the single-threaded case, the LLVM must intialize
2741 certain data structures necessary to provide guards around its internals. To do
2742 so, the client program must invoke <tt>llvm_start_multithreaded()</tt> before
2743 making any concurrent LLVM API calls. To subsequently tear down these
2744 structures, use the <tt>llvm_stop_multithreaded()</tt> call. You can also use
2745 the <tt>llvm_is_multithreaded()</tt> call to check the status of multithreaded
2750 Note that both of these calls must be made <em>in isolation</em>. That is to
2751 say that no other LLVM API calls may be executing at any time during the
2752 execution of <tt>llvm_start_multithreaded()</tt> or <tt>llvm_stop_multithreaded
2753 </tt>. It's is the client's responsibility to enforce this isolation.
2757 The return value of <tt>llvm_start_multithreaded()</tt> indicates the success or
2758 failure of the initialization. Failure typically indicates that your copy of
2759 LLVM was built without multithreading support, typically because GCC atomic
2760 intrinsics were not found in your system compiler. In this case, the LLVM API
2761 will not be safe for concurrent calls. However, it <em>will</em> be safe for
2762 hosting threaded applications in the JIT, though <a href="#jitthreading">care
2763 must be taken</a> to ensure that side exits and the like do not accidentally
2764 result in concurrent LLVM API calls.
2768 <!-- ======================================================================= -->
2770 <a name="shutdown">Ending Execution with <tt>llvm_shutdown()</tt></a>
2775 When you are done using the LLVM APIs, you should call <tt>llvm_shutdown()</tt>
2776 to deallocate memory used for internal structures. This will also invoke
2777 <tt>llvm_stop_multithreaded()</tt> if LLVM is operating in multithreaded mode.
2778 As such, <tt>llvm_shutdown()</tt> requires the same isolation guarantees as
2779 <tt>llvm_stop_multithreaded()</tt>.
2783 Note that, if you use scope-based shutdown, you can use the
2784 <tt>llvm_shutdown_obj</tt> class, which calls <tt>llvm_shutdown()</tt> in its
2788 <!-- ======================================================================= -->
2790 <a name="managedstatic">Lazy Initialization with <tt>ManagedStatic</tt></a>
2795 <tt>ManagedStatic</tt> is a utility class in LLVM used to implement static
2796 initialization of static resources, such as the global type tables. Before the
2797 invocation of <tt>llvm_shutdown()</tt>, it implements a simple lazy
2798 initialization scheme. Once <tt>llvm_start_multithreaded()</tt> returns,
2799 however, it uses double-checked locking to implement thread-safe lazy
2804 Note that, because no other threads are allowed to issue LLVM API calls before
2805 <tt>llvm_start_multithreaded()</tt> returns, it is possible to have
2806 <tt>ManagedStatic</tt>s of <tt>llvm::sys::Mutex</tt>s.
2810 The <tt>llvm_acquire_global_lock()</tt> and <tt>llvm_release_global_lock</tt>
2811 APIs provide access to the global lock used to implement the double-checked
2812 locking for lazy initialization. These should only be used internally to LLVM,
2813 and only if you know what you're doing!
2817 <!-- ======================================================================= -->
2819 <a name="llvmcontext">Achieving Isolation with <tt>LLVMContext</tt></a>
2824 <tt>LLVMContext</tt> is an opaque class in the LLVM API which clients can use
2825 to operate multiple, isolated instances of LLVM concurrently within the same
2826 address space. For instance, in a hypothetical compile-server, the compilation
2827 of an individual translation unit is conceptually independent from all the
2828 others, and it would be desirable to be able to compile incoming translation
2829 units concurrently on independent server threads. Fortunately,
2830 <tt>LLVMContext</tt> exists to enable just this kind of scenario!
2834 Conceptually, <tt>LLVMContext</tt> provides isolation. Every LLVM entity
2835 (<tt>Module</tt>s, <tt>Value</tt>s, <tt>Type</tt>s, <tt>Constant</tt>s, etc.)
2836 in LLVM's in-memory IR belongs to an <tt>LLVMContext</tt>. Entities in
2837 different contexts <em>cannot</em> interact with each other: <tt>Module</tt>s in
2838 different contexts cannot be linked together, <tt>Function</tt>s cannot be added
2839 to <tt>Module</tt>s in different contexts, etc. What this means is that is is
2840 safe to compile on multiple threads simultaneously, as long as no two threads
2841 operate on entities within the same context.
2845 In practice, very few places in the API require the explicit specification of a
2846 <tt>LLVMContext</tt>, other than the <tt>Type</tt> creation/lookup APIs.
2847 Because every <tt>Type</tt> carries a reference to its owning context, most
2848 other entities can determine what context they belong to by looking at their
2849 own <tt>Type</tt>. If you are adding new entities to LLVM IR, please try to
2850 maintain this interface design.
2854 For clients that do <em>not</em> require the benefits of isolation, LLVM
2855 provides a convenience API <tt>getGlobalContext()</tt>. This returns a global,
2856 lazily initialized <tt>LLVMContext</tt> that may be used in situations where
2857 isolation is not a concern.
2861 <!-- ======================================================================= -->
2863 <a name="jitthreading">Threads and the JIT</a>
2868 LLVM's "eager" JIT compiler is safe to use in threaded programs. Multiple
2869 threads can call <tt>ExecutionEngine::getPointerToFunction()</tt> or
2870 <tt>ExecutionEngine::runFunction()</tt> concurrently, and multiple threads can
2871 run code output by the JIT concurrently. The user must still ensure that only
2872 one thread accesses IR in a given <tt>LLVMContext</tt> while another thread
2873 might be modifying it. One way to do that is to always hold the JIT lock while
2874 accessing IR outside the JIT (the JIT <em>modifies</em> the IR by adding
2875 <tt>CallbackVH</tt>s). Another way is to only
2876 call <tt>getPointerToFunction()</tt> from the <tt>LLVMContext</tt>'s thread.
2879 <p>When the JIT is configured to compile lazily (using
2880 <tt>ExecutionEngine::DisableLazyCompilation(false)</tt>), there is currently a
2881 <a href="http://llvm.org/bugs/show_bug.cgi?id=5184">race condition</a> in
2882 updating call sites after a function is lazily-jitted. It's still possible to
2883 use the lazy JIT in a threaded program if you ensure that only one thread at a
2884 time can call any particular lazy stub and that the JIT lock guards any IR
2885 access, but we suggest using only the eager JIT in threaded programs.
2891 <!-- *********************************************************************** -->
2893 <a name="advanced">Advanced Topics</a>
2895 <!-- *********************************************************************** -->
2899 This section describes some of the advanced or obscure API's that most clients
2900 do not need to be aware of. These API's tend manage the inner workings of the
2901 LLVM system, and only need to be accessed in unusual circumstances.
2905 <!-- ======================================================================= -->
2907 <a name="SymbolTable">The <tt>ValueSymbolTable</tt> class</a>
2911 <p>The <tt><a href="http://llvm.org/doxygen/classllvm_1_1ValueSymbolTable.html">
2912 ValueSymbolTable</a></tt> class provides a symbol table that the <a
2913 href="#Function"><tt>Function</tt></a> and <a href="#Module">
2914 <tt>Module</tt></a> classes use for naming value definitions. The symbol table
2915 can provide a name for any <a href="#Value"><tt>Value</tt></a>.
