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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_FoldingSet">"llvm/ADT/FoldingSet.h"</a></li>
85 <li><a href="#dss_set"><set></a></li>
86 <li><a href="#dss_setvector">"llvm/ADT/SetVector.h"</a></li>
87 <li><a href="#dss_uniquevector">"llvm/ADT/UniqueVector.h"</a></li>
88 <li><a href="#dss_otherset">Other Set-Like ContainerOptions</a></li>
90 <li><a href="#ds_map">Map-Like Containers (std::map, DenseMap, etc)</a>
92 <li><a href="#dss_sortedvectormap">A sorted 'vector'</a></li>
93 <li><a href="#dss_stringmap">"llvm/ADT/StringMap.h"</a></li>
94 <li><a href="#dss_indexedmap">"llvm/ADT/IndexedMap.h"</a></li>
95 <li><a href="#dss_densemap">"llvm/ADT/DenseMap.h"</a></li>
96 <li><a href="#dss_valuemap">"llvm/ADT/ValueMap.h"</a></li>
97 <li><a href="#dss_intervalmap">"llvm/ADT/IntervalMap.h"</a></li>
98 <li><a href="#dss_map"><map></a></li>
99 <li><a href="#dss_inteqclasses">"llvm/ADT/IntEqClasses.h"</a></li>
100 <li><a href="#dss_othermap">Other Map-Like Container Options</a></li>
102 <li><a href="#ds_bit">BitVector-like containers</a>
104 <li><a href="#dss_bitvector">A dense bitvector</a></li>
105 <li><a href="#dss_smallbitvector">A "small" dense bitvector</a></li>
106 <li><a href="#dss_sparsebitvector">A sparse bitvector</a></li>
110 <li><a href="#common">Helpful Hints for Common Operations</a>
112 <li><a href="#inspection">Basic Inspection and Traversal Routines</a>
114 <li><a href="#iterate_function">Iterating over the <tt>BasicBlock</tt>s
115 in a <tt>Function</tt></a> </li>
116 <li><a href="#iterate_basicblock">Iterating over the <tt>Instruction</tt>s
117 in a <tt>BasicBlock</tt></a> </li>
118 <li><a href="#iterate_institer">Iterating over the <tt>Instruction</tt>s
119 in a <tt>Function</tt></a> </li>
120 <li><a href="#iterate_convert">Turning an iterator into a
121 class pointer</a> </li>
122 <li><a href="#iterate_complex">Finding call sites: a more
123 complex example</a> </li>
124 <li><a href="#calls_and_invokes">Treating calls and invokes
125 the same way</a> </li>
126 <li><a href="#iterate_chains">Iterating over def-use &
127 use-def chains</a> </li>
128 <li><a href="#iterate_preds">Iterating over predecessors &
129 successors of blocks</a></li>
132 <li><a href="#simplechanges">Making simple changes</a>
134 <li><a href="#schanges_creating">Creating and inserting new
135 <tt>Instruction</tt>s</a> </li>
136 <li><a href="#schanges_deleting">Deleting <tt>Instruction</tt>s</a> </li>
137 <li><a href="#schanges_replacing">Replacing an <tt>Instruction</tt>
138 with another <tt>Value</tt></a> </li>
139 <li><a href="#schanges_deletingGV">Deleting <tt>GlobalVariable</tt>s</a> </li>
142 <li><a href="#create_types">How to Create Types</a></li>
144 <li>Working with the Control Flow Graph
146 <li>Accessing predecessors and successors of a <tt>BasicBlock</tt>
154 <li><a href="#threading">Threads and LLVM</a>
156 <li><a href="#startmultithreaded">Entering and Exiting Multithreaded Mode
158 <li><a href="#shutdown">Ending execution with <tt>llvm_shutdown()</tt></a></li>
159 <li><a href="#managedstatic">Lazy initialization with <tt>ManagedStatic</tt></a></li>
160 <li><a href="#llvmcontext">Achieving Isolation with <tt>LLVMContext</tt></a></li>
161 <li><a href="#jitthreading">Threads and the JIT</a></li>
165 <li><a href="#advanced">Advanced Topics</a>
168 <li><a href="#SymbolTable">The <tt>ValueSymbolTable</tt> class</a></li>
169 <li><a href="#UserLayout">The <tt>User</tt> and owned <tt>Use</tt> classes' memory layout</a></li>
172 <li><a href="#coreclasses">The Core LLVM Class Hierarchy Reference</a>
174 <li><a href="#Type">The <tt>Type</tt> class</a> </li>
175 <li><a href="#Module">The <tt>Module</tt> class</a></li>
176 <li><a href="#Value">The <tt>Value</tt> class</a>
178 <li><a href="#User">The <tt>User</tt> class</a>
180 <li><a href="#Instruction">The <tt>Instruction</tt> class</a></li>
181 <li><a href="#Constant">The <tt>Constant</tt> class</a>
183 <li><a href="#GlobalValue">The <tt>GlobalValue</tt> class</a>
185 <li><a href="#Function">The <tt>Function</tt> class</a></li>
186 <li><a href="#GlobalVariable">The <tt>GlobalVariable</tt> class</a></li>
193 <li><a href="#BasicBlock">The <tt>BasicBlock</tt> class</a></li>
194 <li><a href="#Argument">The <tt>Argument</tt> class</a></li>
201 <div class="doc_author">
202 <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a>,
203 <a href="mailto:dhurjati@cs.uiuc.edu">Dinakar Dhurjati</a>,
204 <a href="mailto:ggreif@gmail.com">Gabor Greif</a>,
205 <a href="mailto:jstanley@cs.uiuc.edu">Joel Stanley</a>,
206 <a href="mailto:rspencer@x10sys.com">Reid Spencer</a> and
207 <a href="mailto:owen@apple.com">Owen Anderson</a></p>
210 <!-- *********************************************************************** -->
212 <a name="introduction">Introduction </a>
214 <!-- *********************************************************************** -->
218 <p>This document is meant to highlight some of the important classes and
219 interfaces available in the LLVM source-base. This manual is not
220 intended to explain what LLVM is, how it works, and what LLVM code looks
221 like. It assumes that you know the basics of LLVM and are interested
222 in writing transformations or otherwise analyzing or manipulating the
225 <p>This document should get you oriented so that you can find your
226 way in the continuously growing source code that makes up the LLVM
227 infrastructure. Note that this manual is not intended to serve as a
228 replacement for reading the source code, so if you think there should be
229 a method in one of these classes to do something, but it's not listed,
230 check the source. Links to the <a href="/doxygen/">doxygen</a> sources
231 are provided to make this as easy as possible.</p>
233 <p>The first section of this document describes general information that is
234 useful to know when working in the LLVM infrastructure, and the second describes
235 the Core LLVM classes. In the future this manual will be extended with
236 information describing how to use extension libraries, such as dominator
237 information, CFG traversal routines, and useful utilities like the <tt><a
238 href="/doxygen/InstVisitor_8h-source.html">InstVisitor</a></tt> template.</p>
242 <!-- *********************************************************************** -->
244 <a name="general">General Information</a>
246 <!-- *********************************************************************** -->
250 <p>This section contains general information that is useful if you are working
251 in the LLVM source-base, but that isn't specific to any particular API.</p>
253 <!-- ======================================================================= -->
255 <a name="stl">The C++ Standard Template Library</a>
260 <p>LLVM makes heavy use of the C++ Standard Template Library (STL),
261 perhaps much more than you are used to, or have seen before. Because of
262 this, you might want to do a little background reading in the
263 techniques used and capabilities of the library. There are many good
264 pages that discuss the STL, and several books on the subject that you
265 can get, so it will not be discussed in this document.</p>
267 <p>Here are some useful links:</p>
271 <li><a href="http://www.dinkumware.com/manuals/#Standard C++ Library">Dinkumware
272 C++ Library reference</a> - an excellent reference for the STL and other parts
273 of the standard C++ library.</li>
275 <li><a href="http://www.tempest-sw.com/cpp/">C++ In a Nutshell</a> - This is an
276 O'Reilly book in the making. It has a decent Standard Library
277 Reference that rivals Dinkumware's, and is unfortunately no longer free since the
278 book has been published.</li>
280 <li><a href="http://www.parashift.com/c++-faq-lite/">C++ Frequently Asked
283 <li><a href="http://www.sgi.com/tech/stl/">SGI's STL Programmer's Guide</a> -
285 href="http://www.sgi.com/tech/stl/stl_introduction.html">Introduction to the
288 <li><a href="http://www.research.att.com/%7Ebs/C++.html">Bjarne Stroustrup's C++
291 <li><a href="http://64.78.49.204/">
292 Bruce Eckel's Thinking in C++, 2nd ed. Volume 2 Revision 4.0 (even better, get
297 <p>You are also encouraged to take a look at the <a
298 href="CodingStandards.html">LLVM Coding Standards</a> guide which focuses on how
299 to write maintainable code more than where to put your curly braces.</p>
303 <!-- ======================================================================= -->
305 <a name="stl">Other useful references</a>
311 <li><a href="http://www.fortran-2000.com/ArnaudRecipes/sharedlib.html">Using
312 static and shared libraries across platforms</a></li>
319 <!-- *********************************************************************** -->
321 <a name="apis">Important and useful LLVM APIs</a>
323 <!-- *********************************************************************** -->
327 <p>Here we highlight some LLVM APIs that are generally useful and good to
328 know about when writing transformations.</p>
330 <!-- ======================================================================= -->
332 <a name="isa">The <tt>isa<></tt>, <tt>cast<></tt> and
333 <tt>dyn_cast<></tt> templates</a>
338 <p>The LLVM source-base makes extensive use of a custom form of RTTI.
339 These templates have many similarities to the C++ <tt>dynamic_cast<></tt>
340 operator, but they don't have some drawbacks (primarily stemming from
341 the fact that <tt>dynamic_cast<></tt> only works on classes that
342 have a v-table). Because they are used so often, you must know what they
343 do and how they work. All of these templates are defined in the <a
344 href="/doxygen/Casting_8h-source.html"><tt>llvm/Support/Casting.h</tt></a>
345 file (note that you very rarely have to include this file directly).</p>
348 <dt><tt>isa<></tt>: </dt>
350 <dd><p>The <tt>isa<></tt> operator works exactly like the Java
351 "<tt>instanceof</tt>" operator. It returns true or false depending on whether
352 a reference or pointer points to an instance of the specified class. This can
353 be very useful for constraint checking of various sorts (example below).</p>
356 <dt><tt>cast<></tt>: </dt>
358 <dd><p>The <tt>cast<></tt> operator is a "checked cast" operation. It
359 converts a pointer or reference from a base class to a derived class, causing
360 an assertion failure if it is not really an instance of the right type. This
361 should be used in cases where you have some information that makes you believe
362 that something is of the right type. An example of the <tt>isa<></tt>
363 and <tt>cast<></tt> template is:</p>
365 <div class="doc_code">
367 static bool isLoopInvariant(const <a href="#Value">Value</a> *V, const Loop *L) {
368 if (isa<<a href="#Constant">Constant</a>>(V) || isa<<a href="#Argument">Argument</a>>(V) || isa<<a href="#GlobalValue">GlobalValue</a>>(V))
371 // <i>Otherwise, it must be an instruction...</i>
372 return !L->contains(cast<<a href="#Instruction">Instruction</a>>(V)->getParent());
377 <p>Note that you should <b>not</b> use an <tt>isa<></tt> test followed
378 by a <tt>cast<></tt>, for that use the <tt>dyn_cast<></tt>
383 <dt><tt>dyn_cast<></tt>:</dt>
385 <dd><p>The <tt>dyn_cast<></tt> operator is a "checking cast" operation.
386 It checks to see if the operand is of the specified type, and if so, returns a
387 pointer to it (this operator does not work with references). If the operand is
388 not of the correct type, a null pointer is returned. Thus, this works very
389 much like the <tt>dynamic_cast<></tt> operator in C++, and should be
390 used in the same circumstances. Typically, the <tt>dyn_cast<></tt>
391 operator is used in an <tt>if</tt> statement or some other flow control
392 statement like this:</p>
394 <div class="doc_code">
396 if (<a href="#AllocationInst">AllocationInst</a> *AI = dyn_cast<<a href="#AllocationInst">AllocationInst</a>>(Val)) {
402 <p>This form of the <tt>if</tt> statement effectively combines together a call
403 to <tt>isa<></tt> and a call to <tt>cast<></tt> into one
404 statement, which is very convenient.</p>
406 <p>Note that the <tt>dyn_cast<></tt> operator, like C++'s
407 <tt>dynamic_cast<></tt> or Java's <tt>instanceof</tt> operator, can be
408 abused. In particular, you should not use big chained <tt>if/then/else</tt>
409 blocks to check for lots of different variants of classes. If you find
410 yourself wanting to do this, it is much cleaner and more efficient to use the
411 <tt>InstVisitor</tt> class to dispatch over the instruction type directly.</p>
415 <dt><tt>cast_or_null<></tt>: </dt>
417 <dd><p>The <tt>cast_or_null<></tt> operator works just like the
418 <tt>cast<></tt> operator, except that it allows for a null pointer as an
419 argument (which it then propagates). This can sometimes be useful, allowing
420 you to combine several null checks into one.</p></dd>
422 <dt><tt>dyn_cast_or_null<></tt>: </dt>
424 <dd><p>The <tt>dyn_cast_or_null<></tt> operator works just like the
425 <tt>dyn_cast<></tt> operator, except that it allows for a null pointer
426 as an argument (which it then propagates). This can sometimes be useful,
427 allowing you to combine several null checks into one.</p></dd>
431 <p>These five templates can be used with any classes, whether they have a
432 v-table or not. To add support for these templates, you simply need to add
433 <tt>classof</tt> static methods to the class you are interested casting
434 to. Describing this is currently outside the scope of this document, but there
435 are lots of examples in the LLVM source base.</p>
440 <!-- ======================================================================= -->
442 <a name="string_apis">Passing strings (the <tt>StringRef</tt>
443 and <tt>Twine</tt> classes)</a>
448 <p>Although LLVM generally does not do much string manipulation, we do have
449 several important APIs which take strings. Two important examples are the
450 Value class -- which has names for instructions, functions, etc. -- and the
451 StringMap class which is used extensively in LLVM and Clang.</p>
453 <p>These are generic classes, and they need to be able to accept strings which
454 may have embedded null characters. Therefore, they cannot simply take
455 a <tt>const char *</tt>, and taking a <tt>const std::string&</tt> requires
456 clients to perform a heap allocation which is usually unnecessary. Instead,
457 many LLVM APIs use a <tt>StringRef</tt> or a <tt>const Twine&</tt> for
458 passing strings efficiently.</p>
460 <!-- _______________________________________________________________________ -->
462 <a name="StringRef">The <tt>StringRef</tt> class</a>
467 <p>The <tt>StringRef</tt> data type represents a reference to a constant string
468 (a character array and a length) and supports the common operations available
469 on <tt>std:string</tt>, but does not require heap allocation.</p>
471 <p>It can be implicitly constructed using a C style null-terminated string,
472 an <tt>std::string</tt>, or explicitly with a character pointer and length.