2918 <p>Note that the <tt>SymbolTable</tt> class should not be directly accessed
2919 by most clients. It should only be used when iteration over the symbol table
2920 names themselves are required, which is very special purpose. Note that not
2922 <tt><a href="#Value">Value</a></tt>s have names, and those without names (i.e. they have
2923 an empty name) do not exist in the symbol table.
2926 <p>Symbol tables support iteration over the values in the symbol
2927 table with <tt>begin/end/iterator</tt> and supports querying to see if a
2928 specific name is in the symbol table (with <tt>lookup</tt>). The
2929 <tt>ValueSymbolTable</tt> class exposes no public mutator methods, instead,
2930 simply call <tt>setName</tt> on a value, which will autoinsert it into the
2931 appropriate symbol table.</p>
2937 <!-- ======================================================================= -->
2939 <a name="UserLayout">The <tt>User</tt> and owned <tt>Use</tt> classes' memory layout</a>
2943 <p>The <tt><a href="http://llvm.org/doxygen/classllvm_1_1User.html">
2944 User</a></tt> class provides a basis for expressing the ownership of <tt>User</tt>
2945 towards other <tt><a href="http://llvm.org/doxygen/classllvm_1_1Value.html">
2946 Value</a></tt>s. The <tt><a href="http://llvm.org/doxygen/classllvm_1_1Use.html">
2947 Use</a></tt> helper class is employed to do the bookkeeping and to facilitate <i>O(1)</i>
2948 addition and removal.</p>
2950 <!-- ______________________________________________________________________ -->
2953 Interaction and relationship between <tt>User</tt> and <tt>Use</tt> objects
2959 A subclass of <tt>User</tt> can choose between incorporating its <tt>Use</tt> objects
2960 or refer to them out-of-line by means of a pointer. A mixed variant
2961 (some <tt>Use</tt>s inline others hung off) is impractical and breaks the invariant
2962 that the <tt>Use</tt> objects belonging to the same <tt>User</tt> form a contiguous array.
2966 We have 2 different layouts in the <tt>User</tt> (sub)classes:
2969 The <tt>Use</tt> object(s) are inside (resp. at fixed offset) of the <tt>User</tt>
2970 object and there are a fixed number of them.</p>
2973 The <tt>Use</tt> object(s) are referenced by a pointer to an
2974 array from the <tt>User</tt> object and there may be a variable
2978 As of v2.4 each layout still possesses a direct pointer to the
2979 start of the array of <tt>Use</tt>s. Though not mandatory for layout a),
2980 we stick to this redundancy for the sake of simplicity.
2981 The <tt>User</tt> object also stores the number of <tt>Use</tt> objects it
2982 has. (Theoretically this information can also be calculated
2983 given the scheme presented below.)</p>
2985 Special forms of allocation operators (<tt>operator new</tt>)
2986 enforce the following memory layouts:</p>
2989 <li><p>Layout a) is modelled by prepending the <tt>User</tt> object by the <tt>Use[]</tt> array.</p>
2992 ...---.---.---.---.-------...
2993 | P | P | P | P | User
2994 '''---'---'---'---'-------'''
2997 <li><p>Layout b) is modelled by pointing at the <tt>Use[]</tt> array.</p>
3009 <i>(In the above figures '<tt>P</tt>' stands for the <tt>Use**</tt> that
3010 is stored in each <tt>Use</tt> object in the member <tt>Use::Prev</tt>)</i>
3014 <!-- ______________________________________________________________________ -->
3016 <a name="Waymarking">The waymarking algorithm</a>
3021 Since the <tt>Use</tt> objects are deprived of the direct (back)pointer to
3022 their <tt>User</tt> objects, there must be a fast and exact method to
3023 recover it. This is accomplished by the following scheme:</p>
3025 A bit-encoding in the 2 LSBits (least significant bits) of the <tt>Use::Prev</tt> allows to find the
3026 start of the <tt>User</tt> object:
3028 <li><tt>00</tt> —> binary digit 0</li>
3029 <li><tt>01</tt> —> binary digit 1</li>
3030 <li><tt>10</tt> —> stop and calculate (<tt>s</tt>)</li>
3031 <li><tt>11</tt> —> full stop (<tt>S</tt>)</li>
3034 Given a <tt>Use*</tt>, all we have to do is to walk till we get
3035 a stop and we either have a <tt>User</tt> immediately behind or
3036 we have to walk to the next stop picking up digits
3037 and calculating the offset:</p>
3039 .---.---.---.---.---.---.---.---.---.---.---.---.---.---.---.---.----------------
3040 | 1 | s | 1 | 0 | 1 | 0 | s | 1 | 1 | 0 | s | 1 | 1 | s | 1 | S | User (or User*)
3041 '---'---'---'---'---'---'---'---'---'---'---'---'---'---'---'---'----------------
3042 |+15 |+10 |+6 |+3 |+1
3045 | | |______________________>
3046 | |______________________________________>
3047 |__________________________________________________________>
3050 Only the significant number of bits need to be stored between the
3051 stops, so that the <i>worst case is 20 memory accesses</i> when there are
3052 1000 <tt>Use</tt> objects associated with a <tt>User</tt>.</p>
3056 <!-- ______________________________________________________________________ -->
3058 <a name="ReferenceImpl">Reference implementation</a>
3063 The following literate Haskell fragment demonstrates the concept:</p>
3065 <div class="doc_code">
3067 > import Test.QuickCheck
3069 > digits :: Int -> [Char] -> [Char]
3070 > digits 0 acc = '0' : acc
3071 > digits 1 acc = '1' : acc
3072 > digits n acc = digits (n `div` 2) $ digits (n `mod` 2) acc
3074 > dist :: Int -> [Char] -> [Char]
3077 > dist 1 acc = let r = dist 0 acc in 's' : digits (length r) r
3078 > dist n acc = dist (n - 1) $ dist 1 acc
3080 > takeLast n ss = reverse $ take n $ reverse ss
3082 > test = takeLast 40 $ dist 20 []
3087 Printing <test> gives: <tt>"1s100000s11010s10100s1111s1010s110s11s1S"</tt></p>
3089 The reverse algorithm computes the length of the string just by examining
3090 a certain prefix:</p>
3092 <div class="doc_code">
3094 > pref :: [Char] -> Int
3096 > pref ('s':'1':rest) = decode 2 1 rest
3097 > pref (_:rest) = 1 + pref rest
3099 > decode walk acc ('0':rest) = decode (walk + 1) (acc * 2) rest
3100 > decode walk acc ('1':rest) = decode (walk + 1) (acc * 2 + 1) rest
3101 > decode walk acc _ = walk + acc
3106 Now, as expected, printing <pref test> gives <tt>40</tt>.</p>
3108 We can <i>quickCheck</i> this with following property:</p>
3110 <div class="doc_code">
3112 > testcase = dist 2000 []
3113 > testcaseLength = length testcase
3115 > identityProp n = n > 0 && n <= testcaseLength ==> length arr == pref arr
3116 > where arr = takeLast n testcase
3121 As expected <quickCheck identityProp> gives:</p>
3124 *Main> quickCheck identityProp
3125 OK, passed 100 tests.
3128 Let's be a bit more exhaustive:</p>
3130 <div class="doc_code">
3133 > deepCheck p = check (defaultConfig { configMaxTest = 500 }) p
3138 And here is the result of <deepCheck identityProp>:</p>
3141 *Main> deepCheck identityProp
3142 OK, passed 500 tests.
3147 <!-- ______________________________________________________________________ -->
3149 <a name="Tagging">Tagging considerations</a>
3155 To maintain the invariant that the 2 LSBits of each <tt>Use**</tt> in <tt>Use</tt>
3156 never change after being set up, setters of <tt>Use::Prev</tt> must re-tag the
3157 new <tt>Use**</tt> on every modification. Accordingly getters must strip the
3160 For layout b) instead of the <tt>User</tt> we find a pointer (<tt>User*</tt> with LSBit set).
3161 Following this pointer brings us to the <tt>User</tt>. A portable trick ensures
3162 that the first bytes of <tt>User</tt> (if interpreted as a pointer) never has
3163 the LSBit set. (Portability is relying on the fact that all known compilers place the
3164 <tt>vptr</tt> in the first word of the instances.)</p>
3172 <!-- *********************************************************************** -->
3174 <a name="coreclasses">The Core LLVM Class Hierarchy Reference </a>
3176 <!-- *********************************************************************** -->
3179 <p><tt>#include "<a href="/doxygen/Type_8h-source.html">llvm/Type.h</a>"</tt>
3180 <br>doxygen info: <a href="/doxygen/classllvm_1_1Type.html">Type Class</a></p>
3182 <p>The Core LLVM classes are the primary means of representing the program
3183 being inspected or transformed. The core LLVM classes are defined in
3184 header files in the <tt>include/llvm/</tt> directory, and implemented in
3185 the <tt>lib/VMCore</tt> directory.</p>
3187 <!-- ======================================================================= -->
3189 <a name="Type">The <tt>Type</tt> class and Derived Types</a>
3194 <p><tt>Type</tt> is a superclass of all type classes. Every <tt>Value</tt> has
3195 a <tt>Type</tt>. <tt>Type</tt> cannot be instantiated directly but only
3196 through its subclasses. Certain primitive types (<tt>VoidType</tt>,
3197 <tt>LabelType</tt>, <tt>FloatType</tt> and <tt>DoubleType</tt>) have hidden
3198 subclasses. They are hidden because they offer no useful functionality beyond
3199 what the <tt>Type</tt> class offers except to distinguish themselves from
3200 other subclasses of <tt>Type</tt>.</p>
3201 <p>All other types are subclasses of <tt>DerivedType</tt>. Types can be
3202 named, but this is not a requirement. There exists exactly
3203 one instance of a given shape at any one time. This allows type equality to
3204 be performed with address equality of the Type Instance. That is, given two
3205 <tt>Type*</tt> values, the types are identical if the pointers are identical.
3208 <!-- _______________________________________________________________________ -->
3210 <a name="m_Type">Important Public Methods</a>
3216 <li><tt>bool isIntegerTy() const</tt>: Returns true for any integer type.</li>
3218 <li><tt>bool isFloatingPointTy()</tt>: Return true if this is one of the five
3219 floating point types.</li>
3221 <li><tt>bool isSized()</tt>: Return true if the type has known size. Things
3222 that don't have a size are abstract types, labels and void.</li>
3227 <!-- _______________________________________________________________________ -->
3229 <a name="derivedtypes">Important Derived Types</a>
3233 <dt><tt>IntegerType</tt></dt>
3234 <dd>Subclass of DerivedType that represents integer types of any bit width.
3235 Any bit width between <tt>IntegerType::MIN_INT_BITS</tt> (1) and
3236 <tt>IntegerType::MAX_INT_BITS</tt> (~8 million) can be represented.
3238 <li><tt>static const IntegerType* get(unsigned NumBits)</tt>: get an integer
3239 type of a specific bit width.</li>
3240 <li><tt>unsigned getBitWidth() const</tt>: Get the bit width of an integer
3244 <dt><tt>SequentialType</tt></dt>
3245 <dd>This is subclassed by ArrayType, PointerType and VectorType.
3247 <li><tt>const Type * getElementType() const</tt>: Returns the type of each
3248 of the elements in the sequential type. </li>
3251 <dt><tt>ArrayType</tt></dt>
3252 <dd>This is a subclass of SequentialType and defines the interface for array
3255 <li><tt>unsigned getNumElements() const</tt>: Returns the number of
3256 elements in the array. </li>
3259 <dt><tt>PointerType</tt></dt>
3260 <dd>Subclass of SequentialType for pointer types.</dd>
3261 <dt><tt>VectorType</tt></dt>
3262 <dd>Subclass of SequentialType for vector types. A
3263 vector type is similar to an ArrayType but is distinguished because it is
3264 a first class type whereas ArrayType is not. Vector types are used for
3265 vector operations and are usually small vectors of of an integer or floating
3267 <dt><tt>StructType</tt></dt>
3268 <dd>Subclass of DerivedTypes for struct types.</dd>
3269 <dt><tt><a name="FunctionType">FunctionType</a></tt></dt>
3270 <dd>Subclass of DerivedTypes for function types.