473 For example, the <tt>StringRef</tt> find function is declared as:</p>
475 <pre class="doc_code">
476 iterator find(StringRef Key);
479 <p>and clients can call it using any one of:</p>
481 <pre class="doc_code">
482 Map.find("foo"); <i>// Lookup "foo"</i>
483 Map.find(std::string("bar")); <i>// Lookup "bar"</i>
484 Map.find(StringRef("\0baz", 4)); <i>// Lookup "\0baz"</i>
487 <p>Similarly, APIs which need to return a string may return a <tt>StringRef</tt>
488 instance, which can be used directly or converted to an <tt>std::string</tt>
489 using the <tt>str</tt> member function. See
490 "<tt><a href="/doxygen/classllvm_1_1StringRef_8h-source.html">llvm/ADT/StringRef.h</a></tt>"
491 for more information.</p>
493 <p>You should rarely use the <tt>StringRef</tt> class directly, because it contains
494 pointers to external memory it is not generally safe to store an instance of the
495 class (unless you know that the external storage will not be freed). StringRef is
496 small and pervasive enough in LLVM that it should always be passed by value.</p>
500 <!-- _______________________________________________________________________ -->
502 <a name="Twine">The <tt>Twine</tt> class</a>
507 <p>The <tt>Twine</tt> class is an efficient way for APIs to accept concatenated
508 strings. For example, a common LLVM paradigm is to name one instruction based on
509 the name of another instruction with a suffix, for example:</p>
511 <div class="doc_code">
513 New = CmpInst::Create(<i>...</i>, SO->getName() + ".cmp");
517 <p>The <tt>Twine</tt> class is effectively a
518 lightweight <a href="http://en.wikipedia.org/wiki/Rope_(computer_science)">rope</a>
519 which points to temporary (stack allocated) objects. Twines can be implicitly
520 constructed as the result of the plus operator applied to strings (i.e., a C
521 strings, an <tt>std::string</tt>, or a <tt>StringRef</tt>). The twine delays the
522 actual concatenation of strings until it is actually required, at which point
523 it can be efficiently rendered directly into a character array. This avoids
524 unnecessary heap allocation involved in constructing the temporary results of
525 string concatenation. See
526 "<tt><a href="/doxygen/classllvm_1_1Twine_8h-source.html">llvm/ADT/Twine.h</a></tt>"
527 for more information.</p>
529 <p>As with a <tt>StringRef</tt>, <tt>Twine</tt> objects point to external memory
530 and should almost never be stored or mentioned directly. They are intended
531 solely for use when defining a function which should be able to efficiently
532 accept concatenated strings.</p>
538 <!-- ======================================================================= -->
540 <a name="DEBUG">The <tt>DEBUG()</tt> macro and <tt>-debug</tt> option</a>
545 <p>Often when working on your pass you will put a bunch of debugging printouts
546 and other code into your pass. After you get it working, you want to remove
547 it, but you may need it again in the future (to work out new bugs that you run
550 <p> Naturally, because of this, you don't want to delete the debug printouts,
551 but you don't want them to always be noisy. A standard compromise is to comment
552 them out, allowing you to enable them if you need them in the future.</p>
554 <p>The "<tt><a href="/doxygen/Debug_8h-source.html">llvm/Support/Debug.h</a></tt>"
555 file provides a macro named <tt>DEBUG()</tt> that is a much nicer solution to
556 this problem. Basically, you can put arbitrary code into the argument of the
557 <tt>DEBUG</tt> macro, and it is only executed if '<tt>opt</tt>' (or any other
558 tool) is run with the '<tt>-debug</tt>' command line argument:</p>
560 <div class="doc_code">
562 DEBUG(errs() << "I am here!\n");
566 <p>Then you can run your pass like this:</p>
568 <div class="doc_code">
570 $ opt < a.bc > /dev/null -mypass
571 <i><no output></i>
572 $ opt < a.bc > /dev/null -mypass -debug
577 <p>Using the <tt>DEBUG()</tt> macro instead of a home-brewed solution allows you
578 to not have to create "yet another" command line option for the debug output for
579 your pass. Note that <tt>DEBUG()</tt> macros are disabled for optimized builds,
580 so they do not cause a performance impact at all (for the same reason, they
581 should also not contain side-effects!).</p>
583 <p>One additional nice thing about the <tt>DEBUG()</tt> macro is that you can
584 enable or disable it directly in gdb. Just use "<tt>set DebugFlag=0</tt>" or
585 "<tt>set DebugFlag=1</tt>" from the gdb if the program is running. If the
586 program hasn't been started yet, you can always just run it with
589 <!-- _______________________________________________________________________ -->
591 <a name="DEBUG_TYPE">Fine grained debug info with <tt>DEBUG_TYPE</tt> and
592 the <tt>-debug-only</tt> option</a>
597 <p>Sometimes you may find yourself in a situation where enabling <tt>-debug</tt>
598 just turns on <b>too much</b> information (such as when working on the code
599 generator). If you want to enable debug information with more fine-grained
600 control, you define the <tt>DEBUG_TYPE</tt> macro and the <tt>-debug</tt> only
601 option as follows:</p>
603 <div class="doc_code">
606 DEBUG(errs() << "No debug type\n");
607 #define DEBUG_TYPE "foo"
608 DEBUG(errs() << "'foo' debug type\n");
610 #define DEBUG_TYPE "bar"
611 DEBUG(errs() << "'bar' debug type\n"));
613 #define DEBUG_TYPE ""
614 DEBUG(errs() << "No debug type (2)\n");
618 <p>Then you can run your pass like this:</p>
620 <div class="doc_code">
622 $ opt < a.bc > /dev/null -mypass
623 <i><no output></i>
624 $ opt < a.bc > /dev/null -mypass -debug
629 $ opt < a.bc > /dev/null -mypass -debug-only=foo
631 $ opt < a.bc > /dev/null -mypass -debug-only=bar
636 <p>Of course, in practice, you should only set <tt>DEBUG_TYPE</tt> at the top of
637 a file, to specify the debug type for the entire module (if you do this before
638 you <tt>#include "llvm/Support/Debug.h"</tt>, you don't have to insert the ugly
639 <tt>#undef</tt>'s). Also, you should use names more meaningful than "foo" and
640 "bar", because there is no system in place to ensure that names do not
641 conflict. If two different modules use the same string, they will all be turned
642 on when the name is specified. This allows, for example, all debug information
643 for instruction scheduling to be enabled with <tt>-debug-type=InstrSched</tt>,
644 even if the source lives in multiple files.</p>
646 <p>The <tt>DEBUG_WITH_TYPE</tt> macro is also available for situations where you
647 would like to set <tt>DEBUG_TYPE</tt>, but only for one specific <tt>DEBUG</tt>
648 statement. It takes an additional first parameter, which is the type to use. For
649 example, the preceding example could be written as:</p>
652 <div class="doc_code">
654 DEBUG_WITH_TYPE("", errs() << "No debug type\n");
655 DEBUG_WITH_TYPE("foo", errs() << "'foo' debug type\n");
656 DEBUG_WITH_TYPE("bar", errs() << "'bar' debug type\n"));
657 DEBUG_WITH_TYPE("", errs() << "No debug type (2)\n");
665 <!-- ======================================================================= -->
667 <a name="Statistic">The <tt>Statistic</tt> class & <tt>-stats</tt>
674 href="/doxygen/Statistic_8h-source.html">llvm/ADT/Statistic.h</a></tt>" file
675 provides a class named <tt>Statistic</tt> that is used as a unified way to
676 keep track of what the LLVM compiler is doing and how effective various
677 optimizations are. It is useful to see what optimizations are contributing to
678 making a particular program run faster.</p>
680 <p>Often you may run your pass on some big program, and you're interested to see
681 how many times it makes a certain transformation. Although you can do this with
682 hand inspection, or some ad-hoc method, this is a real pain and not very useful
683 for big programs. Using the <tt>Statistic</tt> class makes it very easy to
684 keep track of this information, and the calculated information is presented in a
685 uniform manner with the rest of the passes being executed.</p>
687 <p>There are many examples of <tt>Statistic</tt> uses, but the basics of using
688 it are as follows:</p>
691 <li><p>Define your statistic like this:</p>
693 <div class="doc_code">
695 #define <a href="#DEBUG_TYPE">DEBUG_TYPE</a> "mypassname" <i>// This goes before any #includes.</i>
696 STATISTIC(NumXForms, "The # of times I did stuff");
700 <p>The <tt>STATISTIC</tt> macro defines a static variable, whose name is
701 specified by the first argument. The pass name is taken from the DEBUG_TYPE
702 macro, and the description is taken from the second argument. The variable
703 defined ("NumXForms" in this case) acts like an unsigned integer.</p></li>
705 <li><p>Whenever you make a transformation, bump the counter:</p>
707 <div class="doc_code">
709 ++NumXForms; // <i>I did stuff!</i>
716 <p>That's all you have to do. To get '<tt>opt</tt>' to print out the
717 statistics gathered, use the '<tt>-stats</tt>' option:</p>
719 <div class="doc_code">
721 $ opt -stats -mypassname < program.bc > /dev/null
722 <i>... statistics output ...</i>
726 <p> When running <tt>opt</tt> on a C file from the SPEC benchmark
727 suite, it gives a report that looks like this:</p>
729 <div class="doc_code">
731 7646 bitcodewriter - Number of normal instructions
732 725 bitcodewriter - Number of oversized instructions
733 129996 bitcodewriter - Number of bitcode bytes written
734 2817 raise - Number of insts DCEd or constprop'd
735 3213 raise - Number of cast-of-self removed
736 5046 raise - Number of expression trees converted
737 75 raise - Number of other getelementptr's formed
738 138 raise - Number of load/store peepholes
739 42 deadtypeelim - Number of unused typenames removed from symtab
740 392 funcresolve - Number of varargs functions resolved
741 27 globaldce - Number of global variables removed
742 2 adce - Number of basic blocks removed
743 134 cee - Number of branches revectored
744 49 cee - Number of setcc instruction eliminated
745 532 gcse - Number of loads removed
746 2919 gcse - Number of instructions removed
747 86 indvars - Number of canonical indvars added
748 87 indvars - Number of aux indvars removed
749 25 instcombine - Number of dead inst eliminate
750 434 instcombine - Number of insts combined
751 248 licm - Number of load insts hoisted
752 1298 licm - Number of insts hoisted to a loop pre-header
753 3 licm - Number of insts hoisted to multiple loop preds (bad, no loop pre-header)
754 75 mem2reg - Number of alloca's promoted
755 1444 cfgsimplify - Number of blocks simplified
759 <p>Obviously, with so many optimizations, having a unified framework for this
760 stuff is very nice. Making your pass fit well into the framework makes it more
761 maintainable and useful.</p>
765 <!-- ======================================================================= -->
767 <a name="ViewGraph">Viewing graphs while debugging code</a>
772 <p>Several of the important data structures in LLVM are graphs: for example
773 CFGs made out of LLVM <a href="#BasicBlock">BasicBlock</a>s, CFGs made out of
774 LLVM <a href="CodeGenerator.html#machinebasicblock">MachineBasicBlock</a>s, and
775 <a href="CodeGenerator.html#selectiondag_intro">Instruction Selection
776 DAGs</a>. In many cases, while debugging various parts of the compiler, it is
777 nice to instantly visualize these graphs.</p>
779 <p>LLVM provides several callbacks that are available in a debug build to do
780 exactly that. If you call the <tt>Function::viewCFG()</tt> method, for example,
781 the current LLVM tool will pop up a window containing the CFG for the function
782 where each basic block is a node in the graph, and each node contains the
783 instructions in the block. Similarly, there also exists
784 <tt>Function::viewCFGOnly()</tt> (does not include the instructions), the
785 <tt>MachineFunction::viewCFG()</tt> and <tt>MachineFunction::viewCFGOnly()</tt>,
786 and the <tt>SelectionDAG::viewGraph()</tt> methods. Within GDB, for example,
787 you can usually use something like <tt>call DAG.viewGraph()</tt> to pop
788 up a window. Alternatively, you can sprinkle calls to these functions in your
789 code in places you want to debug.</p>
791 <p>Getting this to work requires a small amount of configuration. On Unix
792 systems with X11, install the <a href="http://www.graphviz.org">graphviz</a>
793 toolkit, and make sure 'dot' and 'gv' are in your path. If you are running on
794 Mac OS/X, download and install the Mac OS/X <a
795 href="http://www.pixelglow.com/graphviz/">Graphviz program</a>, and add
796 <tt>/Applications/Graphviz.app/Contents/MacOS/</tt> (or wherever you install
797 it) to your path. Once in your system and path are set up, rerun the LLVM
798 configure script and rebuild LLVM to enable this functionality.</p>
800 <p><tt>SelectionDAG</tt> has been extended to make it easier to locate
801 <i>interesting</i> nodes in large complex graphs. From gdb, if you
802 <tt>call DAG.setGraphColor(<i>node</i>, "<i>color</i>")</tt>, then the
803 next <tt>call DAG.viewGraph()</tt> would highlight the node in the
804 specified color (choices of colors can be found at <a
805 href="http://www.graphviz.org/doc/info/colors.html">colors</a>.) More
806 complex node attributes can be provided with <tt>call
807 DAG.setGraphAttrs(<i>node</i>, "<i>attributes</i>")</tt> (choices can be
808 found at <a href="http://www.graphviz.org/doc/info/attrs.html">Graph
809 Attributes</a>.) If you want to restart and clear all the current graph
810 attributes, then you can <tt>call DAG.clearGraphAttrs()</tt>. </p>
812 <p>Note that graph visualization features are compiled out of Release builds
813 to reduce file size. This means that you need a Debug+Asserts or
814 Release+Asserts build to use these features.</p>
820 <!-- *********************************************************************** -->
822 <a name="datastructure">Picking the Right Data Structure for a Task</a>
824 <!-- *********************************************************************** -->
828 <p>LLVM has a plethora of data structures in the <tt>llvm/ADT/</tt> directory,
829 and we commonly use STL data structures. This section describes the trade-offs
830 you should consider when you pick one.</p>
833 The first step is a choose your own adventure: do you want a sequential
834 container, a set-like container, or a map-like container? The most important
835 thing when choosing a container is the algorithmic properties of how you plan to
836 access the container. Based on that, you should use:</p>
839 <li>a <a href="#ds_map">map-like</a> container if you need efficient look-up
840 of an value based on another value. Map-like containers also support
841 efficient queries for containment (whether a key is in the map). Map-like
842 containers generally do not support efficient reverse mapping (values to
843 keys). If you need that, use two maps. Some map-like containers also
844 support efficient iteration through the keys in sorted order. Map-like
845 containers are the most expensive sort, only use them if you need one of
846 these capabilities.</li>
848 <li>a <a href="#ds_set">set-like</a> container if you need to put a bunch of
849 stuff into a container that automatically eliminates duplicates. Some
850 set-like containers support efficient iteration through the elements in
851 sorted order. Set-like containers are more expensive than sequential
855 <li>a <a href="#ds_sequential">sequential</a> container provides
856 the most efficient way to add elements and keeps track of the order they are
857 added to the collection. They permit duplicates and support efficient
858 iteration, but do not support efficient look-up based on a key.
861 <li>a <a href="#ds_string">string</a> container is a specialized sequential
862 container or reference structure that is used for character or byte
865 <li>a <a href="#ds_bit">bit</a> container provides an efficient way to store and
866 perform set operations on sets of numeric id's, while automatically
867 eliminating duplicates. Bit containers require a maximum of 1 bit for each
868 identifier you want to store.
873 Once the proper category of container is determined, you can fine tune the
874 memory use, constant factors, and cache behaviors of access by intelligently
875 picking a member of the category. Note that constant factors and cache behavior
876 can be a big deal. If you have a vector that usually only contains a few
877 elements (but could contain many), for example, it's much better to use
878 <a href="#dss_smallvector">SmallVector</a> than <a href="#dss_vector">vector</a>
879 . Doing so avoids (relatively) expensive malloc/free calls, which dwarf the
880 cost of adding the elements to the container. </p>
885 <!-- ======================================================================= -->
887 <a name="ds_sequential">Sequential Containers (std::vector, std::list, etc)</a>
891 There are a variety of sequential containers available for you, based on your
892 needs. Pick the first in this section that will do what you want.
894 <!-- _______________________________________________________________________ -->
896 <a name="dss_arrayref">llvm/ADT/ArrayRef.h</a>
900 <p>The llvm::ArrayRef class is the preferred class to use in an interface that
901 accepts a sequential list of elements in memory and just reads from them. By
902 taking an ArrayRef, the API can be passed a fixed size array, an std::vector,
903 an llvm::SmallVector and anything else that is contiguous in memory.