3272 <li><tt>bool isVarArg() const</tt>: Returns true if it's a vararg
3274 <li><tt> const Type * getReturnType() const</tt>: Returns the
3275 return type of the function.</li>
3276 <li><tt>const Type * getParamType (unsigned i)</tt>: Returns
3277 the type of the ith parameter.</li>
3278 <li><tt> const unsigned getNumParams() const</tt>: Returns the
3279 number of formal parameters.</li>
3287 <!-- ======================================================================= -->
3289 <a name="Module">The <tt>Module</tt> class</a>
3295 href="/doxygen/Module_8h-source.html">llvm/Module.h</a>"</tt><br> doxygen info:
3296 <a href="/doxygen/classllvm_1_1Module.html">Module Class</a></p>
3298 <p>The <tt>Module</tt> class represents the top level structure present in LLVM
3299 programs. An LLVM module is effectively either a translation unit of the
3300 original program or a combination of several translation units merged by the
3301 linker. The <tt>Module</tt> class keeps track of a list of <a
3302 href="#Function"><tt>Function</tt></a>s, a list of <a
3303 href="#GlobalVariable"><tt>GlobalVariable</tt></a>s, and a <a
3304 href="#SymbolTable"><tt>SymbolTable</tt></a>. Additionally, it contains a few
3305 helpful member functions that try to make common operations easy.</p>
3307 <!-- _______________________________________________________________________ -->
3309 <a name="m_Module">Important Public Members of the <tt>Module</tt> class</a>
3315 <li><tt>Module::Module(std::string name = "")</tt>
3317 <p>Constructing a <a href="#Module">Module</a> is easy. You can optionally
3318 provide a name for it (probably based on the name of the translation unit).</p>
3321 <li><tt>Module::iterator</tt> - Typedef for function list iterator<br>
3322 <tt>Module::const_iterator</tt> - Typedef for const_iterator.<br>
3324 <tt>begin()</tt>, <tt>end()</tt>
3325 <tt>size()</tt>, <tt>empty()</tt>
3327 <p>These are forwarding methods that make it easy to access the contents of
3328 a <tt>Module</tt> object's <a href="#Function"><tt>Function</tt></a>
3331 <li><tt>Module::FunctionListType &getFunctionList()</tt>
3333 <p> Returns the list of <a href="#Function"><tt>Function</tt></a>s. This is
3334 necessary to use when you need to update the list or perform a complex
3335 action that doesn't have a forwarding method.</p>
3337 <p><!-- Global Variable --></p></li>
3343 <li><tt>Module::global_iterator</tt> - Typedef for global variable list iterator<br>
3345 <tt>Module::const_global_iterator</tt> - Typedef for const_iterator.<br>
3347 <tt>global_begin()</tt>, <tt>global_end()</tt>
3348 <tt>global_size()</tt>, <tt>global_empty()</tt>
3350 <p> These are forwarding methods that make it easy to access the contents of
3351 a <tt>Module</tt> object's <a
3352 href="#GlobalVariable"><tt>GlobalVariable</tt></a> list.</p></li>
3354 <li><tt>Module::GlobalListType &getGlobalList()</tt>
3356 <p>Returns the list of <a
3357 href="#GlobalVariable"><tt>GlobalVariable</tt></a>s. This is necessary to
3358 use when you need to update the list or perform a complex action that
3359 doesn't have a forwarding method.</p>
3361 <p><!-- Symbol table stuff --> </p></li>
3367 <li><tt><a href="#SymbolTable">SymbolTable</a> *getSymbolTable()</tt>
3369 <p>Return a reference to the <a href="#SymbolTable"><tt>SymbolTable</tt></a>
3370 for this <tt>Module</tt>.</p>
3372 <p><!-- Convenience methods --></p></li>
3379 <li><tt><a href="#Function">Function</a> *getFunction(StringRef Name) const
3382 <p>Look up the specified function in the <tt>Module</tt> <a
3383 href="#SymbolTable"><tt>SymbolTable</tt></a>. If it does not exist, return
3384 <tt>null</tt>.</p></li>
3386 <li><tt><a href="#Function">Function</a> *getOrInsertFunction(const
3387 std::string &Name, const <a href="#FunctionType">FunctionType</a> *T)</tt>
3389 <p>Look up the specified function in the <tt>Module</tt> <a
3390 href="#SymbolTable"><tt>SymbolTable</tt></a>. If it does not exist, add an
3391 external declaration for the function and return it.</p></li>
3393 <li><tt>std::string getTypeName(const <a href="#Type">Type</a> *Ty)</tt>
3395 <p>If there is at least one entry in the <a
3396 href="#SymbolTable"><tt>SymbolTable</tt></a> for the specified <a
3397 href="#Type"><tt>Type</tt></a>, return it. Otherwise return the empty
3400 <li><tt>bool addTypeName(const std::string &Name, const <a
3401 href="#Type">Type</a> *Ty)</tt>
3403 <p>Insert an entry in the <a href="#SymbolTable"><tt>SymbolTable</tt></a>
3404 mapping <tt>Name</tt> to <tt>Ty</tt>. If there is already an entry for this
3405 name, true is returned and the <a
3406 href="#SymbolTable"><tt>SymbolTable</tt></a> is not modified.</p></li>
3413 <!-- ======================================================================= -->
3415 <a name="Value">The <tt>Value</tt> class</a>
3420 <p><tt>#include "<a href="/doxygen/Value_8h-source.html">llvm/Value.h</a>"</tt>
3422 doxygen info: <a href="/doxygen/classllvm_1_1Value.html">Value Class</a></p>
3424 <p>The <tt>Value</tt> class is the most important class in the LLVM Source
3425 base. It represents a typed value that may be used (among other things) as an
3426 operand to an instruction. There are many different types of <tt>Value</tt>s,
3427 such as <a href="#Constant"><tt>Constant</tt></a>s,<a
3428 href="#Argument"><tt>Argument</tt></a>s. Even <a
3429 href="#Instruction"><tt>Instruction</tt></a>s and <a
3430 href="#Function"><tt>Function</tt></a>s are <tt>Value</tt>s.</p>
3432 <p>A particular <tt>Value</tt> may be used many times in the LLVM representation
3433 for a program. For example, an incoming argument to a function (represented
3434 with an instance of the <a href="#Argument">Argument</a> class) is "used" by
3435 every instruction in the function that references the argument. To keep track
3436 of this relationship, the <tt>Value</tt> class keeps a list of all of the <a
3437 href="#User"><tt>User</tt></a>s that is using it (the <a
3438 href="#User"><tt>User</tt></a> class is a base class for all nodes in the LLVM
3439 graph that can refer to <tt>Value</tt>s). This use list is how LLVM represents
3440 def-use information in the program, and is accessible through the <tt>use_</tt>*
3441 methods, shown below.</p>
3443 <p>Because LLVM is a typed representation, every LLVM <tt>Value</tt> is typed,
3444 and this <a href="#Type">Type</a> is available through the <tt>getType()</tt>
3445 method. In addition, all LLVM values can be named. The "name" of the
3446 <tt>Value</tt> is a symbolic string printed in the LLVM code:</p>
3448 <div class="doc_code">
3450 %<b>foo</b> = add i32 1, 2
3454 <p><a name="nameWarning">The name of this instruction is "foo".</a> <b>NOTE</b>
3455 that the name of any value may be missing (an empty string), so names should
3456 <b>ONLY</b> be used for debugging (making the source code easier to read,
3457 debugging printouts), they should not be used to keep track of values or map
3458 between them. For this purpose, use a <tt>std::map</tt> of pointers to the
3459 <tt>Value</tt> itself instead.</p>
3461 <p>One important aspect of LLVM is that there is no distinction between an SSA
3462 variable and the operation that produces it. Because of this, any reference to
3463 the value produced by an instruction (or the value available as an incoming
3464 argument, for example) is represented as a direct pointer to the instance of
3466 represents this value. Although this may take some getting used to, it
3467 simplifies the representation and makes it easier to manipulate.</p>
3469 <!-- _______________________________________________________________________ -->
3471 <a name="m_Value">Important Public Members of the <tt>Value</tt> class</a>
3477 <li><tt>Value::use_iterator</tt> - Typedef for iterator over the
3479 <tt>Value::const_use_iterator</tt> - Typedef for const_iterator over
3481 <tt>unsigned use_size()</tt> - Returns the number of users of the
3483 <tt>bool use_empty()</tt> - Returns true if there are no users.<br>
3484 <tt>use_iterator use_begin()</tt> - Get an iterator to the start of
3486 <tt>use_iterator use_end()</tt> - Get an iterator to the end of the
3488 <tt><a href="#User">User</a> *use_back()</tt> - Returns the last
3489 element in the list.