909 <!-- _______________________________________________________________________ -->
911 <a name="dss_fixedarrays">Fixed Size Arrays</a>
915 <p>Fixed size arrays are very simple and very fast. They are good if you know
916 exactly how many elements you have, or you have a (low) upper bound on how many
920 <!-- _______________________________________________________________________ -->
922 <a name="dss_heaparrays">Heap Allocated Arrays</a>
926 <p>Heap allocated arrays (new[] + delete[]) are also simple. They are good if
927 the number of elements is variable, if you know how many elements you will need
928 before the array is allocated, and if the array is usually large (if not,
929 consider a <a href="#dss_smallvector">SmallVector</a>). The cost of a heap
930 allocated array is the cost of the new/delete (aka malloc/free). Also note that
931 if you are allocating an array of a type with a constructor, the constructor and
932 destructors will be run for every element in the array (re-sizable vectors only
933 construct those elements actually used).</p>
936 <!-- _______________________________________________________________________ -->
938 <a name="dss_tinyptrvector">"llvm/ADT/TinyPtrVector.h"</a>
943 <p><tt>TinyPtrVector<Type></tt> is a highly specialized collection class
944 that is optimized to avoid allocation in the case when a vector has zero or one
945 elements. It has two major restrictions: 1) it can only hold values of pointer
946 type, and 2) it cannot hold a null pointer.</p>
948 <p>Since this container is highly specialized, it is rarely used.</p>
952 <!-- _______________________________________________________________________ -->
954 <a name="dss_smallvector">"llvm/ADT/SmallVector.h"</a>
958 <p><tt>SmallVector<Type, N></tt> is a simple class that looks and smells
959 just like <tt>vector<Type></tt>:
960 it supports efficient iteration, lays out elements in memory order (so you can
961 do pointer arithmetic between elements), supports efficient push_back/pop_back
962 operations, supports efficient random access to its elements, etc.</p>
964 <p>The advantage of SmallVector is that it allocates space for
965 some number of elements (N) <b>in the object itself</b>. Because of this, if
966 the SmallVector is dynamically smaller than N, no malloc is performed. This can
967 be a big win in cases where the malloc/free call is far more expensive than the
968 code that fiddles around with the elements.</p>
970 <p>This is good for vectors that are "usually small" (e.g. the number of
971 predecessors/successors of a block is usually less than 8). On the other hand,
972 this makes the size of the SmallVector itself large, so you don't want to
973 allocate lots of them (doing so will waste a lot of space). As such,
974 SmallVectors are most useful when on the stack.</p>
976 <p>SmallVector also provides a nice portable and efficient replacement for
981 <!-- _______________________________________________________________________ -->
983 <a name="dss_vector"><vector></a>
988 std::vector is well loved and respected. It is useful when SmallVector isn't:
989 when the size of the vector is often large (thus the small optimization will
990 rarely be a benefit) or if you will be allocating many instances of the vector
991 itself (which would waste space for elements that aren't in the container).
992 vector is also useful when interfacing with code that expects vectors :).
995 <p>One worthwhile note about std::vector: avoid code like this:</p>
997 <div class="doc_code">
1000 std::vector<foo> V;
1006 <p>Instead, write this as:</p>
1008 <div class="doc_code">
1010 std::vector<foo> V;
1018 <p>Doing so will save (at least) one heap allocation and free per iteration of
1023 <!-- _______________________________________________________________________ -->
1025 <a name="dss_deque"><deque></a>
1029 <p>std::deque is, in some senses, a generalized version of std::vector. Like
1030 std::vector, it provides constant time random access and other similar
1031 properties, but it also provides efficient access to the front of the list. It
1032 does not guarantee continuity of elements within memory.</p>
1034 <p>In exchange for this extra flexibility, std::deque has significantly higher
1035 constant factor costs than std::vector. If possible, use std::vector or
1036 something cheaper.</p>
1039 <!-- _______________________________________________________________________ -->
1041 <a name="dss_list"><list></a>
1045 <p>std::list is an extremely inefficient class that is rarely useful.
1046 It performs a heap allocation for every element inserted into it, thus having an
1047 extremely high constant factor, particularly for small data types. std::list
1048 also only supports bidirectional iteration, not random access iteration.</p>
1050 <p>In exchange for this high cost, std::list supports efficient access to both
1051 ends of the list (like std::deque, but unlike std::vector or SmallVector). In
1052 addition, the iterator invalidation characteristics of std::list are stronger
1053 than that of a vector class: inserting or removing an element into the list does
1054 not invalidate iterator or pointers to other elements in the list.</p>
1057 <!-- _______________________________________________________________________ -->
1059 <a name="dss_ilist">llvm/ADT/ilist.h</a>
1063 <p><tt>ilist<T></tt> implements an 'intrusive' doubly-linked list. It is
1064 intrusive, because it requires the element to store and provide access to the
1065 prev/next pointers for the list.</p>
1067 <p><tt>ilist</tt> has the same drawbacks as <tt>std::list</tt>, and additionally
1068 requires an <tt>ilist_traits</tt> implementation for the element type, but it
1069 provides some novel characteristics. In particular, it can efficiently store
1070 polymorphic objects, the traits class is informed when an element is inserted or
1071 removed from the list, and <tt>ilist</tt>s are guaranteed to support a
1072 constant-time splice operation.</p>
1074 <p>These properties are exactly what we want for things like
1075 <tt>Instruction</tt>s and basic blocks, which is why these are implemented with
1076 <tt>ilist</tt>s.</p>
1078 Related classes of interest are explained in the following subsections:
1080 <li><a href="#dss_ilist_traits">ilist_traits</a></li>
1081 <li><a href="#dss_iplist">iplist</a></li>
1082 <li><a href="#dss_ilist_node">llvm/ADT/ilist_node.h</a></li>
1083 <li><a href="#dss_ilist_sentinel">Sentinels</a></li>
1087 <!-- _______________________________________________________________________ -->
1089 <a name="dss_packedvector">llvm/ADT/PackedVector.h</a>
1094 Useful for storing a vector of values using only a few number of bits for each
1095 value. Apart from the standard operations of a vector-like container, it can
1096 also perform an 'or' set operation.
1101 <div class="doc_code">
1105 FirstCondition = 0x1,
1106 SecondCondition = 0x2,
1111 PackedVector<State, 2> Vec1;
1112 Vec1.push_back(FirstCondition);
1114 PackedVector<State, 2> Vec2;
1115 Vec2.push_back(SecondCondition);
1118 return Vec1[0]; // returns 'Both'.
1125 <!-- _______________________________________________________________________ -->
1127 <a name="dss_ilist_traits">ilist_traits</a>
1131 <p><tt>ilist_traits<T></tt> is <tt>ilist<T></tt>'s customization
1132 mechanism. <tt>iplist<T></tt> (and consequently <tt>ilist<T></tt>)
1133 publicly derive from this traits class.</p>
1136 <!-- _______________________________________________________________________ -->
1138 <a name="dss_iplist">iplist</a>
1142 <p><tt>iplist<T></tt> is <tt>ilist<T></tt>'s base and as such
1143 supports a slightly narrower interface. Notably, inserters from
1144 <tt>T&</tt> are absent.</p>
1146 <p><tt>ilist_traits<T></tt> is a public base of this class and can be
1147 used for a wide variety of customizations.</p>
1150 <!-- _______________________________________________________________________ -->
1152 <a name="dss_ilist_node">llvm/ADT/ilist_node.h</a>
1156 <p><tt>ilist_node<T></tt> implements a the forward and backward links
1157 that are expected by the <tt>ilist<T></tt> (and analogous containers)
1158 in the default manner.</p>
1160 <p><tt>ilist_node<T></tt>s are meant to be embedded in the node type
1161 <tt>T</tt>, usually <tt>T</tt> publicly derives from
1162 <tt>ilist_node<T></tt>.</p>
1165 <!-- _______________________________________________________________________ -->
1167 <a name="dss_ilist_sentinel">Sentinels</a>
1171 <p><tt>ilist</tt>s have another specialty that must be considered. To be a good
1172 citizen in the C++ ecosystem, it needs to support the standard container
1173 operations, such as <tt>begin</tt> and <tt>end</tt> iterators, etc. Also, the
1174 <tt>operator--</tt> must work correctly on the <tt>end</tt> iterator in the
1175 case of non-empty <tt>ilist</tt>s.</p>
1177 <p>The only sensible solution to this problem is to allocate a so-called
1178 <i>sentinel</i> along with the intrusive list, which serves as the <tt>end</tt>
1179 iterator, providing the back-link to the last element. However conforming to the
1180 C++ convention it is illegal to <tt>operator++</tt> beyond the sentinel and it
1181 also must not be dereferenced.</p>
1183 <p>These constraints allow for some implementation freedom to the <tt>ilist</tt>
1184 how to allocate and store the sentinel. The corresponding policy is dictated
1185 by <tt>ilist_traits<T></tt>. By default a <tt>T</tt> gets heap-allocated
1186 whenever the need for a sentinel arises.</p>
1188 <p>While the default policy is sufficient in most cases, it may break down when
1189 <tt>T</tt> does not provide a default constructor. Also, in the case of many
1190 instances of <tt>ilist</tt>s, the memory overhead of the associated sentinels
1191 is wasted. To alleviate the situation with numerous and voluminous
1192 <tt>T</tt>-sentinels, sometimes a trick is employed, leading to <i>ghostly
1195 <p>Ghostly sentinels are obtained by specially-crafted <tt>ilist_traits<T></tt>
1196 which superpose the sentinel with the <tt>ilist</tt> instance in memory. Pointer
1197 arithmetic is used to obtain the sentinel, which is relative to the
1198 <tt>ilist</tt>'s <tt>this</tt> pointer. The <tt>ilist</tt> is augmented by an
1199 extra pointer, which serves as the back-link of the sentinel. This is the only
1200 field in the ghostly sentinel which can be legally accessed.</p>
1203 <!-- _______________________________________________________________________ -->
1205 <a name="dss_other">Other Sequential Container options</a>
1209 <p>Other STL containers are available, such as std::string.</p>
1211 <p>There are also various STL adapter classes such as std::queue,
1212 std::priority_queue, std::stack, etc. These provide simplified access to an
1213 underlying container but don't affect the cost of the container itself.</p>
1218 <!-- ======================================================================= -->
1220 <a name="ds_string">String-like containers</a>
1226 There are a variety of ways to pass around and use strings in C and C++, and
1227 LLVM adds a few new options to choose from. Pick the first option on this list
1228 that will do what you need, they are ordered according to their relative cost.
1231 Note that is is generally preferred to <em>not</em> pass strings around as
1232 "<tt>const char*</tt>"'s. These have a number of problems, including the fact
1233 that they cannot represent embedded nul ("\0") characters, and do not have a
1234 length available efficiently. The general replacement for '<tt>const
1235 char*</tt>' is StringRef.
1238 <p>For more information on choosing string containers for APIs, please see
1239 <a href="#string_apis">Passing strings</a>.</p>
1242 <!-- _______________________________________________________________________ -->
1244 <a name="dss_stringref">llvm/ADT/StringRef.h</a>
1249 The StringRef class is a simple value class that contains a pointer to a
1250 character and a length, and is quite related to the <a
1251 href="#dss_arrayref">ArrayRef</a> class (but specialized for arrays of
1252 characters). Because StringRef carries a length with it, it safely handles
1253 strings with embedded nul characters in it, getting the length does not require
1254 a strlen call, and it even has very convenient APIs for slicing and dicing the
1255 character range that it represents.
1259 StringRef is ideal for passing simple strings around that are known to be live,
1260 either because they are C string literals, std::string, a C array, or a
1261 SmallVector. Each of these cases has an efficient implicit conversion to
1262 StringRef, which doesn't result in a dynamic strlen being executed.
1265 <p>StringRef has a few major limitations which make more powerful string
1266 containers useful:</p>
1269 <li>You cannot directly convert a StringRef to a 'const char*' because there is
1270 no way to add a trailing nul (unlike the .c_str() method on various stronger
1274 <li>StringRef doesn't own or keep alive the underlying string bytes.
1275 As such it can easily lead to dangling pointers, and is not suitable for
1276 embedding in datastructures in most cases (instead, use an std::string or
1277 something like that).</li>
1279 <li>For the same reason, StringRef cannot be used as the return value of a
1280 method if the method "computes" the result string. Instead, use
1283 <li>StringRef's do not allow you to mutate the pointed-to string bytes and it
1284 doesn't allow you to insert or remove bytes from the range. For editing
1285 operations like this, it interoperates with the <a
1286 href="#dss_twine">Twine</a> class.</li>
1289 <p>Because of its strengths and limitations, it is very common for a function to
1290 take a StringRef and for a method on an object to return a StringRef that
1291 points into some string that it owns.</p>
1295 <!-- _______________________________________________________________________ -->
1297 <a name="dss_twine">llvm/ADT/Twine.h</a>
1302 The Twine class is used as an intermediary datatype for APIs that want to take
1303 a string that can be constructed inline with a series of concatenations.
1304 Twine works by forming recursive instances of the Twine datatype (a simple
1305 value object) on the stack as temporary objects, linking them together into a
1306 tree which is then linearized when the Twine is consumed. Twine is only safe
1307 to use as the argument to a function, and should always be a const reference,
1312 void foo(const Twine &T);
1316 foo(X + "." + Twine(i));
1319 <p>This example forms a string like "blarg.42" by concatenating the values
1320 together, and does not form intermediate strings containing "blarg" or
1324 <p>Because Twine is constructed with temporary objects on the stack, and
1325 because these instances are destroyed at the end of the current statement,
1326 it is an inherently dangerous API. For example, this simple variant contains
1327 undefined behavior and will probably crash:</p>
1330 void foo(const Twine &T);
1334 const Twine &Tmp = X + "." + Twine(i);
1338 <p>... because the temporaries are destroyed before the call. That said,
1339 Twine's are much more efficient than intermediate std::string temporaries, and
1340 they work really well with StringRef. Just be aware of their limitations.</p>
1345 <!-- _______________________________________________________________________ -->
1347 <a name="dss_smallstring">llvm/ADT/SmallString.h</a>
1352 <p>SmallString is a subclass of <a href="#dss_smallvector">SmallVector</a> that
1353 adds some convenience APIs like += that takes StringRef's. SmallString avoids
1354 allocating memory in the case when the preallocated space is enough to hold its
1355 data, and it calls back to general heap allocation when required. Since it owns
1356 its data, it is very safe to use and supports full mutation of the string.</p>
1358 <p>Like SmallVector's, the big downside to SmallString is their sizeof. While
1359 they are optimized for small strings, they themselves are not particularly
1360 small. This means that they work great for temporary scratch buffers on the
1361 stack, but should not generally be put into the heap: it is very rare to
1362 see a SmallString as the member of a frequently-allocated heap data structure
1363 or returned by-value.
1368 <!-- _______________________________________________________________________ -->
1370 <a name="dss_stdstring">std::string</a>
1375 <p>The standard C++ std::string class is a very general class that (like
1376 SmallString) owns its underlying data. sizeof(std::string) is very reasonable
1377 so it can be embedded into heap data structures and returned by-value.
1378 On the other hand, std::string is highly inefficient for inline editing (e.g.
1379 concatenating a bunch of stuff together) and because it is provided by the
1380 standard library, its performance characteristics depend a lot of the host
1381 standard library (e.g. libc++ and MSVC provide a highly optimized string
1382 class, GCC contains a really slow implementation).
1385 <p>The major disadvantage of std::string is that almost every operation that
1386 makes them larger can allocate memory, which is slow. As such, it is better
1387 to use SmallVector or Twine as a scratch buffer, but then use std::string to
1388 persist the result.</p>
1393 <!-- end of strings -->
1397 <!-- ======================================================================= -->
1399 <a name="ds_set">Set-Like Containers (std::set, SmallSet, SetVector, etc)</a>
1404 <p>Set-like containers are useful when you need to canonicalize multiple values
1405 into a single representation. There are several different choices for how to do
1406 this, providing various trade-offs.</p>
1408 <!-- _______________________________________________________________________ -->
1410 <a name="dss_sortedvectorset">A sorted 'vector'</a>
1415 <p>If you intend to insert a lot of elements, then do a lot of queries, a
1416 great approach is to use a vector (or other sequential container) with
1417 std::sort+std::unique to remove duplicates. This approach works really well if
1418 your usage pattern has these two distinct phases (insert then query), and can be
1419 coupled with a good choice of <a href="#ds_sequential">sequential container</a>.