3490 <p> These methods are the interface to access the def-use
3491 information in LLVM. As with all other iterators in LLVM, the naming
3492 conventions follow the conventions defined by the <a href="#stl">STL</a>.</p>
3494 <li><tt><a href="#Type">Type</a> *getType() const</tt>
3495 <p>This method returns the Type of the Value.</p>
3497 <li><tt>bool hasName() const</tt><br>
3498 <tt>std::string getName() const</tt><br>
3499 <tt>void setName(const std::string &Name)</tt>
3500 <p> This family of methods is used to access and assign a name to a <tt>Value</tt>,
3501 be aware of the <a href="#nameWarning">precaution above</a>.</p>
3503 <li><tt>void replaceAllUsesWith(Value *V)</tt>
3505 <p>This method traverses the use list of a <tt>Value</tt> changing all <a
3506 href="#User"><tt>User</tt>s</a> of the current value to refer to
3507 "<tt>V</tt>" instead. For example, if you detect that an instruction always
3508 produces a constant value (for example through constant folding), you can
3509 replace all uses of the instruction with the constant like this:</p>
3511 <div class="doc_code">
3513 Inst->replaceAllUsesWith(ConstVal);
3523 <!-- ======================================================================= -->
3525 <a name="User">The <tt>User</tt> class</a>
3531 <tt>#include "<a href="/doxygen/User_8h-source.html">llvm/User.h</a>"</tt><br>
3532 doxygen info: <a href="/doxygen/classllvm_1_1User.html">User Class</a><br>
3533 Superclass: <a href="#Value"><tt>Value</tt></a></p>
3535 <p>The <tt>User</tt> class is the common base class of all LLVM nodes that may
3536 refer to <a href="#Value"><tt>Value</tt></a>s. It exposes a list of "Operands"
3537 that are all of the <a href="#Value"><tt>Value</tt></a>s that the User is
3538 referring to. The <tt>User</tt> class itself is a subclass of
3541 <p>The operands of a <tt>User</tt> point directly to the LLVM <a
3542 href="#Value"><tt>Value</tt></a> that it refers to. Because LLVM uses Static
3543 Single Assignment (SSA) form, there can only be one definition referred to,
3544 allowing this direct connection. This connection provides the use-def
3545 information in LLVM.</p>
3547 <!-- _______________________________________________________________________ -->
3549 <a name="m_User">Important Public Members of the <tt>User</tt> class</a>
3554 <p>The <tt>User</tt> class exposes the operand list in two ways: through
3555 an index access interface and through an iterator based interface.</p>
3558 <li><tt>Value *getOperand(unsigned i)</tt><br>
3559 <tt>unsigned getNumOperands()</tt>
3560 <p> These two methods expose the operands of the <tt>User</tt> in a
3561 convenient form for direct access.</p></li>
3563 <li><tt>User::op_iterator</tt> - Typedef for iterator over the operand
3565 <tt>op_iterator op_begin()</tt> - Get an iterator to the start of
3566 the operand list.<br>
3567 <tt>op_iterator op_end()</tt> - Get an iterator to the end of the
3569 <p> Together, these methods make up the iterator based interface to
3570 the operands of a <tt>User</tt>.</p></li>
3577 <!-- ======================================================================= -->
3579 <a name="Instruction">The <tt>Instruction</tt> class</a>
3584 <p><tt>#include "</tt><tt><a
3585 href="/doxygen/Instruction_8h-source.html">llvm/Instruction.h</a>"</tt><br>
3586 doxygen info: <a href="/doxygen/classllvm_1_1Instruction.html">Instruction Class</a><br>
3587 Superclasses: <a href="#User"><tt>User</tt></a>, <a
3588 href="#Value"><tt>Value</tt></a></p>
3590 <p>The <tt>Instruction</tt> class is the common base class for all LLVM
3591 instructions. It provides only a few methods, but is a very commonly used
3592 class. The primary data tracked by the <tt>Instruction</tt> class itself is the
3593 opcode (instruction type) and the parent <a
3594 href="#BasicBlock"><tt>BasicBlock</tt></a> the <tt>Instruction</tt> is embedded
3595 into. To represent a specific type of instruction, one of many subclasses of
3596 <tt>Instruction</tt> are used.</p>
3598 <p> Because the <tt>Instruction</tt> class subclasses the <a
3599 href="#User"><tt>User</tt></a> class, its operands can be accessed in the same
3600 way as for other <a href="#User"><tt>User</tt></a>s (with the
3601 <tt>getOperand()</tt>/<tt>getNumOperands()</tt> and
3602 <tt>op_begin()</tt>/<tt>op_end()</tt> methods).</p> <p> An important file for
3603 the <tt>Instruction</tt> class is the <tt>llvm/Instruction.def</tt> file. This
3604 file contains some meta-data about the various different types of instructions
3605 in LLVM. It describes the enum values that are used as opcodes (for example
3606 <tt>Instruction::Add</tt> and <tt>Instruction::ICmp</tt>), as well as the
3607 concrete sub-classes of <tt>Instruction</tt> that implement the instruction (for
3608 example <tt><a href="#BinaryOperator">BinaryOperator</a></tt> and <tt><a
3609 href="#CmpInst">CmpInst</a></tt>). Unfortunately, the use of macros in
3610 this file confuses doxygen, so these enum values don't show up correctly in the
3611 <a href="/doxygen/classllvm_1_1Instruction.html">doxygen output</a>.</p>
3613 <!-- _______________________________________________________________________ -->
3615 <a name="s_Instruction">
3616 Important Subclasses of the <tt>Instruction</tt> class
3621 <li><tt><a name="BinaryOperator">BinaryOperator</a></tt>
3622 <p>This subclasses represents all two operand instructions whose operands
3623 must be the same type, except for the comparison instructions.</p></li>
3624 <li><tt><a name="CastInst">CastInst</a></tt>
3625 <p>This subclass is the parent of the 12 casting instructions. It provides
3626 common operations on cast instructions.</p>
3627 <li><tt><a name="CmpInst">CmpInst</a></tt>
3628 <p>This subclass respresents the two comparison instructions,
3629 <a href="LangRef.html#i_icmp">ICmpInst</a> (integer opreands), and
3630 <a href="LangRef.html#i_fcmp">FCmpInst</a> (floating point operands).</p>
3631 <li><tt><a name="TerminatorInst">TerminatorInst</a></tt>
3632 <p>This subclass is the parent of all terminator instructions (those which
3633 can terminate a block).</p>
3637 <!-- _______________________________________________________________________ -->
3639 <a name="m_Instruction">
3640 Important Public Members of the <tt>Instruction</tt> class
3647 <li><tt><a href="#BasicBlock">BasicBlock</a> *getParent()</tt>
3648 <p>Returns the <a href="#BasicBlock"><tt>BasicBlock</tt></a> that
3649 this <tt>Instruction</tt> is embedded into.</p></li>
3650 <li><tt>bool mayWriteToMemory()</tt>
3651 <p>Returns true if the instruction writes to memory, i.e. it is a
3652 <tt>call</tt>,<tt>free</tt>,<tt>invoke</tt>, or <tt>store</tt>.</p></li>
3653 <li><tt>unsigned getOpcode()</tt>
3654 <p>Returns the opcode for the <tt>Instruction</tt>.</p></li>
3655 <li><tt><a href="#Instruction">Instruction</a> *clone() const</tt>
3656 <p>Returns another instance of the specified instruction, identical
3657 in all ways to the original except that the instruction has no parent
3658 (ie it's not embedded into a <a href="#BasicBlock"><tt>BasicBlock</tt></a>),
3659 and it has no name</p></li>
3666 <!-- ======================================================================= -->
3668 <a name="Constant">The <tt>Constant</tt> class and subclasses</a>
3673 <p>Constant represents a base class for different types of constants. It
3674 is subclassed by ConstantInt, ConstantArray, etc. for representing
3675 the various types of Constants. <a href="#GlobalValue">GlobalValue</a> is also
3676 a subclass, which represents the address of a global variable or function.