1423 This combination provides the several nice properties: the result data is
1424 contiguous in memory (good for cache locality), has few allocations, is easy to
1425 address (iterators in the final vector are just indices or pointers), and can be
1426 efficiently queried with a standard binary or radix search.</p>
1430 <!-- _______________________________________________________________________ -->
1432 <a name="dss_smallset">"llvm/ADT/SmallSet.h"</a>
1437 <p>If you have a set-like data structure that is usually small and whose elements
1438 are reasonably small, a <tt>SmallSet<Type, N></tt> is a good choice. This set
1439 has space for N elements in place (thus, if the set is dynamically smaller than
1440 N, no malloc traffic is required) and accesses them with a simple linear search.
1441 When the set grows beyond 'N' elements, it allocates a more expensive representation that
1442 guarantees efficient access (for most types, it falls back to std::set, but for
1443 pointers it uses something far better, <a
1444 href="#dss_smallptrset">SmallPtrSet</a>).</p>
1446 <p>The magic of this class is that it handles small sets extremely efficiently,
1447 but gracefully handles extremely large sets without loss of efficiency. The
1448 drawback is that the interface is quite small: it supports insertion, queries
1449 and erasing, but does not support iteration.</p>
1453 <!-- _______________________________________________________________________ -->
1455 <a name="dss_smallptrset">"llvm/ADT/SmallPtrSet.h"</a>
1460 <p>SmallPtrSet has all the advantages of <tt>SmallSet</tt> (and a <tt>SmallSet</tt> of pointers is
1461 transparently implemented with a <tt>SmallPtrSet</tt>), but also supports iterators. If
1462 more than 'N' insertions are performed, a single quadratically
1463 probed hash table is allocated and grows as needed, providing extremely
1464 efficient access (constant time insertion/deleting/queries with low constant
1465 factors) and is very stingy with malloc traffic.</p>
1467 <p>Note that, unlike <tt>std::set</tt>, the iterators of <tt>SmallPtrSet</tt> are invalidated
1468 whenever an insertion occurs. Also, the values visited by the iterators are not
1469 visited in sorted order.</p>
1473 <!-- _______________________________________________________________________ -->
1475 <a name="dss_denseset">"llvm/ADT/DenseSet.h"</a>
1481 DenseSet is a simple quadratically probed hash table. It excels at supporting
1482 small values: it uses a single allocation to hold all of the pairs that
1483 are currently inserted in the set. DenseSet is a great way to unique small
1484 values that are not simple pointers (use <a
1485 href="#dss_smallptrset">SmallPtrSet</a> for pointers). Note that DenseSet has
1486 the same requirements for the value type that <a
1487 href="#dss_densemap">DenseMap</a> has.
1492 <!-- _______________________________________________________________________ -->
1494 <a name="dss_FoldingSet">"llvm/ADT/FoldingSet.h"</a>
1500 FoldingSet is an aggregate class that is really good at uniquing
1501 expensive-to-create or polymorphic objects. It is a combination of a chained
1502 hash table with intrusive links (uniqued objects are required to inherit from
1503 FoldingSetNode) that uses <a href="#dss_smallvector">SmallVector</a> as part of
1506 <p>Consider a case where you want to implement a "getOrCreateFoo" method for
1507 a complex object (for example, a node in the code generator). The client has a
1508 description of *what* it wants to generate (it knows the opcode and all the
1509 operands), but we don't want to 'new' a node, then try inserting it into a set
1510 only to find out it already exists, at which point we would have to delete it
1511 and return the node that already exists.
1514 <p>To support this style of client, FoldingSet perform a query with a
1515 FoldingSetNodeID (which wraps SmallVector) that can be used to describe the
1516 element that we want to query for. The query either returns the element
1517 matching the ID or it returns an opaque ID that indicates where insertion should
1518 take place. Construction of the ID usually does not require heap traffic.</p>
1520 <p>Because FoldingSet uses intrusive links, it can support polymorphic objects
1521 in the set (for example, you can have SDNode instances mixed with LoadSDNodes).
1522 Because the elements are individually allocated, pointers to the elements are
1523 stable: inserting or removing elements does not invalidate any pointers to other
1529 <!-- _______________________________________________________________________ -->
1531 <a name="dss_set"><set></a>
1536 <p><tt>std::set</tt> is a reasonable all-around set class, which is decent at
1537 many things but great at nothing. std::set allocates memory for each element
1538 inserted (thus it is very malloc intensive) and typically stores three pointers
1539 per element in the set (thus adding a large amount of per-element space
1540 overhead). It offers guaranteed log(n) performance, which is not particularly
1541 fast from a complexity standpoint (particularly if the elements of the set are
1542 expensive to compare, like strings), and has extremely high constant factors for
1543 lookup, insertion and removal.</p>
1545 <p>The advantages of std::set are that its iterators are stable (deleting or
1546 inserting an element from the set does not affect iterators or pointers to other
1547 elements) and that iteration over the set is guaranteed to be in sorted order.
1548 If the elements in the set are large, then the relative overhead of the pointers
1549 and malloc traffic is not a big deal, but if the elements of the set are small,
1550 std::set is almost never a good choice.</p>
1554 <!-- _______________________________________________________________________ -->
1556 <a name="dss_setvector">"llvm/ADT/SetVector.h"</a>
1560 <p>LLVM's SetVector<Type> is an adapter class that combines your choice of
1561 a set-like container along with a <a href="#ds_sequential">Sequential
1562 Container</a>. The important property
1563 that this provides is efficient insertion with uniquing (duplicate elements are
1564 ignored) with iteration support. It implements this by inserting elements into
1565 both a set-like container and the sequential container, using the set-like
1566 container for uniquing and the sequential container for iteration.
1569 <p>The difference between SetVector and other sets is that the order of
1570 iteration is guaranteed to match the order of insertion into the SetVector.
1571 This property is really important for things like sets of pointers. Because
1572 pointer values are non-deterministic (e.g. vary across runs of the program on
1573 different machines), iterating over the pointers in the set will
1574 not be in a well-defined order.</p>
1577 The drawback of SetVector is that it requires twice as much space as a normal
1578 set and has the sum of constant factors from the set-like container and the
1579 sequential container that it uses. Use it *only* if you need to iterate over
1580 the elements in a deterministic order. SetVector is also expensive to delete
1581 elements out of (linear time), unless you use it's "pop_back" method, which is
1585 <p><tt>SetVector</tt> is an adapter class that defaults to
1586 using <tt>std::vector</tt> and a size 16 <tt>SmallSet</tt> for the underlying
1587 containers, so it is quite expensive. However,
1588 <tt>"llvm/ADT/SetVector.h"</tt> also provides a <tt>SmallSetVector</tt>
1589 class, which defaults to using a <tt>SmallVector</tt> and <tt>SmallSet</tt>
1590 of a specified size. If you use this, and if your sets are dynamically
1591 smaller than <tt>N</tt>, you will save a lot of heap traffic.</p>
1595 <!-- _______________________________________________________________________ -->
1597 <a name="dss_uniquevector">"llvm/ADT/UniqueVector.h"</a>
1603 UniqueVector is similar to <a href="#dss_setvector">SetVector</a>, but it
1604 retains a unique ID for each element inserted into the set. It internally
1605 contains a map and a vector, and it assigns a unique ID for each value inserted
1608 <p>UniqueVector is very expensive: its cost is the sum of the cost of
1609 maintaining both the map and vector, it has high complexity, high constant
1610 factors, and produces a lot of malloc traffic. It should be avoided.</p>
1615 <!-- _______________________________________________________________________ -->
1617 <a name="dss_otherset">Other Set-Like Container Options</a>
1623 The STL provides several other options, such as std::multiset and the various
1624 "hash_set" like containers (whether from C++ TR1 or from the SGI library). We
1625 never use hash_set and unordered_set because they are generally very expensive
1626 (each insertion requires a malloc) and very non-portable.
1629 <p>std::multiset is useful if you're not interested in elimination of
1630 duplicates, but has all the drawbacks of std::set. A sorted vector (where you
1631 don't delete duplicate entries) or some other approach is almost always
1638 <!-- ======================================================================= -->
1640 <a name="ds_map">Map-Like Containers (std::map, DenseMap, etc)</a>
1644 Map-like containers are useful when you want to associate data to a key. As
1645 usual, there are a lot of different ways to do this. :)
1647 <!-- _______________________________________________________________________ -->
1649 <a name="dss_sortedvectormap">A sorted 'vector'</a>
1655 If your usage pattern follows a strict insert-then-query approach, you can
1656 trivially use the same approach as <a href="#dss_sortedvectorset">sorted vectors
1657 for set-like containers</a>. The only difference is that your query function
1658 (which uses std::lower_bound to get efficient log(n) lookup) should only compare
1659 the key, not both the key and value. This yields the same advantages as sorted
1664 <!-- _______________________________________________________________________ -->
1666 <a name="dss_stringmap">"llvm/ADT/StringMap.h"</a>
1672 Strings are commonly used as keys in maps, and they are difficult to support
1673 efficiently: they are variable length, inefficient to hash and compare when
1674 long, expensive to copy, etc. StringMap is a specialized container designed to
1675 cope with these issues. It supports mapping an arbitrary range of bytes to an
1676 arbitrary other object.</p>
1678 <p>The StringMap implementation uses a quadratically-probed hash table, where
1679 the buckets store a pointer to the heap allocated entries (and some other
1680 stuff). The entries in the map must be heap allocated because the strings are
1681 variable length. The string data (key) and the element object (value) are
1682 stored in the same allocation with the string data immediately after the element
1683 object. This container guarantees the "<tt>(char*)(&Value+1)</tt>" points
1684 to the key string for a value.</p>
1686 <p>The StringMap is very fast for several reasons: quadratic probing is very
1687 cache efficient for lookups, the hash value of strings in buckets is not
1688 recomputed when looking up an element, StringMap rarely has to touch the
1689 memory for unrelated objects when looking up a value (even when hash collisions
1690 happen), hash table growth does not recompute the hash values for strings
1691 already in the table, and each pair in the map is store in a single allocation
1692 (the string data is stored in the same allocation as the Value of a pair).</p>
1694 <p>StringMap also provides query methods that take byte ranges, so it only ever
1695 copies a string if a value is inserted into the table.</p>
1698 <!-- _______________________________________________________________________ -->
1700 <a name="dss_indexedmap">"llvm/ADT/IndexedMap.h"</a>
1705 IndexedMap is a specialized container for mapping small dense integers (or
1706 values that can be mapped to small dense integers) to some other type. It is
1707 internally implemented as a vector with a mapping function that maps the keys to
1708 the dense integer range.
1712 This is useful for cases like virtual registers in the LLVM code generator: they
1713 have a dense mapping that is offset by a compile-time constant (the first
1714 virtual register ID).</p>
1718 <!-- _______________________________________________________________________ -->
1720 <a name="dss_densemap">"llvm/ADT/DenseMap.h"</a>
1726 DenseMap is a simple quadratically probed hash table. It excels at supporting
1727 small keys and values: it uses a single allocation to hold all of the pairs that
1728 are currently inserted in the map. DenseMap is a great way to map pointers to
1729 pointers, or map other small types to each other.
1733 There are several aspects of DenseMap that you should be aware of, however. The
1734 iterators in a densemap are invalidated whenever an insertion occurs, unlike
1735 map. Also, because DenseMap allocates space for a large number of key/value
1736 pairs (it starts with 64 by default), it will waste a lot of space if your keys
1737 or values are large. Finally, you must implement a partial specialization of
1738 DenseMapInfo for the key that you want, if it isn't already supported. This
1739 is required to tell DenseMap about two special marker values (which can never be
1740 inserted into the map) that it needs internally.</p>
1744 <!-- _______________________________________________________________________ -->
1746 <a name="dss_valuemap">"llvm/ADT/ValueMap.h"</a>
1752 ValueMap is a wrapper around a <a href="#dss_densemap">DenseMap</a> mapping
1753 Value*s (or subclasses) to another type. When a Value is deleted or RAUW'ed,
1754 ValueMap will update itself so the new version of the key is mapped to the same
1755 value, just as if the key were a WeakVH. You can configure exactly how this
1756 happens, and what else happens on these two events, by passing
1757 a <code>Config</code> parameter to the ValueMap template.</p>
1761 <!-- _______________________________________________________________________ -->
1763 <a name="dss_intervalmap">"llvm/ADT/IntervalMap.h"</a>
1768 <p> IntervalMap is a compact map for small keys and values. It maps key
1769 intervals instead of single keys, and it will automatically coalesce adjacent
1770 intervals. When then map only contains a few intervals, they are stored in the
1771 map object itself to avoid allocations.</p>
1773 <p> The IntervalMap iterators are quite big, so they should not be passed around
1774 as STL iterators. The heavyweight iterators allow a smaller data structure.</p>
1778 <!-- _______________________________________________________________________ -->
1780 <a name="dss_map"><map></a>
1786 std::map has similar characteristics to <a href="#dss_set">std::set</a>: it uses
1787 a single allocation per pair inserted into the map, it offers log(n) lookup with
1788 an extremely large constant factor, imposes a space penalty of 3 pointers per
1789 pair in the map, etc.</p>
1791 <p>std::map is most useful when your keys or values are very large, if you need
1792 to iterate over the collection in sorted order, or if you need stable iterators
1793 into the map (i.e. they don't get invalidated if an insertion or deletion of
1794 another element takes place).</p>
1798 <!-- _______________________________________________________________________ -->
1800 <a name="dss_inteqclasses">"llvm/ADT/IntEqClasses.h"</a>
1805 <p>IntEqClasses provides a compact representation of equivalence classes of
1806 small integers. Initially, each integer in the range 0..n-1 has its own
1807 equivalence class. Classes can be joined by passing two class representatives to
1808 the join(a, b) method. Two integers are in the same class when findLeader()
1809 returns the same representative.</p>
1811 <p>Once all equivalence classes are formed, the map can be compressed so each
1812 integer 0..n-1 maps to an equivalence class number in the range 0..m-1, where m
1813 is the total number of equivalence classes. The map must be uncompressed before
1814 it can be edited again.</p>
1818 <!-- _______________________________________________________________________ -->
1820 <a name="dss_othermap">Other Map-Like Container Options</a>
1826 The STL provides several other options, such as std::multimap and the various
1827 "hash_map" like containers (whether from C++ TR1 or from the SGI library). We
1828 never use hash_set and unordered_set because they are generally very expensive
1829 (each insertion requires a malloc) and very non-portable.</p>
1831 <p>std::multimap is useful if you want to map a key to multiple values, but has
1832 all the drawbacks of std::map. A sorted vector or some other approach is almost
1839 <!-- ======================================================================= -->
1841 <a name="ds_bit">Bit storage containers (BitVector, SparseBitVector)</a>
1845 <p>Unlike the other containers, there are only two bit storage containers, and
1846 choosing when to use each is relatively straightforward.</p>
1848 <p>One additional option is
1849 <tt>std::vector<bool></tt>: we discourage its use for two reasons 1) the
1850 implementation in many common compilers (e.g. commonly available versions of
1851 GCC) is extremely inefficient and 2) the C++ standards committee is likely to
1852 deprecate this container and/or change it significantly somehow. In any case,
1853 please don't use it.</p>
1855 <!-- _______________________________________________________________________ -->
1857 <a name="dss_bitvector">BitVector</a>
1861 <p> The BitVector container provides a dynamic size set of bits for manipulation.
1862 It supports individual bit setting/testing, as well as set operations. The set
1863 operations take time O(size of bitvector), but operations are performed one word
1864 at a time, instead of one bit at a time. This makes the BitVector very fast for
1865 set operations compared to other containers. Use the BitVector when you expect
1866 the number of set bits to be high (IE a dense set).
1870 <!-- _______________________________________________________________________ -->
1872 <a name="dss_smallbitvector">SmallBitVector</a>
1876 <p> The SmallBitVector container provides the same interface as BitVector, but
1877 it is optimized for the case where only a small number of bits, less than
1878 25 or so, are needed. It also transparently supports larger bit counts, but
1879 slightly less efficiently than a plain BitVector, so SmallBitVector should
1880 only be used when larger counts are rare.
1884 At this time, SmallBitVector does not support set operations (and, or, xor),
1885 and its operator[] does not provide an assignable lvalue.
1889 <!-- _______________________________________________________________________ -->
1891 <a name="dss_sparsebitvector">SparseBitVector</a>
1895 <p> The SparseBitVector container is much like BitVector, with one major
1896 difference: Only the bits that are set, are stored. This makes the
1897 SparseBitVector much more space efficient than BitVector when the set is sparse,
1898 as well as making set operations O(number of set bits) instead of O(size of
1899 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
1900 (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).