3679 <!-- _______________________________________________________________________ -->
3680 <h4>Important Subclasses of Constant</h4>
3683 <li>ConstantInt : This subclass of Constant represents an integer constant of
3686 <li><tt>const APInt& getValue() const</tt>: Returns the underlying
3687 value of this constant, an APInt value.</li>
3688 <li><tt>int64_t getSExtValue() const</tt>: Converts the underlying APInt
3689 value to an int64_t via sign extension. If the value (not the bit width)
3690 of the APInt is too large to fit in an int64_t, an assertion will result.
3691 For this reason, use of this method is discouraged.</li>
3692 <li><tt>uint64_t getZExtValue() const</tt>: Converts the underlying APInt
3693 value to a uint64_t via zero extension. IF the value (not the bit width)
3694 of the APInt is too large to fit in a uint64_t, an assertion will result.
3695 For this reason, use of this method is discouraged.</li>
3696 <li><tt>static ConstantInt* get(const APInt& Val)</tt>: Returns the
3697 ConstantInt object that represents the value provided by <tt>Val</tt>.
3698 The type is implied as the IntegerType that corresponds to the bit width
3699 of <tt>Val</tt>.</li>
3700 <li><tt>static ConstantInt* get(const Type *Ty, uint64_t Val)</tt>:
3701 Returns the ConstantInt object that represents the value provided by
3702 <tt>Val</tt> for integer type <tt>Ty</tt>.</li>
3705 <li>ConstantFP : This class represents a floating point constant.
3707 <li><tt>double getValue() const</tt>: Returns the underlying value of
3708 this constant. </li>
3711 <li>ConstantArray : This represents a constant array.
3713 <li><tt>const std::vector<Use> &getValues() const</tt>: Returns
3714 a vector of component constants that makeup this array. </li>
3717 <li>ConstantStruct : This represents a constant struct.
3719 <li><tt>const std::vector<Use> &getValues() const</tt>: Returns
3720 a vector of component constants that makeup this array. </li>
3723 <li>GlobalValue : This represents either a global variable or a function. In
3724 either case, the value is a constant fixed address (after linking).
3731 <!-- ======================================================================= -->
3733 <a name="GlobalValue">The <tt>GlobalValue</tt> class</a>
3739 href="/doxygen/GlobalValue_8h-source.html">llvm/GlobalValue.h</a>"</tt><br>
3740 doxygen info: <a href="/doxygen/classllvm_1_1GlobalValue.html">GlobalValue
3742 Superclasses: <a href="#Constant"><tt>Constant</tt></a>,
3743 <a href="#User"><tt>User</tt></a>, <a href="#Value"><tt>Value</tt></a></p>
3745 <p>Global values (<a href="#GlobalVariable"><tt>GlobalVariable</tt></a>s or <a
3746 href="#Function"><tt>Function</tt></a>s) are the only LLVM values that are
3747 visible in the bodies of all <a href="#Function"><tt>Function</tt></a>s.
3748 Because they are visible at global scope, they are also subject to linking with
3749 other globals defined in different translation units. To control the linking
3750 process, <tt>GlobalValue</tt>s know their linkage rules. Specifically,
3751 <tt>GlobalValue</tt>s know whether they have internal or external linkage, as
3752 defined by the <tt>LinkageTypes</tt> enumeration.</p>
3754 <p>If a <tt>GlobalValue</tt> has internal linkage (equivalent to being
3755 <tt>static</tt> in C), it is not visible to code outside the current translation
3756 unit, and does not participate in linking. If it has external linkage, it is
3757 visible to external code, and does participate in linking. In addition to
3758 linkage information, <tt>GlobalValue</tt>s keep track of which <a
3759 href="#Module"><tt>Module</tt></a> they are currently part of.</p>
3761 <p>Because <tt>GlobalValue</tt>s are memory objects, they are always referred to
3762 by their <b>address</b>. As such, the <a href="#Type"><tt>Type</tt></a> of a
3763 global is always a pointer to its contents. It is important to remember this
3764 when using the <tt>GetElementPtrInst</tt> instruction because this pointer must
3765 be dereferenced first. For example, if you have a <tt>GlobalVariable</tt> (a
3766 subclass of <tt>GlobalValue)</tt> that is an array of 24 ints, type <tt>[24 x
3767 i32]</tt>, then the <tt>GlobalVariable</tt> is a pointer to that array. Although
3768 the address of the first element of this array and the value of the
3769 <tt>GlobalVariable</tt> are the same, they have different types. The
3770 <tt>GlobalVariable</tt>'s type is <tt>[24 x i32]</tt>. The first element's type
3771 is <tt>i32.</tt> Because of this, accessing a global value requires you to
3772 dereference the pointer with <tt>GetElementPtrInst</tt> first, then its elements
3773 can be accessed. This is explained in the <a href="LangRef.html#globalvars">LLVM
3774 Language Reference Manual</a>.</p>
3776 <!-- _______________________________________________________________________ -->
3778 <a name="m_GlobalValue">
3779 Important Public Members of the <tt>GlobalValue</tt> class
3786 <li><tt>bool hasInternalLinkage() const</tt><br>
3787 <tt>bool hasExternalLinkage() const</tt><br>
3788 <tt>void setInternalLinkage(bool HasInternalLinkage)</tt>
3789 <p> These methods manipulate the linkage characteristics of the <tt>GlobalValue</tt>.</p>
3792 <li><tt><a href="#Module">Module</a> *getParent()</tt>
3793 <p> This returns the <a href="#Module"><tt>Module</tt></a> that the
3794 GlobalValue is currently embedded into.</p></li>
3801 <!-- ======================================================================= -->
3803 <a name="Function">The <tt>Function</tt> class</a>
3809 href="/doxygen/Function_8h-source.html">llvm/Function.h</a>"</tt><br> doxygen
3810 info: <a href="/doxygen/classllvm_1_1Function.html">Function Class</a><br>
3811 Superclasses: <a href="#GlobalValue"><tt>GlobalValue</tt></a>,
3812 <a href="#Constant"><tt>Constant</tt></a>,
3813 <a href="#User"><tt>User</tt></a>,
3814 <a href="#Value"><tt>Value</tt></a></p>
3816 <p>The <tt>Function</tt> class represents a single procedure in LLVM. It is
3817 actually one of the more complex classes in the LLVM hierarchy because it must
3818 keep track of a large amount of data. The <tt>Function</tt> class keeps track
3819 of a list of <a href="#BasicBlock"><tt>BasicBlock</tt></a>s, a list of formal
3820 <a href="#Argument"><tt>Argument</tt></a>s, and a
3821 <a href="#SymbolTable"><tt>SymbolTable</tt></a>.</p>
3823 <p>The list of <a href="#BasicBlock"><tt>BasicBlock</tt></a>s is the most
3824 commonly used part of <tt>Function</tt> objects. The list imposes an implicit
3825 ordering of the blocks in the function, which indicate how the code will be
3826 laid out by the backend. Additionally, the first <a
3827 href="#BasicBlock"><tt>BasicBlock</tt></a> is the implicit entry node for the
3828 <tt>Function</tt>. It is not legal in LLVM to explicitly branch to this initial
3829 block. There are no implicit exit nodes, and in fact there may be multiple exit