1908 <!-- *********************************************************************** -->
1910 <a name="common">Helpful Hints for Common Operations</a>
1912 <!-- *********************************************************************** -->
1916 <p>This section describes how to perform some very simple transformations of
1917 LLVM code. This is meant to give examples of common idioms used, showing the
1918 practical side of LLVM transformations. <p> Because this is a "how-to" section,
1919 you should also read about the main classes that you will be working with. The
1920 <a href="#coreclasses">Core LLVM Class Hierarchy Reference</a> contains details
1921 and descriptions of the main classes that you should know about.</p>
1923 <!-- NOTE: this section should be heavy on example code -->
1924 <!-- ======================================================================= -->
1926 <a name="inspection">Basic Inspection and Traversal Routines</a>
1931 <p>The LLVM compiler infrastructure have many different data structures that may
1932 be traversed. Following the example of the C++ standard template library, the
1933 techniques used to traverse these various data structures are all basically the
1934 same. For a enumerable sequence of values, the <tt>XXXbegin()</tt> function (or
1935 method) returns an iterator to the start of the sequence, the <tt>XXXend()</tt>
1936 function returns an iterator pointing to one past the last valid element of the
1937 sequence, and there is some <tt>XXXiterator</tt> data type that is common
1938 between the two operations.</p>
1940 <p>Because the pattern for iteration is common across many different aspects of
1941 the program representation, the standard template library algorithms may be used
1942 on them, and it is easier to remember how to iterate. First we show a few common
1943 examples of the data structures that need to be traversed. Other data
1944 structures are traversed in very similar ways.</p>
1946 <!-- _______________________________________________________________________ -->
1948 <a name="iterate_function">Iterating over the </a><a
1949 href="#BasicBlock"><tt>BasicBlock</tt></a>s in a <a
1950 href="#Function"><tt>Function</tt></a>
1955 <p>It's quite common to have a <tt>Function</tt> instance that you'd like to
1956 transform in some way; in particular, you'd like to manipulate its
1957 <tt>BasicBlock</tt>s. To facilitate this, you'll need to iterate over all of
1958 the <tt>BasicBlock</tt>s that constitute the <tt>Function</tt>. The following is
1959 an example that prints the name of a <tt>BasicBlock</tt> and the number of
1960 <tt>Instruction</tt>s it contains:</p>
1962 <div class="doc_code">
1964 // <i>func is a pointer to a Function instance</i>
1965 for (Function::iterator i = func->begin(), e = func->end(); i != e; ++i)
1966 // <i>Print out the name of the basic block if it has one, and then the</i>
1967 // <i>number of instructions that it contains</i>
1968 errs() << "Basic block (name=" << i->getName() << ") has "
1969 << i->size() << " instructions.\n";
1973 <p>Note that i can be used as if it were a pointer for the purposes of
1974 invoking member functions of the <tt>Instruction</tt> class. This is
1975 because the indirection operator is overloaded for the iterator
1976 classes. In the above code, the expression <tt>i->size()</tt> is
1977 exactly equivalent to <tt>(*i).size()</tt> just like you'd expect.</p>
1981 <!-- _______________________________________________________________________ -->
1983 <a name="iterate_basicblock">Iterating over the </a><a
1984 href="#Instruction"><tt>Instruction</tt></a>s in a <a
1985 href="#BasicBlock"><tt>BasicBlock</tt></a>
1990 <p>Just like when dealing with <tt>BasicBlock</tt>s in <tt>Function</tt>s, it's
1991 easy to iterate over the individual instructions that make up
1992 <tt>BasicBlock</tt>s. Here's a code snippet that prints out each instruction in
1993 a <tt>BasicBlock</tt>:</p>
1995 <div class="doc_code">
1997 // <i>blk is a pointer to a BasicBlock instance</i>
1998 for (BasicBlock::iterator i = blk->begin(), e = blk->end(); i != e; ++i)
1999 // <i>The next statement works since operator<<(ostream&,...)</i>
2000 // <i>is overloaded for Instruction&</i>
2001 errs() << *i << "\n";
2005 <p>However, this isn't really the best way to print out the contents of a
2006 <tt>BasicBlock</tt>! Since the ostream operators are overloaded for virtually
2007 anything you'll care about, you could have just invoked the print routine on the
2008 basic block itself: <tt>errs() << *blk << "\n";</tt>.</p>
2012 <!-- _______________________________________________________________________ -->
2014 <a name="iterate_institer">Iterating over the </a><a
2015 href="#Instruction"><tt>Instruction</tt></a>s in a <a
2016 href="#Function"><tt>Function</tt></a>
2021 <p>If you're finding that you commonly iterate over a <tt>Function</tt>'s
2022 <tt>BasicBlock</tt>s and then that <tt>BasicBlock</tt>'s <tt>Instruction</tt>s,
2023 <tt>InstIterator</tt> should be used instead. You'll need to include <a
2024 href="/doxygen/InstIterator_8h-source.html"><tt>llvm/Support/InstIterator.h</tt></a>,
2025 and then instantiate <tt>InstIterator</tt>s explicitly in your code. Here's a
2026 small example that shows how to dump all instructions in a function to the standard error stream:<p>
2028 <div class="doc_code">
2030 #include "<a href="/doxygen/InstIterator_8h-source.html">llvm/Support/InstIterator.h</a>"
2032 // <i>F is a pointer to a Function instance</i>
2033 for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I)
2034 errs() << *I << "\n";
2038 <p>Easy, isn't it? You can also use <tt>InstIterator</tt>s to fill a
2039 work list with its initial contents. For example, if you wanted to
2040 initialize a work list to contain all instructions in a <tt>Function</tt>
2041 F, all you would need to do is something like:</p>
2043 <div class="doc_code">
2045 std::set<Instruction*> worklist;
2046 // or better yet, SmallPtrSet<Instruction*, 64> worklist;
2048 for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I)
2049 worklist.insert(&*I);
2053 <p>The STL set <tt>worklist</tt> would now contain all instructions in the
2054 <tt>Function</tt> pointed to by F.</p>
2058 <!-- _______________________________________________________________________ -->
2060 <a name="iterate_convert">Turning an iterator into a class pointer (and
2066 <p>Sometimes, it'll be useful to grab a reference (or pointer) to a class
2067 instance when all you've got at hand is an iterator. Well, extracting
2068 a reference or a pointer from an iterator is very straight-forward.
2069 Assuming that <tt>i</tt> is a <tt>BasicBlock::iterator</tt> and <tt>j</tt>
2070 is a <tt>BasicBlock::const_iterator</tt>:</p>
2072 <div class="doc_code">
2074 Instruction& inst = *i; // <i>Grab reference to instruction reference</i>
2075 Instruction* pinst = &*i; // <i>Grab pointer to instruction reference</i>
2076 const Instruction& inst = *j;
2080 <p>However, the iterators you'll be working with in the LLVM framework are
2081 special: they will automatically convert to a ptr-to-instance type whenever they
2082 need to. Instead of dereferencing the iterator and then taking the address of
2083 the result, you can simply assign the iterator to the proper pointer type and
2084 you get the dereference and address-of operation as a result of the assignment
2085 (behind the scenes, this is a result of overloading casting mechanisms). Thus
2086 the last line of the last example,</p>
2088 <div class="doc_code">
2090 Instruction *pinst = &*i;
2094 <p>is semantically equivalent to</p>
2096 <div class="doc_code">
2098 Instruction *pinst = i;
2102 <p>It's also possible to turn a class pointer into the corresponding iterator,
2103 and this is a constant time operation (very efficient). The following code
2104 snippet illustrates use of the conversion constructors provided by LLVM
2105 iterators. By using these, you can explicitly grab the iterator of something
2106 without actually obtaining it via iteration over some structure:</p>
2108 <div class="doc_code">
2110 void printNextInstruction(Instruction* inst) {
2111 BasicBlock::iterator it(inst);
2112 ++it; // <i>After this line, it refers to the instruction after *inst</i>
2113 if (it != inst->getParent()->end()) errs() << *it << "\n";
2118 <p>Unfortunately, these implicit conversions come at a cost; they prevent
2119 these iterators from conforming to standard iterator conventions, and thus
2120 from being usable with standard algorithms and containers. For example, they
2121 prevent the following code, where <tt>B</tt> is a <tt>BasicBlock</tt>,
2124 <div class="doc_code">
2126 llvm::SmallVector<llvm::Instruction *, 16>(B->begin(), B->end());
2130 <p>Because of this, these implicit conversions may be removed some day,
2131 and <tt>operator*</tt> changed to return a pointer instead of a reference.</p>
2135 <!--_______________________________________________________________________-->
2137 <a name="iterate_complex">Finding call sites: a slightly more complex
2143 <p>Say that you're writing a FunctionPass and would like to count all the
2144 locations in the entire module (that is, across every <tt>Function</tt>) where a
2145 certain function (i.e., some <tt>Function</tt>*) is already in scope. As you'll
2146 learn later, you may want to use an <tt>InstVisitor</tt> to accomplish this in a
2147 much more straight-forward manner, but this example will allow us to explore how
2148 you'd do it if you didn't have <tt>InstVisitor</tt> around. In pseudo-code, this
2149 is what we want to do:</p>
2151 <div class="doc_code">
2153 initialize callCounter to zero
2154 for each Function f in the Module
2155 for each BasicBlock b in f
2156 for each Instruction i in b
2157 if (i is a CallInst and calls the given function)
2158 increment callCounter
2162 <p>And the actual code is (remember, because we're writing a
2163 <tt>FunctionPass</tt>, our <tt>FunctionPass</tt>-derived class simply has to
2164 override the <tt>runOnFunction</tt> method):</p>
2166 <div class="doc_code">
2168 Function* targetFunc = ...;
2170 class OurFunctionPass : public FunctionPass {
2172 OurFunctionPass(): callCounter(0) { }
2174 virtual runOnFunction(Function& F) {
2175 for (Function::iterator b = F.begin(), be = F.end(); b != be; ++b) {
2176 for (BasicBlock::iterator i = b->begin(), ie = b->end(); i != ie; ++i) {
2177 if (<a href="#CallInst">CallInst</a>* callInst = <a href="#isa">dyn_cast</a><<a
2178 href="#CallInst">CallInst</a>>(&*i)) {
2179 // <i>We know we've encountered a call instruction, so we</i>
2180 // <i>need to determine if it's a call to the</i>
2181 // <i>function pointed to by m_func or not.</i>
2182 if (callInst->getCalledFunction() == targetFunc)
2190 unsigned callCounter;
2197 <!--_______________________________________________________________________-->
2199 <a name="calls_and_invokes">Treating calls and invokes the same way</a>
2204 <p>You may have noticed that the previous example was a bit oversimplified in
2205 that it did not deal with call sites generated by 'invoke' instructions. In
2206 this, and in other situations, you may find that you want to treat
2207 <tt>CallInst</tt>s and <tt>InvokeInst</tt>s the same way, even though their
2208 most-specific common base class is <tt>Instruction</tt>, which includes lots of
2209 less closely-related things. For these cases, LLVM provides a handy wrapper
2211 href="http://llvm.org/doxygen/classllvm_1_1CallSite.html"><tt>CallSite</tt></a>.
2212 It is essentially a wrapper around an <tt>Instruction</tt> pointer, with some
2213 methods that provide functionality common to <tt>CallInst</tt>s and
2214 <tt>InvokeInst</tt>s.</p>
2216 <p>This class has "value semantics": it should be passed by value, not by
2217 reference and it should not be dynamically allocated or deallocated using
2218 <tt>operator new</tt> or <tt>operator delete</tt>. It is efficiently copyable,
2219 assignable and constructable, with costs equivalents to that of a bare pointer.
2220 If you look at its definition, it has only a single pointer member.</p>
2224 <!--_______________________________________________________________________-->
2226 <a name="iterate_chains">Iterating over def-use & use-def chains</a>
2231 <p>Frequently, we might have an instance of the <a
2232 href="/doxygen/classllvm_1_1Value.html">Value Class</a> and we want to
2233 determine which <tt>User</tt>s use the <tt>Value</tt>. The list of all
2234 <tt>User</tt>s of a particular <tt>Value</tt> is called a <i>def-use</i> chain.
2235 For example, let's say we have a <tt>Function*</tt> named <tt>F</tt> to a
2236 particular function <tt>foo</tt>. Finding all of the instructions that
2237 <i>use</i> <tt>foo</tt> is as simple as iterating over the <i>def-use</i> chain
2240 <div class="doc_code">
2244 for (Value::use_iterator i = F->use_begin(), e = F->use_end(); i != e; ++i)
2245 if (Instruction *Inst = dyn_cast<Instruction>(*i)) {
2246 errs() << "F is used in instruction:\n";
2247 errs() << *Inst << "\n";
2252 <p>Note that dereferencing a <tt>Value::use_iterator</tt> is not a very cheap
2253 operation. Instead of performing <tt>*i</tt> above several times, consider
2254 doing it only once in the loop body and reusing its result.</p>
2256 <p>Alternatively, it's common to have an instance of the <a
2257 href="/doxygen/classllvm_1_1User.html">User Class</a> and need to know what
2258 <tt>Value</tt>s are used by it. The list of all <tt>Value</tt>s used by a
2259 <tt>User</tt> is known as a <i>use-def</i> chain. Instances of class
2260 <tt>Instruction</tt> are common <tt>User</tt>s, so we might want to iterate over
2261 all of the values that a particular instruction uses (that is, the operands of
2262 the particular <tt>Instruction</tt>):</p>
2264 <div class="doc_code">
2266 Instruction *pi = ...;
2268 for (User::op_iterator i = pi->op_begin(), e = pi->op_end(); i != e; ++i) {
2275 <p>Declaring objects as <tt>const</tt> is an important tool of enforcing
2276 mutation free algorithms (such as analyses, etc.). For this purpose above
2277 iterators come in constant flavors as <tt>Value::const_use_iterator</tt>
2278 and <tt>Value::const_op_iterator</tt>. They automatically arise when
2279 calling <tt>use/op_begin()</tt> on <tt>const Value*</tt>s or
2280 <tt>const User*</tt>s respectively. Upon dereferencing, they return
2281 <tt>const Use*</tt>s. Otherwise the above patterns remain unchanged.</p>
2285 <!--_______________________________________________________________________-->
2287 <a name="iterate_preds">Iterating over predecessors &
2288 successors of blocks</a>
2293 <p>Iterating over the predecessors and successors of a block is quite easy
2294 with the routines defined in <tt>"llvm/Support/CFG.h"</tt>. Just use code like
2295 this to iterate over all predecessors of BB:</p>
2297 <div class="doc_code">
2299 #include "llvm/Support/CFG.h"
2300 BasicBlock *BB = ...;
2302 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
2303 BasicBlock *Pred = *PI;
2309 <p>Similarly, to iterate over successors use
2310 succ_iterator/succ_begin/succ_end.</p>
2316 <!-- ======================================================================= -->
2318 <a name="simplechanges">Making simple changes</a>
2323 <p>There are some primitive transformation operations present in the LLVM
2324 infrastructure that are worth knowing about. When performing
2325 transformations, it's fairly common to manipulate the contents of basic
2326 blocks. This section describes some of the common methods for doing so
2327 and gives example code.</p>
2329 <!--_______________________________________________________________________-->
2331 <a name="schanges_creating">Creating and inserting new
2332 <tt>Instruction</tt>s</a>
2337 <p><i>Instantiating Instructions</i></p>
2339 <p>Creation of <tt>Instruction</tt>s is straight-forward: simply call the
2340 constructor for the kind of instruction to instantiate and provide the necessary
2341 parameters. For example, an <tt>AllocaInst</tt> only <i>requires</i> a
2342 (const-ptr-to) <tt>Type</tt>. Thus:</p>
2344 <div class="doc_code">
2346 AllocaInst* ai = new AllocaInst(Type::Int32Ty);
2350 <p>will create an <tt>AllocaInst</tt> instance that represents the allocation of
2351 one integer in the current stack frame, at run time. Each <tt>Instruction</tt>
2352 subclass is likely to have varying default parameters which change the semantics
2353 of the instruction, so refer to the <a
2354 href="/doxygen/classllvm_1_1Instruction.html">doxygen documentation for the subclass of
2355 Instruction</a> that you're interested in instantiating.</p>
2357 <p><i>Naming values</i></p>
2359 <p>It is very useful to name the values of instructions when you're able to, as
2360 this facilitates the debugging of your transformations. If you end up looking
2361 at generated LLVM machine code, you definitely want to have logical names
2362 associated with the results of instructions! By supplying a value for the
2363 <tt>Name</tt> (default) parameter of the <tt>Instruction</tt> constructor, you
2364 associate a logical name with the result of the instruction's execution at
2365 run time. For example, say that I'm writing a transformation that dynamically
2366 allocates space for an integer on the stack, and that integer is going to be