3830 nodes from a single <tt>Function</tt>. If the <a
3831 href="#BasicBlock"><tt>BasicBlock</tt></a> list is empty, this indicates that
3832 the <tt>Function</tt> is actually a function declaration: the actual body of the
3833 function hasn't been linked in yet.</p>
3835 <p>In addition to a list of <a href="#BasicBlock"><tt>BasicBlock</tt></a>s, the
3836 <tt>Function</tt> class also keeps track of the list of formal <a
3837 href="#Argument"><tt>Argument</tt></a>s that the function receives. This
3838 container manages the lifetime of the <a href="#Argument"><tt>Argument</tt></a>
3839 nodes, just like the <a href="#BasicBlock"><tt>BasicBlock</tt></a> list does for
3840 the <a href="#BasicBlock"><tt>BasicBlock</tt></a>s.</p>
3842 <p>The <a href="#SymbolTable"><tt>SymbolTable</tt></a> is a very rarely used
3843 LLVM feature that is only used when you have to look up a value by name. Aside
3844 from that, the <a href="#SymbolTable"><tt>SymbolTable</tt></a> is used
3845 internally to make sure that there are not conflicts between the names of <a
3846 href="#Instruction"><tt>Instruction</tt></a>s, <a
3847 href="#BasicBlock"><tt>BasicBlock</tt></a>s, or <a
3848 href="#Argument"><tt>Argument</tt></a>s in the function body.</p>
3850 <p>Note that <tt>Function</tt> is a <a href="#GlobalValue">GlobalValue</a>
3851 and therefore also a <a href="#Constant">Constant</a>. The value of the function
3852 is its address (after linking) which is guaranteed to be constant.</p>
3854 <!-- _______________________________________________________________________ -->
3856 <a name="m_Function">
3857 Important Public Members of the <tt>Function</tt> class
3864 <li><tt>Function(const </tt><tt><a href="#FunctionType">FunctionType</a>
3865 *Ty, LinkageTypes Linkage, const std::string &N = "", Module* Parent = 0)</tt>
3867 <p>Constructor used when you need to create new <tt>Function</tt>s to add
3868 the program. The constructor must specify the type of the function to
3869 create and what type of linkage the function should have. The <a
3870 href="#FunctionType"><tt>FunctionType</tt></a> argument
3871 specifies the formal arguments and return value for the function. The same
3872 <a href="#FunctionType"><tt>FunctionType</tt></a> value can be used to
3873 create multiple functions. The <tt>Parent</tt> argument specifies the Module
3874 in which the function is defined. If this argument is provided, the function
3875 will automatically be inserted into that module's list of
3878 <li><tt>bool isDeclaration()</tt>
3880 <p>Return whether or not the <tt>Function</tt> has a body defined. If the
3881 function is "external", it does not have a body, and thus must be resolved
3882 by linking with a function defined in a different translation unit.</p></li>
3884 <li><tt>Function::iterator</tt> - Typedef for basic block list iterator<br>
3885 <tt>Function::const_iterator</tt> - Typedef for const_iterator.<br>
3887 <tt>begin()</tt>, <tt>end()</tt>
3888 <tt>size()</tt>, <tt>empty()</tt>
3890 <p>These are forwarding methods that make it easy to access the contents of
3891 a <tt>Function</tt> object's <a href="#BasicBlock"><tt>BasicBlock</tt></a>
3894 <li><tt>Function::BasicBlockListType &getBasicBlockList()</tt>
3896 <p>Returns the list of <a href="#BasicBlock"><tt>BasicBlock</tt></a>s. This
3897 is necessary to use when you need to update the list or perform a complex
3898 action that doesn't have a forwarding method.</p></li>
3900 <li><tt>Function::arg_iterator</tt> - Typedef for the argument list
3902 <tt>Function::const_arg_iterator</tt> - Typedef for const_iterator.<br>
3904 <tt>arg_begin()</tt>, <tt>arg_end()</tt>
3905 <tt>arg_size()</tt>, <tt>arg_empty()</tt>
3907 <p>These are forwarding methods that make it easy to access the contents of
3908 a <tt>Function</tt> object's <a href="#Argument"><tt>Argument</tt></a>
3911 <li><tt>Function::ArgumentListType &getArgumentList()</tt>
3913 <p>Returns the list of <a href="#Argument"><tt>Argument</tt></a>s. This is
3914 necessary to use when you need to update the list or perform a complex
3915 action that doesn't have a forwarding method.</p></li>
3917 <li><tt><a href="#BasicBlock">BasicBlock</a> &getEntryBlock()</tt>
3919 <p>Returns the entry <a href="#BasicBlock"><tt>BasicBlock</tt></a> for the
3920 function. Because the entry block for the function is always the first
3921 block, this returns the first block of the <tt>Function</tt>.</p></li>
3923 <li><tt><a href="#Type">Type</a> *getReturnType()</tt><br>
3924 <tt><a href="#FunctionType">FunctionType</a> *getFunctionType()</tt>
3926 <p>This traverses the <a href="#Type"><tt>Type</tt></a> of the
3927 <tt>Function</tt> and returns the return type of the function, or the <a
3928 href="#FunctionType"><tt>FunctionType</tt></a> of the actual
3931 <li><tt><a href="#SymbolTable">SymbolTable</a> *getSymbolTable()</tt>
3933 <p> Return a pointer to the <a href="#SymbolTable"><tt>SymbolTable</tt></a>
3934 for this <tt>Function</tt>.</p></li>
3941 <!-- ======================================================================= -->
3943 <a name="GlobalVariable">The <tt>GlobalVariable</tt> class</a>
3949 href="/doxygen/GlobalVariable_8h-source.html">llvm/GlobalVariable.h</a>"</tt>
3951 doxygen info: <a href="/doxygen/classllvm_1_1GlobalVariable.html">GlobalVariable
3953 Superclasses: <a href="#GlobalValue"><tt>GlobalValue</tt></a>,
3954 <a href="#Constant"><tt>Constant</tt></a>,
3955 <a href="#User"><tt>User</tt></a>,
3956 <a href="#Value"><tt>Value</tt></a></p>
3958 <p>Global variables are represented with the (surprise surprise)
3959 <tt>GlobalVariable</tt> class. Like functions, <tt>GlobalVariable</tt>s are also
3960 subclasses of <a href="#GlobalValue"><tt>GlobalValue</tt></a>, and as such are
3961 always referenced by their address (global values must live in memory, so their
3962 "name" refers to their constant address). See
3963 <a href="#GlobalValue"><tt>GlobalValue</tt></a> for more on this. Global
3964 variables may have an initial value (which must be a
3965 <a href="#Constant"><tt>Constant</tt></a>), and if they have an initializer,
3966 they may be marked as "constant" themselves (indicating that their contents
3967 never change at runtime).</p>
3969 <!-- _______________________________________________________________________ -->
3971 <a name="m_GlobalVariable">
3972 Important Public Members of the <tt>GlobalVariable</tt> class
3979 <li><tt>GlobalVariable(const </tt><tt><a href="#Type">Type</a> *Ty, bool
3980 isConstant, LinkageTypes& Linkage, <a href="#Constant">Constant</a>
3981 *Initializer = 0, const std::string &Name = "", Module* Parent = 0)</tt>
3983 <p>Create a new global variable of the specified type. If
3984 <tt>isConstant</tt> is true then the global variable will be marked as
3985 unchanging for the program. The Linkage parameter specifies the type of
3986 linkage (internal, external, weak, linkonce, appending) for the variable.