2367 used as some kind of index by some other code. To accomplish this, I place an
2368 <tt>AllocaInst</tt> at the first point in the first <tt>BasicBlock</tt> of some
2369 <tt>Function</tt>, and I'm intending to use it within the same
2370 <tt>Function</tt>. I might do:</p>
2372 <div class="doc_code">
2374 AllocaInst* pa = new AllocaInst(Type::Int32Ty, 0, "indexLoc");
2378 <p>where <tt>indexLoc</tt> is now the logical name of the instruction's
2379 execution value, which is a pointer to an integer on the run time stack.</p>
2381 <p><i>Inserting instructions</i></p>
2383 <p>There are essentially two ways to insert an <tt>Instruction</tt>
2384 into an existing sequence of instructions that form a <tt>BasicBlock</tt>:</p>
2387 <li>Insertion into an explicit instruction list
2389 <p>Given a <tt>BasicBlock* pb</tt>, an <tt>Instruction* pi</tt> within that
2390 <tt>BasicBlock</tt>, and a newly-created instruction we wish to insert
2391 before <tt>*pi</tt>, we do the following: </p>
2393 <div class="doc_code">
2395 BasicBlock *pb = ...;
2396 Instruction *pi = ...;
2397 Instruction *newInst = new Instruction(...);
2399 pb->getInstList().insert(pi, newInst); // <i>Inserts newInst before pi in pb</i>
2403 <p>Appending to the end of a <tt>BasicBlock</tt> is so common that
2404 the <tt>Instruction</tt> class and <tt>Instruction</tt>-derived
2405 classes provide constructors which take a pointer to a
2406 <tt>BasicBlock</tt> to be appended to. For example code that
2409 <div class="doc_code">
2411 BasicBlock *pb = ...;
2412 Instruction *newInst = new Instruction(...);
2414 pb->getInstList().push_back(newInst); // <i>Appends newInst to pb</i>
2420 <div class="doc_code">
2422 BasicBlock *pb = ...;
2423 Instruction *newInst = new Instruction(..., pb);
2427 <p>which is much cleaner, especially if you are creating
2428 long instruction streams.</p></li>
2430 <li>Insertion into an implicit instruction list
2432 <p><tt>Instruction</tt> instances that are already in <tt>BasicBlock</tt>s
2433 are implicitly associated with an existing instruction list: the instruction
2434 list of the enclosing basic block. Thus, we could have accomplished the same
2435 thing as the above code without being given a <tt>BasicBlock</tt> by doing:
2438 <div class="doc_code">
2440 Instruction *pi = ...;
2441 Instruction *newInst = new Instruction(...);
2443 pi->getParent()->getInstList().insert(pi, newInst);
2447 <p>In fact, this sequence of steps occurs so frequently that the
2448 <tt>Instruction</tt> class and <tt>Instruction</tt>-derived classes provide
2449 constructors which take (as a default parameter) a pointer to an
2450 <tt>Instruction</tt> which the newly-created <tt>Instruction</tt> should
2451 precede. That is, <tt>Instruction</tt> constructors are capable of
2452 inserting the newly-created instance into the <tt>BasicBlock</tt> of a
2453 provided instruction, immediately before that instruction. Using an
2454 <tt>Instruction</tt> constructor with a <tt>insertBefore</tt> (default)
2455 parameter, the above code becomes:</p>
2457 <div class="doc_code">
2459 Instruction* pi = ...;
2460 Instruction* newInst = new Instruction(..., pi);
2464 <p>which is much cleaner, especially if you're creating a lot of
2465 instructions and adding them to <tt>BasicBlock</tt>s.</p></li>
2470 <!--_______________________________________________________________________-->
2472 <a name="schanges_deleting">Deleting <tt>Instruction</tt>s</a>
2477 <p>Deleting an instruction from an existing sequence of instructions that form a
2478 <a href="#BasicBlock"><tt>BasicBlock</tt></a> is very straight-forward: just
2479 call the instruction's eraseFromParent() method. For example:</p>
2481 <div class="doc_code">
2483 <a href="#Instruction">Instruction</a> *I = .. ;
2484 I->eraseFromParent();
2488 <p>This unlinks the instruction from its containing basic block and deletes
2489 it. If you'd just like to unlink the instruction from its containing basic
2490 block but not delete it, you can use the <tt>removeFromParent()</tt> method.</p>
2494 <!--_______________________________________________________________________-->
2496 <a name="schanges_replacing">Replacing an <tt>Instruction</tt> with another
2502 <p><i>Replacing individual instructions</i></p>
2504 <p>Including "<a href="/doxygen/BasicBlockUtils_8h-source.html">llvm/Transforms/Utils/BasicBlockUtils.h</a>"
2505 permits use of two very useful replace functions: <tt>ReplaceInstWithValue</tt>
2506 and <tt>ReplaceInstWithInst</tt>.</p>
2508 <h5><a name="schanges_deleting">Deleting <tt>Instruction</tt>s</a></h5>
2511 <li><tt>ReplaceInstWithValue</tt>
2513 <p>This function replaces all uses of a given instruction with a value,
2514 and then removes the original instruction. The following example
2515 illustrates the replacement of the result of a particular
2516 <tt>AllocaInst</tt> that allocates memory for a single integer with a null
2517 pointer to an integer.</p>
2519 <div class="doc_code">
2521 AllocaInst* instToReplace = ...;
2522 BasicBlock::iterator ii(instToReplace);
2524 ReplaceInstWithValue(instToReplace->getParent()->getInstList(), ii,
2525 Constant::getNullValue(PointerType::getUnqual(Type::Int32Ty)));
2528 <li><tt>ReplaceInstWithInst</tt>
2530 <p>This function replaces a particular instruction with another
2531 instruction, inserting the new instruction into the basic block at the
2532 location where the old instruction was, and replacing any uses of the old
2533 instruction with the new instruction. The following example illustrates
2534 the replacement of one <tt>AllocaInst</tt> with another.</p>
2536 <div class="doc_code">
2538 AllocaInst* instToReplace = ...;
2539 BasicBlock::iterator ii(instToReplace);
2541 ReplaceInstWithInst(instToReplace->getParent()->getInstList(), ii,
2542 new AllocaInst(Type::Int32Ty, 0, "ptrToReplacedInt"));
2546 <p><i>Replacing multiple uses of <tt>User</tt>s and <tt>Value</tt>s</i></p>
2548 <p>You can use <tt>Value::replaceAllUsesWith</tt> and
2549 <tt>User::replaceUsesOfWith</tt> to change more than one use at a time. See the
2550 doxygen documentation for the <a href="/doxygen/classllvm_1_1Value.html">Value Class</a>
2551 and <a href="/doxygen/classllvm_1_1User.html">User Class</a>, respectively, for more
2554 <!-- Value::replaceAllUsesWith User::replaceUsesOfWith Point out:
2555 include/llvm/Transforms/Utils/ especially BasicBlockUtils.h with:
2556 ReplaceInstWithValue, ReplaceInstWithInst -->
2560 <!--_______________________________________________________________________-->
2562 <a name="schanges_deletingGV">Deleting <tt>GlobalVariable</tt>s</a>
2567 <p>Deleting a global variable from a module is just as easy as deleting an
2568 Instruction. First, you must have a pointer to the global variable that you wish
2569 to delete. You use this pointer to erase it from its parent, the module.
2572 <div class="doc_code">
2574 <a href="#GlobalVariable">GlobalVariable</a> *GV = .. ;
2576 GV->eraseFromParent();
2584 <!-- ======================================================================= -->
2586 <a name="create_types">How to Create Types</a>
2591 <p>In generating IR, you may need some complex types. If you know these types
2592 statically, you can use <tt>TypeBuilder<...>::get()</tt>, defined
2593 in <tt>llvm/Support/TypeBuilder.h</tt>, to retrieve them. <tt>TypeBuilder</tt>
2594 has two forms depending on whether you're building types for cross-compilation
2595 or native library use. <tt>TypeBuilder<T, true></tt> requires
2596 that <tt>T</tt> be independent of the host environment, meaning that it's built
2598 the <a href="/doxygen/namespacellvm_1_1types.html"><tt>llvm::types</tt></a>
2599 namespace and pointers, functions, arrays, etc. built of
2600 those. <tt>TypeBuilder<T, false></tt> additionally allows native C types
2601 whose size may depend on the host compiler. For example,</p>
2603 <div class="doc_code">
2605 FunctionType *ft = TypeBuilder<types::i<8>(types::i<32>*), true>::get();
2609 <p>is easier to read and write than the equivalent</p>
2611 <div class="doc_code">
2613 std::vector<const Type*> params;
2614 params.push_back(PointerType::getUnqual(Type::Int32Ty));
2615 FunctionType *ft = FunctionType::get(Type::Int8Ty, params, false);
2619 <p>See the <a href="/doxygen/TypeBuilder_8h-source.html#l00001">class
2620 comment</a> for more details.</p>
2626 <!-- *********************************************************************** -->
2628 <a name="threading">Threads and LLVM</a>
2630 <!-- *********************************************************************** -->
2634 This section describes the interaction of the LLVM APIs with multithreading,
2635 both on the part of client applications, and in the JIT, in the hosted
2640 Note that LLVM's support for multithreading is still relatively young. Up
2641 through version 2.5, the execution of threaded hosted applications was
2642 supported, but not threaded client access to the APIs. While this use case is
2643 now supported, clients <em>must</em> adhere to the guidelines specified below to
2644 ensure proper operation in multithreaded mode.
2648 Note that, on Unix-like platforms, LLVM requires the presence of GCC's atomic
2649 intrinsics in order to support threaded operation. If you need a
2650 multhreading-capable LLVM on a platform without a suitably modern system
2651 compiler, consider compiling LLVM and LLVM-GCC in single-threaded mode, and
2652 using the resultant compiler to build a copy of LLVM with multithreading
2656 <!-- ======================================================================= -->
2658 <a name="startmultithreaded">Entering and Exiting Multithreaded Mode</a>
2664 In order to properly protect its internal data structures while avoiding
2665 excessive locking overhead in the single-threaded case, the LLVM must intialize
2666 certain data structures necessary to provide guards around its internals. To do
2667 so, the client program must invoke <tt>llvm_start_multithreaded()</tt> before
2668 making any concurrent LLVM API calls. To subsequently tear down these
2669 structures, use the <tt>llvm_stop_multithreaded()</tt> call. You can also use
2670 the <tt>llvm_is_multithreaded()</tt> call to check the status of multithreaded
2675 Note that both of these calls must be made <em>in isolation</em>. That is to
2676 say that no other LLVM API calls may be executing at any time during the
2677 execution of <tt>llvm_start_multithreaded()</tt> or <tt>llvm_stop_multithreaded
2678 </tt>. It's is the client's responsibility to enforce this isolation.
2682 The return value of <tt>llvm_start_multithreaded()</tt> indicates the success or
2683 failure of the initialization. Failure typically indicates that your copy of
2684 LLVM was built without multithreading support, typically because GCC atomic
2685 intrinsics were not found in your system compiler. In this case, the LLVM API
2686 will not be safe for concurrent calls. However, it <em>will</em> be safe for
2687 hosting threaded applications in the JIT, though <a href="#jitthreading">care
2688 must be taken</a> to ensure that side exits and the like do not accidentally
2689 result in concurrent LLVM API calls.
2693 <!-- ======================================================================= -->
2695 <a name="shutdown">Ending Execution with <tt>llvm_shutdown()</tt></a>
2700 When you are done using the LLVM APIs, you should call <tt>llvm_shutdown()</tt>
2701 to deallocate memory used for internal structures. This will also invoke
2702 <tt>llvm_stop_multithreaded()</tt> if LLVM is operating in multithreaded mode.
2703 As such, <tt>llvm_shutdown()</tt> requires the same isolation guarantees as
2704 <tt>llvm_stop_multithreaded()</tt>.
2708 Note that, if you use scope-based shutdown, you can use the
2709 <tt>llvm_shutdown_obj</tt> class, which calls <tt>llvm_shutdown()</tt> in its
2713 <!-- ======================================================================= -->
2715 <a name="managedstatic">Lazy Initialization with <tt>ManagedStatic</tt></a>
2720 <tt>ManagedStatic</tt> is a utility class in LLVM used to implement static
2721 initialization of static resources, such as the global type tables. Before the
2722 invocation of <tt>llvm_shutdown()</tt>, it implements a simple lazy
2723 initialization scheme. Once <tt>llvm_start_multithreaded()</tt> returns,
2724 however, it uses double-checked locking to implement thread-safe lazy
2729 Note that, because no other threads are allowed to issue LLVM API calls before
2730 <tt>llvm_start_multithreaded()</tt> returns, it is possible to have
2731 <tt>ManagedStatic</tt>s of <tt>llvm::sys::Mutex</tt>s.
2735 The <tt>llvm_acquire_global_lock()</tt> and <tt>llvm_release_global_lock</tt>
2736 APIs provide access to the global lock used to implement the double-checked
2737 locking for lazy initialization. These should only be used internally to LLVM,
2738 and only if you know what you're doing!
2742 <!-- ======================================================================= -->
2744 <a name="llvmcontext">Achieving Isolation with <tt>LLVMContext</tt></a>
2749 <tt>LLVMContext</tt> is an opaque class in the LLVM API which clients can use
2750 to operate multiple, isolated instances of LLVM concurrently within the same
2751 address space. For instance, in a hypothetical compile-server, the compilation
2752 of an individual translation unit is conceptually independent from all the
2753 others, and it would be desirable to be able to compile incoming translation
2754 units concurrently on independent server threads. Fortunately,
2755 <tt>LLVMContext</tt> exists to enable just this kind of scenario!
2759 Conceptually, <tt>LLVMContext</tt> provides isolation. Every LLVM entity
2760 (<tt>Module</tt>s, <tt>Value</tt>s, <tt>Type</tt>s, <tt>Constant</tt>s, etc.)
2761 in LLVM's in-memory IR belongs to an <tt>LLVMContext</tt>. Entities in
2762 different contexts <em>cannot</em> interact with each other: <tt>Module</tt>s in
2763 different contexts cannot be linked together, <tt>Function</tt>s cannot be added
2764 to <tt>Module</tt>s in different contexts, etc. What this means is that is is
2765 safe to compile on multiple threads simultaneously, as long as no two threads
2766 operate on entities within the same context.
2770 In practice, very few places in the API require the explicit specification of a
2771 <tt>LLVMContext</tt>, other than the <tt>Type</tt> creation/lookup APIs.
2772 Because every <tt>Type</tt> carries a reference to its owning context, most
2773 other entities can determine what context they belong to by looking at their
2774 own <tt>Type</tt>. If you are adding new entities to LLVM IR, please try to
2775 maintain this interface design.
2779 For clients that do <em>not</em> require the benefits of isolation, LLVM
2780 provides a convenience API <tt>getGlobalContext()</tt>. This returns a global,
2781 lazily initialized <tt>LLVMContext</tt> that may be used in situations where
2782 isolation is not a concern.
2786 <!-- ======================================================================= -->
2788 <a name="jitthreading">Threads and the JIT</a>
2793 LLVM's "eager" JIT compiler is safe to use in threaded programs. Multiple
2794 threads can call <tt>ExecutionEngine::getPointerToFunction()</tt> or
2795 <tt>ExecutionEngine::runFunction()</tt> concurrently, and multiple threads can
2796 run code output by the JIT concurrently. The user must still ensure that only
2797 one thread accesses IR in a given <tt>LLVMContext</tt> while another thread
2798 might be modifying it. One way to do that is to always hold the JIT lock while
2799 accessing IR outside the JIT (the JIT <em>modifies</em> the IR by adding
2800 <tt>CallbackVH</tt>s). Another way is to only
2801 call <tt>getPointerToFunction()</tt> from the <tt>LLVMContext</tt>'s thread.
2804 <p>When the JIT is configured to compile lazily (using
2805 <tt>ExecutionEngine::DisableLazyCompilation(false)</tt>), there is currently a
2806 <a href="http://llvm.org/bugs/show_bug.cgi?id=5184">race condition</a> in
2807 updating call sites after a function is lazily-jitted. It's still possible to
2808 use the lazy JIT in a threaded program if you ensure that only one thread at a
2809 time can call any particular lazy stub and that the JIT lock guards any IR
2810 access, but we suggest using only the eager JIT in threaded programs.
2816 <!-- *********************************************************************** -->
2818 <a name="advanced">Advanced Topics</a>
2820 <!-- *********************************************************************** -->
2824 This section describes some of the advanced or obscure API's that most clients
2825 do not need to be aware of. These API's tend manage the inner workings of the
2826 LLVM system, and only need to be accessed in unusual circumstances.
2830 <!-- ======================================================================= -->
2832 <a name="SymbolTable">The <tt>ValueSymbolTable</tt> class</a>
2836 <p>The <tt><a href="http://llvm.org/doxygen/classllvm_1_1ValueSymbolTable.html">
2837 ValueSymbolTable</a></tt> class provides a symbol table that the <a
2838 href="#Function"><tt>Function</tt></a> and <a href="#Module">
2839 <tt>Module</tt></a> classes use for naming value definitions. The symbol table
2840 can provide a name for any <a href="#Value"><tt>Value</tt></a>.
2843 <p>Note that the <tt>SymbolTable</tt> class should not be directly accessed
2844 by most clients. It should only be used when iteration over the symbol table
2845 names themselves are required, which is very special purpose. Note that not
2847 <tt><a href="#Value">Value</a></tt>s have names, and those without names (i.e. they have
2848 an empty name) do not exist in the symbol table.
2851 <p>Symbol tables support iteration over the values in the symbol
2852 table with <tt>begin/end/iterator</tt> and supports querying to see if a
2853 specific name is in the symbol table (with <tt>lookup</tt>). The
2854 <tt>ValueSymbolTable</tt> class exposes no public mutator methods, instead,
2855 simply call <tt>setName</tt> on a value, which will autoinsert it into the
2856 appropriate symbol table.</p>
2862 <!-- ======================================================================= -->
2864 <a name="UserLayout">The <tt>User</tt> and owned <tt>Use</tt> classes' memory layout</a>
2868 <p>The <tt><a href="http://llvm.org/doxygen/classllvm_1_1User.html">
2869 User</a></tt> class provides a basis for expressing the ownership of <tt>User</tt>
2870 towards other <tt><a href="http://llvm.org/doxygen/classllvm_1_1Value.html">
2871 Value</a></tt>s. The <tt><a href="http://llvm.org/doxygen/classllvm_1_1Use.html">
2872 Use</a></tt> helper class is employed to do the bookkeeping and to facilitate <i>O(1)</i>
2873 addition and removal.</p>
2875 <!-- ______________________________________________________________________ -->
2878 Interaction and relationship between <tt>User</tt> and <tt>Use</tt> objects
2884 A subclass of <tt>User</tt> can choose between incorporating its <tt>Use</tt> objects
2885 or refer to them out-of-line by means of a pointer. A mixed variant
2886 (some <tt>Use</tt>s inline others hung off) is impractical and breaks the invariant
2887 that the <tt>Use</tt> objects belonging to the same <tt>User</tt> form a contiguous array.
2891 We have 2 different layouts in the <tt>User</tt> (sub)classes:
2894 The <tt>Use</tt> object(s) are inside (resp. at fixed offset) of the <tt>User</tt>
2895 object and there are a fixed number of them.</p>
2898 The <tt>Use</tt> object(s) are referenced by a pointer to an
2899 array from the <tt>User</tt> object and there may be a variable
2903 As of v2.4 each layout still possesses a direct pointer to the
2904 start of the array of <tt>Use</tt>s. Though not mandatory for layout a),
2905 we stick to this redundancy for the sake of simplicity.
2906 The <tt>User</tt> object also stores the number of <tt>Use</tt> objects it
2907 has. (Theoretically this information can also be calculated
2908 given the scheme presented below.)</p>
2910 Special forms of allocation operators (<tt>operator new</tt>)
2911 enforce the following memory layouts:</p>
2914 <li><p>Layout a) is modelled by prepending the <tt>User</tt> object by the <tt>Use[]</tt> array.</p>
2917 ...---.---.---.---.-------...
2918 | P | P | P | P | User
2919 '''---'---'---'---'-------'''
2922 <li><p>Layout b) is modelled by pointing at the <tt>Use[]</tt> array.</p>
2934 <i>(In the above figures '<tt>P</tt>' stands for the <tt>Use**</tt> that
2935 is stored in each <tt>Use</tt> object in the member <tt>Use::Prev</tt>)</i>
2939 <!-- ______________________________________________________________________ -->
2941 <a name="Waymarking">The waymarking algorithm</a>
2946 Since the <tt>Use</tt> objects are deprived of the direct (back)pointer to
2947 their <tt>User</tt> objects, there must be a fast and exact method to
2948 recover it. This is accomplished by the following scheme:</p>
2950 A bit-encoding in the 2 LSBits (least significant bits) of the <tt>Use::Prev</tt> allows to find the
2951 start of the <tt>User</tt> object:
2953 <li><tt>00</tt> —> binary digit 0</li>
2954 <li><tt>01</tt> —> binary digit 1</li>
2955 <li><tt>10</tt> —> stop and calculate (<tt>s</tt>)</li>
2956 <li><tt>11</tt> —> full stop (<tt>S</tt>)</li>
2959 Given a <tt>Use*</tt>, all we have to do is to walk till we get
2960 a stop and we either have a <tt>User</tt> immediately behind or
2961 we have to walk to the next stop picking up digits
2962 and calculating the offset:</p>
2964 .---.---.---.---.---.---.---.---.---.---.---.---.---.---.---.---.----------------
2965 | 1 | s | 1 | 0 | 1 | 0 | s | 1 | 1 | 0 | s | 1 | 1 | s | 1 | S | User (or User*)
2966 '---'---'---'---'---'---'---'---'---'---'---'---'---'---'---'---'----------------
2967 |+15 |+10 |+6 |+3 |+1
2970 | | |______________________>
2971 | |______________________________________>
2972 |__________________________________________________________>
2975 Only the significant number of bits need to be stored between the
2976 stops, so that the <i>worst case is 20 memory accesses</i> when there are
2977 1000 <tt>Use</tt> objects associated with a <tt>User</tt>.</p>
2981 <!-- ______________________________________________________________________ -->
2983 <a name="ReferenceImpl">Reference implementation</a>
2988 The following literate Haskell fragment demonstrates the concept:</p>
2990 <div class="doc_code">
2992 > import Test.QuickCheck
2994 > digits :: Int -> [Char] -> [Char]
2995 > digits 0 acc = '0' : acc
2996 > digits 1 acc = '1' : acc
2997 > digits n acc = digits (n `div` 2) $ digits (n `mod` 2) acc
2999 > dist :: Int -> [Char] -> [Char]
3002 > dist 1 acc = let r = dist 0 acc in 's' : digits (length r) r
3003 > dist n acc = dist (n - 1) $ dist 1 acc
3005 > takeLast n ss = reverse $ take n $ reverse ss
3007 > test = takeLast 40 $ dist 20 []
3012 Printing <test> gives: <tt>"1s100000s11010s10100s1111s1010s110s11s1S"</tt></p>
3014 The reverse algorithm computes the length of the string just by examining
3015 a certain prefix:</p>
3017 <div class="doc_code">
3019 > pref :: [Char] -> Int
3021 > pref ('s':'1':rest) = decode 2 1 rest
3022 > pref (_:rest) = 1 + pref rest
3024 > decode walk acc ('0':rest) = decode (walk + 1) (acc * 2) rest
3025 > decode walk acc ('1':rest) = decode (walk + 1) (acc * 2 + 1) rest
3026 > decode walk acc _ = walk + acc
3031 Now, as expected, printing <pref test> gives <tt>40</tt>.</p>
3033 We can <i>quickCheck</i> this with following property:</p>
3035 <div class="doc_code">
3037 > testcase = dist 2000 []
3038 > testcaseLength = length testcase
3040 > identityProp n = n > 0 && n <= testcaseLength ==> length arr == pref arr
3041 > where arr = takeLast n testcase
3046 As expected <quickCheck identityProp> gives:</p>
3049 *Main> quickCheck identityProp
3050 OK, passed 100 tests.
3053 Let's be a bit more exhaustive:</p>
3055 <div class="doc_code">
3058 > deepCheck p = check (defaultConfig { configMaxTest = 500 }) p
3063 And here is the result of <deepCheck identityProp>:</p>
3066 *Main> deepCheck identityProp
3067 OK, passed 500 tests.
3072 <!-- ______________________________________________________________________ -->
3074 <a name="Tagging">Tagging considerations</a>
3080 To maintain the invariant that the 2 LSBits of each <tt>Use**</tt> in <tt>Use</tt>
3081 never change after being set up, setters of <tt>Use::Prev</tt> must re-tag the
3082 new <tt>Use**</tt> on every modification. Accordingly getters must strip the
3085 For layout b) instead of the <tt>User</tt> we find a pointer (<tt>User*</tt> with LSBit set).
3086 Following this pointer brings us to the <tt>User</tt>. A portable trick ensures
3087 that the first bytes of <tt>User</tt> (if interpreted as a pointer) never has
3088 the LSBit set. (Portability is relying on the fact that all known compilers place the
3089 <tt>vptr</tt> in the first word of the instances.)</p>
3097 <!-- *********************************************************************** -->
3099 <a name="coreclasses">The Core LLVM Class Hierarchy Reference </a>
3101 <!-- *********************************************************************** -->
3104 <p><tt>#include "<a href="/doxygen/Type_8h-source.html">llvm/Type.h</a>"</tt>
3105 <br>doxygen info: <a href="/doxygen/classllvm_1_1Type.html">Type Class</a></p>
3107 <p>The Core LLVM classes are the primary means of representing the program
3108 being inspected or transformed. The core LLVM classes are defined in
3109 header files in the <tt>include/llvm/</tt> directory, and implemented in
3110 the <tt>lib/VMCore</tt> directory.</p>
3112 <!-- ======================================================================= -->
3114 <a name="Type">The <tt>Type</tt> class and Derived Types</a>
3119 <p><tt>Type</tt> is a superclass of all type classes. Every <tt>Value</tt> has
3120 a <tt>Type</tt>. <tt>Type</tt> cannot be instantiated directly but only
3121 through its subclasses. Certain primitive types (<tt>VoidType</tt>,
3122 <tt>LabelType</tt>, <tt>FloatType</tt> and <tt>DoubleType</tt>) have hidden
3123 subclasses. They are hidden because they offer no useful functionality beyond
3124 what the <tt>Type</tt> class offers except to distinguish themselves from
3125 other subclasses of <tt>Type</tt>.</p>
3126 <p>All other types are subclasses of <tt>DerivedType</tt>. Types can be
3127 named, but this is not a requirement. There exists exactly
3128 one instance of a given shape at any one time. This allows type equality to
3129 be performed with address equality of the Type Instance. That is, given two
3130 <tt>Type*</tt> values, the types are identical if the pointers are identical.
3133 <!-- _______________________________________________________________________ -->
3135 <a name="m_Type">Important Public Methods</a>
3141 <li><tt>bool isIntegerTy() const</tt>: Returns true for any integer type.</li>
3143 <li><tt>bool isFloatingPointTy()</tt>: Return true if this is one of the five
3144 floating point types.</li>
3146 <li><tt>bool isSized()</tt>: Return true if the type has known size. Things
3147 that don't have a size are abstract types, labels and void.</li>
3152 <!-- _______________________________________________________________________ -->
3154 <a name="derivedtypes">Important Derived Types</a>
3158 <dt><tt>IntegerType</tt></dt>
3159 <dd>Subclass of DerivedType that represents integer types of any bit width.
3160 Any bit width between <tt>IntegerType::MIN_INT_BITS</tt> (1) and
3161 <tt>IntegerType::MAX_INT_BITS</tt> (~8 million) can be represented.
3163 <li><tt>static const IntegerType* get(unsigned NumBits)</tt>: get an integer
3164 type of a specific bit width.</li>
3165 <li><tt>unsigned getBitWidth() const</tt>: Get the bit width of an integer
3169 <dt><tt>SequentialType</tt></dt>
3170 <dd>This is subclassed by ArrayType, PointerType and VectorType.
3172 <li><tt>const Type * getElementType() const</tt>: Returns the type of each
3173 of the elements in the sequential type. </li>
3176 <dt><tt>ArrayType</tt></dt>
3177 <dd>This is a subclass of SequentialType and defines the interface for array
3180 <li><tt>unsigned getNumElements() const</tt>: Returns the number of
3181 elements in the array. </li>
3184 <dt><tt>PointerType</tt></dt>
3185 <dd>Subclass of SequentialType for pointer types.</dd>
3186 <dt><tt>VectorType</tt></dt>
3187 <dd>Subclass of SequentialType for vector types. A
3188 vector type is similar to an ArrayType but is distinguished because it is
3189 a first class type whereas ArrayType is not. Vector types are used for
3190 vector operations and are usually small vectors of of an integer or floating
3192 <dt><tt>StructType</tt></dt>
3193 <dd>Subclass of DerivedTypes for struct types.</dd>
3194 <dt><tt><a name="FunctionType">FunctionType</a></tt></dt>
3195 <dd>Subclass of DerivedTypes for function types.