3987 If the linkage is InternalLinkage, WeakAnyLinkage, WeakODRLinkage,
3988 LinkOnceAnyLinkage or LinkOnceODRLinkage, then the resultant
3989 global variable will have internal linkage. AppendingLinkage concatenates
3990 together all instances (in different translation units) of the variable
3991 into a single variable but is only applicable to arrays. See
3992 the <a href="LangRef.html#modulestructure">LLVM Language Reference</a> for
3993 further details on linkage types. Optionally an initializer, a name, and the
3994 module to put the variable into may be specified for the global variable as
3997 <li><tt>bool isConstant() const</tt>
3999 <p>Returns true if this is a global variable that is known not to
4000 be modified at runtime.</p></li>
4002 <li><tt>bool hasInitializer()</tt>
4004 <p>Returns true if this <tt>GlobalVariable</tt> has an intializer.</p></li>
4006 <li><tt><a href="#Constant">Constant</a> *getInitializer()</tt>
4008 <p>Returns the initial value for a <tt>GlobalVariable</tt>. It is not legal
4009 to call this method if there is no initializer.</p></li>
4016 <!-- ======================================================================= -->
4018 <a name="BasicBlock">The <tt>BasicBlock</tt> class</a>
4024 href="/doxygen/BasicBlock_8h-source.html">llvm/BasicBlock.h</a>"</tt><br>
4025 doxygen info: <a href="/doxygen/classllvm_1_1BasicBlock.html">BasicBlock
4027 Superclass: <a href="#Value"><tt>Value</tt></a></p>
4029 <p>This class represents a single entry single exit section of the code,
4030 commonly known as a basic block by the compiler community. The
4031 <tt>BasicBlock</tt> class maintains a list of <a
4032 href="#Instruction"><tt>Instruction</tt></a>s, which form the body of the block.
4033 Matching the language definition, the last element of this list of instructions
4034 is always a terminator instruction (a subclass of the <a
4035 href="#TerminatorInst"><tt>TerminatorInst</tt></a> class).</p>
4037 <p>In addition to tracking the list of instructions that make up the block, the
4038 <tt>BasicBlock</tt> class also keeps track of the <a
4039 href="#Function"><tt>Function</tt></a> that it is embedded into.</p>
4041 <p>Note that <tt>BasicBlock</tt>s themselves are <a
4042 href="#Value"><tt>Value</tt></a>s, because they are referenced by instructions
4043 like branches and can go in the switch tables. <tt>BasicBlock</tt>s have type
4046 <!-- _______________________________________________________________________ -->
4048 <a name="m_BasicBlock">
4049 Important Public Members of the <tt>BasicBlock</tt> class
4056 <li><tt>BasicBlock(const std::string &Name = "", </tt><tt><a
4057 href="#Function">Function</a> *Parent = 0)</tt>
4059 <p>The <tt>BasicBlock</tt> constructor is used to create new basic blocks for
4060 insertion into a function. The constructor optionally takes a name for the new
4061 block, and a <a href="#Function"><tt>Function</tt></a> to insert it into. If
4062 the <tt>Parent</tt> parameter is specified, the new <tt>BasicBlock</tt> is
4063 automatically inserted at the end of the specified <a
4064 href="#Function"><tt>Function</tt></a>, if not specified, the BasicBlock must be
4065 manually inserted into the <a href="#Function"><tt>Function</tt></a>.</p></li>
4067 <li><tt>BasicBlock::iterator</tt> - Typedef for instruction list iterator<br>
4068 <tt>BasicBlock::const_iterator</tt> - Typedef for const_iterator.<br>
4069 <tt>begin()</tt>, <tt>end()</tt>, <tt>front()</tt>, <tt>back()</tt>,
4070 <tt>size()</tt>, <tt>empty()</tt>
4071 STL-style functions for accessing the instruction list.
4073 <p>These methods and typedefs are forwarding functions that have the same
4074 semantics as the standard library methods of the same names. These methods
4075 expose the underlying instruction list of a basic block in a way that is easy to
4076 manipulate. To get the full complement of container operations (including
4077 operations to update the list), you must use the <tt>getInstList()</tt>
4080 <li><tt>BasicBlock::InstListType &getInstList()</tt>
4082 <p>This method is used to get access to the underlying container that actually
4083 holds the Instructions. This method must be used when there isn't a forwarding
4084 function in the <tt>BasicBlock</tt> class for the operation that you would like
4085 to perform. Because there are no forwarding functions for "updating"
4086 operations, you need to use this if you want to update the contents of a
4087 <tt>BasicBlock</tt>.</p></li>
4089 <li><tt><a href="#Function">Function</a> *getParent()</tt>
4091 <p> Returns a pointer to <a href="#Function"><tt>Function</tt></a> the block is
4092 embedded into, or a null pointer if it is homeless.</p></li>
4094 <li><tt><a href="#TerminatorInst">TerminatorInst</a> *getTerminator()</tt>
4096 <p> Returns a pointer to the terminator instruction that appears at the end of
4097 the <tt>BasicBlock</tt>. If there is no terminator instruction, or if the last
4098 instruction in the block is not a terminator, then a null pointer is
4107 <!-- ======================================================================= -->
4109 <a name="Argument">The <tt>Argument</tt> class</a>
4114 <p>This subclass of Value defines the interface for incoming formal
4115 arguments to a function. A Function maintains a list of its formal
4116 arguments. An argument has a pointer to the parent Function.</p>
4122 <!-- *********************************************************************** -->
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4130 <a href="mailto:dhurjati@cs.uiuc.edu">Dinakar Dhurjati</a> and
4131 <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
4132 <a href="http://llvm.org/">The LLVM Compiler Infrastructure</a><br>
4133 Last modified: $Date$