3197 <li><tt>bool isVarArg() const</tt>: Returns true if it's a vararg
3199 <li><tt> const Type * getReturnType() const</tt>: Returns the
3200 return type of the function.</li>
3201 <li><tt>const Type * getParamType (unsigned i)</tt>: Returns
3202 the type of the ith parameter.</li>
3203 <li><tt> const unsigned getNumParams() const</tt>: Returns the
3204 number of formal parameters.</li>
3212 <!-- ======================================================================= -->
3214 <a name="Module">The <tt>Module</tt> class</a>
3220 href="/doxygen/Module_8h-source.html">llvm/Module.h</a>"</tt><br> doxygen info:
3221 <a href="/doxygen/classllvm_1_1Module.html">Module Class</a></p>
3223 <p>The <tt>Module</tt> class represents the top level structure present in LLVM
3224 programs. An LLVM module is effectively either a translation unit of the
3225 original program or a combination of several translation units merged by the
3226 linker. The <tt>Module</tt> class keeps track of a list of <a
3227 href="#Function"><tt>Function</tt></a>s, a list of <a
3228 href="#GlobalVariable"><tt>GlobalVariable</tt></a>s, and a <a
3229 href="#SymbolTable"><tt>SymbolTable</tt></a>. Additionally, it contains a few
3230 helpful member functions that try to make common operations easy.</p>
3232 <!-- _______________________________________________________________________ -->
3234 <a name="m_Module">Important Public Members of the <tt>Module</tt> class</a>
3240 <li><tt>Module::Module(std::string name = "")</tt></li>
3243 <p>Constructing a <a href="#Module">Module</a> is easy. You can optionally
3244 provide a name for it (probably based on the name of the translation unit).</p>
3247 <li><tt>Module::iterator</tt> - Typedef for function list iterator<br>
3248 <tt>Module::const_iterator</tt> - Typedef for const_iterator.<br>
3250 <tt>begin()</tt>, <tt>end()</tt>
3251 <tt>size()</tt>, <tt>empty()</tt>
3253 <p>These are forwarding methods that make it easy to access the contents of
3254 a <tt>Module</tt> object's <a href="#Function"><tt>Function</tt></a>
3257 <li><tt>Module::FunctionListType &getFunctionList()</tt>
3259 <p> Returns the list of <a href="#Function"><tt>Function</tt></a>s. This is
3260 necessary to use when you need to update the list or perform a complex
3261 action that doesn't have a forwarding method.</p>
3263 <p><!-- Global Variable --></p></li>
3269 <li><tt>Module::global_iterator</tt> - Typedef for global variable list iterator<br>
3271 <tt>Module::const_global_iterator</tt> - Typedef for const_iterator.<br>
3273 <tt>global_begin()</tt>, <tt>global_end()</tt>
3274 <tt>global_size()</tt>, <tt>global_empty()</tt>
3276 <p> These are forwarding methods that make it easy to access the contents of
3277 a <tt>Module</tt> object's <a
3278 href="#GlobalVariable"><tt>GlobalVariable</tt></a> list.</p></li>
3280 <li><tt>Module::GlobalListType &getGlobalList()</tt>
3282 <p>Returns the list of <a
3283 href="#GlobalVariable"><tt>GlobalVariable</tt></a>s. This is necessary to
3284 use when you need to update the list or perform a complex action that
3285 doesn't have a forwarding method.</p>
3287 <p><!-- Symbol table stuff --> </p></li>
3293 <li><tt><a href="#SymbolTable">SymbolTable</a> *getSymbolTable()</tt>
3295 <p>Return a reference to the <a href="#SymbolTable"><tt>SymbolTable</tt></a>
3296 for this <tt>Module</tt>.</p>
3298 <p><!-- Convenience methods --></p></li>
3304 <li><tt><a href="#Function">Function</a> *getFunction(const std::string
3305 &Name, const <a href="#FunctionType">FunctionType</a> *Ty)</tt>
3307 <p>Look up the specified function in the <tt>Module</tt> <a
3308 href="#SymbolTable"><tt>SymbolTable</tt></a>. If it does not exist, return
3309 <tt>null</tt>.</p></li>
3311 <li><tt><a href="#Function">Function</a> *getOrInsertFunction(const
3312 std::string &Name, const <a href="#FunctionType">FunctionType</a> *T)</tt>
3314 <p>Look up the specified function in the <tt>Module</tt> <a
3315 href="#SymbolTable"><tt>SymbolTable</tt></a>. If it does not exist, add an
3316 external declaration for the function and return it.</p></li>
3318 <li><tt>std::string getTypeName(const <a href="#Type">Type</a> *Ty)</tt>
3320 <p>If there is at least one entry in the <a
3321 href="#SymbolTable"><tt>SymbolTable</tt></a> for the specified <a
3322 href="#Type"><tt>Type</tt></a>, return it. Otherwise return the empty
3325 <li><tt>bool addTypeName(const std::string &Name, const <a
3326 href="#Type">Type</a> *Ty)</tt>
3328 <p>Insert an entry in the <a href="#SymbolTable"><tt>SymbolTable</tt></a>
3329 mapping <tt>Name</tt> to <tt>Ty</tt>. If there is already an entry for this
3330 name, true is returned and the <a
3331 href="#SymbolTable"><tt>SymbolTable</tt></a> is not modified.</p></li>
3338 <!-- ======================================================================= -->
3340 <a name="Value">The <tt>Value</tt> class</a>
3345 <p><tt>#include "<a href="/doxygen/Value_8h-source.html">llvm/Value.h</a>"</tt>
3347 doxygen info: <a href="/doxygen/classllvm_1_1Value.html">Value Class</a></p>
3349 <p>The <tt>Value</tt> class is the most important class in the LLVM Source
3350 base. It represents a typed value that may be used (among other things) as an
3351 operand to an instruction. There are many different types of <tt>Value</tt>s,
3352 such as <a href="#Constant"><tt>Constant</tt></a>s,<a
3353 href="#Argument"><tt>Argument</tt></a>s. Even <a
3354 href="#Instruction"><tt>Instruction</tt></a>s and <a
3355 href="#Function"><tt>Function</tt></a>s are <tt>Value</tt>s.</p>
3357 <p>A particular <tt>Value</tt> may be used many times in the LLVM representation
3358 for a program. For example, an incoming argument to a function (represented
3359 with an instance of the <a href="#Argument">Argument</a> class) is "used" by
3360 every instruction in the function that references the argument. To keep track
3361 of this relationship, the <tt>Value</tt> class keeps a list of all of the <a
3362 href="#User"><tt>User</tt></a>s that is using it (the <a
3363 href="#User"><tt>User</tt></a> class is a base class for all nodes in the LLVM
3364 graph that can refer to <tt>Value</tt>s). This use list is how LLVM represents
3365 def-use information in the program, and is accessible through the <tt>use_</tt>*
3366 methods, shown below.</p>
3368 <p>Because LLVM is a typed representation, every LLVM <tt>Value</tt> is typed,
3369 and this <a href="#Type">Type</a> is available through the <tt>getType()</tt>
3370 method. In addition, all LLVM values can be named. The "name" of the
3371 <tt>Value</tt> is a symbolic string printed in the LLVM code:</p>
3373 <div class="doc_code">
3375 %<b>foo</b> = add i32 1, 2
3379 <p><a name="nameWarning">The name of this instruction is "foo".</a> <b>NOTE</b>
3380 that the name of any value may be missing (an empty string), so names should
3381 <b>ONLY</b> be used for debugging (making the source code easier to read,
3382 debugging printouts), they should not be used to keep track of values or map
3383 between them. For this purpose, use a <tt>std::map</tt> of pointers to the
3384 <tt>Value</tt> itself instead.</p>
3386 <p>One important aspect of LLVM is that there is no distinction between an SSA
3387 variable and the operation that produces it. Because of this, any reference to
3388 the value produced by an instruction (or the value available as an incoming
3389 argument, for example) is represented as a direct pointer to the instance of
3391 represents this value. Although this may take some getting used to, it
3392 simplifies the representation and makes it easier to manipulate.</p>
3394 <!-- _______________________________________________________________________ -->
3396 <a name="m_Value">Important Public Members of the <tt>Value</tt> class</a>
3402 <li><tt>Value::use_iterator</tt> - Typedef for iterator over the
3404 <tt>Value::const_use_iterator</tt> - Typedef for const_iterator over
3406 <tt>unsigned use_size()</tt> - Returns the number of users of the
3408 <tt>bool use_empty()</tt> - Returns true if there are no users.<br>
3409 <tt>use_iterator use_begin()</tt> - Get an iterator to the start of
3411 <tt>use_iterator use_end()</tt> - Get an iterator to the end of the
3413 <tt><a href="#User">User</a> *use_back()</tt> - Returns the last
3414 element in the list.
3415 <p> These methods are the interface to access the def-use
3416 information in LLVM. As with all other iterators in LLVM, the naming
3417 conventions follow the conventions defined by the <a href="#stl">STL</a>.</p>
3419 <li><tt><a href="#Type">Type</a> *getType() const</tt>
3420 <p>This method returns the Type of the Value.</p>
3422 <li><tt>bool hasName() const</tt><br>
3423 <tt>std::string getName() const</tt><br>
3424 <tt>void setName(const std::string &Name)</tt>
3425 <p> This family of methods is used to access and assign a name to a <tt>Value</tt>,
3426 be aware of the <a href="#nameWarning">precaution above</a>.</p>
3428 <li><tt>void replaceAllUsesWith(Value *V)</tt>
3430 <p>This method traverses the use list of a <tt>Value</tt> changing all <a
3431 href="#User"><tt>User</tt>s</a> of the current value to refer to
3432 "<tt>V</tt>" instead. For example, if you detect that an instruction always
3433 produces a constant value (for example through constant folding), you can
3434 replace all uses of the instruction with the constant like this:</p>
3436 <div class="doc_code">
3438 Inst->replaceAllUsesWith(ConstVal);
3448 <!-- ======================================================================= -->
3450 <a name="User">The <tt>User</tt> class</a>
3456 <tt>#include "<a href="/doxygen/User_8h-source.html">llvm/User.h</a>"</tt><br>
3457 doxygen info: <a href="/doxygen/classllvm_1_1User.html">User Class</a><br>
3458 Superclass: <a href="#Value"><tt>Value</tt></a></p>
3460 <p>The <tt>User</tt> class is the common base class of all LLVM nodes that may
3461 refer to <a href="#Value"><tt>Value</tt></a>s. It exposes a list of "Operands"
3462 that are all of the <a href="#Value"><tt>Value</tt></a>s that the User is
3463 referring to. The <tt>User</tt> class itself is a subclass of
3466 <p>The operands of a <tt>User</tt> point directly to the LLVM <a
3467 href="#Value"><tt>Value</tt></a> that it refers to. Because LLVM uses Static
3468 Single Assignment (SSA) form, there can only be one definition referred to,
3469 allowing this direct connection. This connection provides the use-def
3470 information in LLVM.</p>
3472 <!-- _______________________________________________________________________ -->
3474 <a name="m_User">Important Public Members of the <tt>User</tt> class</a>
3479 <p>The <tt>User</tt> class exposes the operand list in two ways: through
3480 an index access interface and through an iterator based interface.</p>
3483 <li><tt>Value *getOperand(unsigned i)</tt><br>
3484 <tt>unsigned getNumOperands()</tt>
3485 <p> These two methods expose the operands of the <tt>User</tt> in a
3486 convenient form for direct access.</p></li>
3488 <li><tt>User::op_iterator</tt> - Typedef for iterator over the operand
3490 <tt>op_iterator op_begin()</tt> - Get an iterator to the start of
3491 the operand list.<br>
3492 <tt>op_iterator op_end()</tt> - Get an iterator to the end of the
3494 <p> Together, these methods make up the iterator based interface to
3495 the operands of a <tt>User</tt>.</p></li>
3502 <!-- ======================================================================= -->
3504 <a name="Instruction">The <tt>Instruction</tt> class</a>
3509 <p><tt>#include "</tt><tt><a
3510 href="/doxygen/Instruction_8h-source.html">llvm/Instruction.h</a>"</tt><br>
3511 doxygen info: <a href="/doxygen/classllvm_1_1Instruction.html">Instruction Class</a><br>
3512 Superclasses: <a href="#User"><tt>User</tt></a>, <a
3513 href="#Value"><tt>Value</tt></a></p>
3515 <p>The <tt>Instruction</tt> class is the common base class for all LLVM
3516 instructions. It provides only a few methods, but is a very commonly used
3517 class. The primary data tracked by the <tt>Instruction</tt> class itself is the
3518 opcode (instruction type) and the parent <a
3519 href="#BasicBlock"><tt>BasicBlock</tt></a> the <tt>Instruction</tt> is embedded
3520 into. To represent a specific type of instruction, one of many subclasses of
3521 <tt>Instruction</tt> are used.</p>
3523 <p> Because the <tt>Instruction</tt> class subclasses the <a
3524 href="#User"><tt>User</tt></a> class, its operands can be accessed in the same
3525 way as for other <a href="#User"><tt>User</tt></a>s (with the
3526 <tt>getOperand()</tt>/<tt>getNumOperands()</tt> and
3527 <tt>op_begin()</tt>/<tt>op_end()</tt> methods).</p> <p> An important file for
3528 the <tt>Instruction</tt> class is the <tt>llvm/Instruction.def</tt> file. This
3529 file contains some meta-data about the various different types of instructions
3530 in LLVM. It describes the enum values that are used as opcodes (for example
3531 <tt>Instruction::Add</tt> and <tt>Instruction::ICmp</tt>), as well as the
3532 concrete sub-classes of <tt>Instruction</tt> that implement the instruction (for
3533 example <tt><a href="#BinaryOperator">BinaryOperator</a></tt> and <tt><a
3534 href="#CmpInst">CmpInst</a></tt>). Unfortunately, the use of macros in
3535 this file confuses doxygen, so these enum values don't show up correctly in the
3536 <a href="/doxygen/classllvm_1_1Instruction.html">doxygen output</a>.</p>
3538 <!-- _______________________________________________________________________ -->
3540 <a name="s_Instruction">
3541 Important Subclasses of the <tt>Instruction</tt> class
3546 <li><tt><a name="BinaryOperator">BinaryOperator</a></tt>
3547 <p>This subclasses represents all two operand instructions whose operands
3548 must be the same type, except for the comparison instructions.</p></li>
3549 <li><tt><a name="CastInst">CastInst</a></tt>
3550 <p>This subclass is the parent of the 12 casting instructions. It provides
3551 common operations on cast instructions.</p>
3552 <li><tt><a name="CmpInst">CmpInst</a></tt>
3553 <p>This subclass respresents the two comparison instructions,
3554 <a href="LangRef.html#i_icmp">ICmpInst</a> (integer opreands), and
3555 <a href="LangRef.html#i_fcmp">FCmpInst</a> (floating point operands).</p>
3556 <li><tt><a name="TerminatorInst">TerminatorInst</a></tt>
3557 <p>This subclass is the parent of all terminator instructions (those which
3558 can terminate a block).</p>
3562 <!-- _______________________________________________________________________ -->
3564 <a name="m_Instruction">
3565 Important Public Members of the <tt>Instruction</tt> class
3572 <li><tt><a href="#BasicBlock">BasicBlock</a> *getParent()</tt>
3573 <p>Returns the <a href="#BasicBlock"><tt>BasicBlock</tt></a> that
3574 this <tt>Instruction</tt> is embedded into.</p></li>
3575 <li><tt>bool mayWriteToMemory()</tt>
3576 <p>Returns true if the instruction writes to memory, i.e. it is a
3577 <tt>call</tt>,<tt>free</tt>,<tt>invoke</tt>, or <tt>store</tt>.</p></li>
3578 <li><tt>unsigned getOpcode()</tt>
3579 <p>Returns the opcode for the <tt>Instruction</tt>.</p></li>
3580 <li><tt><a href="#Instruction">Instruction</a> *clone() const</tt>
3581 <p>Returns another instance of the specified instruction, identical
3582 in all ways to the original except that the instruction has no parent
3583 (ie it's not embedded into a <a href="#BasicBlock"><tt>BasicBlock</tt></a>),
3584 and it has no name</p></li>
3591 <!-- ======================================================================= -->
3593 <a name="Constant">The <tt>Constant</tt> class and subclasses</a>
3598 <p>Constant represents a base class for different types of constants. It
3599 is subclassed by ConstantInt, ConstantArray, etc. for representing
3600 the various types of Constants. <a href="#GlobalValue">GlobalValue</a> is also
3601 a subclass, which represents the address of a global variable or function.
3604 <!-- _______________________________________________________________________ -->
3605 <h4>Important Subclasses of Constant</h4>
3608 <li>ConstantInt : This subclass of Constant represents an integer constant of
3611 <li><tt>const APInt& getValue() const</tt>: Returns the underlying
3612 value of this constant, an APInt value.</li>
3613 <li><tt>int64_t getSExtValue() const</tt>: Converts the underlying APInt
3614 value to an int64_t via sign extension. If the value (not the bit width)
3615 of the APInt is too large to fit in an int64_t, an assertion will result.
3616 For this reason, use of this method is discouraged.</li>
3617 <li><tt>uint64_t getZExtValue() const</tt>: Converts the underlying APInt
3618 value to a uint64_t via zero extension. IF the value (not the bit width)
3619 of the APInt is too large to fit in a uint64_t, an assertion will result.
3620 For this reason, use of this method is discouraged.</li>
3621 <li><tt>static ConstantInt* get(const APInt& Val)</tt>: Returns the
3622 ConstantInt object that represents the value provided by <tt>Val</tt>.
3623 The type is implied as the IntegerType that corresponds to the bit width
3624 of <tt>Val</tt>.</li>
3625 <li><tt>static ConstantInt* get(const Type *Ty, uint64_t Val)</tt>:
3626 Returns the ConstantInt object that represents the value provided by
3627 <tt>Val</tt> for integer type <tt>Ty</tt>.</li>
3630 <li>ConstantFP : This class represents a floating point constant.
3632 <li><tt>double getValue() const</tt>: Returns the underlying value of
3633 this constant. </li>
3636 <li>ConstantArray : This represents a constant array.
3638 <li><tt>const std::vector<Use> &getValues() const</tt>: Returns
3639 a vector of component constants that makeup this array. </li>
3642 <li>ConstantStruct : This represents a constant struct.
3644 <li><tt>const std::vector<Use> &getValues() const</tt>: Returns
3645 a vector of component constants that makeup this array. </li>
3648 <li>GlobalValue : This represents either a global variable or a function. In
3649 either case, the value is a constant fixed address (after linking).
3656 <!-- ======================================================================= -->
3658 <a name="GlobalValue">The <tt>GlobalValue</tt> class</a>
3664 href="/doxygen/GlobalValue_8h-source.html">llvm/GlobalValue.h</a>"</tt><br>
3665 doxygen info: <a href="/doxygen/classllvm_1_1GlobalValue.html">GlobalValue
3667 Superclasses: <a href="#Constant"><tt>Constant</tt></a>,
3668 <a href="#User"><tt>User</tt></a>, <a href="#Value"><tt>Value</tt></a></p>
3670 <p>Global values (<a href="#GlobalVariable"><tt>GlobalVariable</tt></a>s or <a
3671 href="#Function"><tt>Function</tt></a>s) are the only LLVM values that are
3672 visible in the bodies of all <a href="#Function"><tt>Function</tt></a>s.
3673 Because they are visible at global scope, they are also subject to linking with
3674 other globals defined in different translation units. To control the linking
3675 process, <tt>GlobalValue</tt>s know their linkage rules. Specifically,
3676 <tt>GlobalValue</tt>s know whether they have internal or external linkage, as
3677 defined by the <tt>LinkageTypes</tt> enumeration.</p>
3679 <p>If a <tt>GlobalValue</tt> has internal linkage (equivalent to being
3680 <tt>static</tt> in C), it is not visible to code outside the current translation
3681 unit, and does not participate in linking. If it has external linkage, it is
3682 visible to external code, and does participate in linking. In addition to
3683 linkage information, <tt>GlobalValue</tt>s keep track of which <a
3684 href="#Module"><tt>Module</tt></a> they are currently part of.</p>
3686 <p>Because <tt>GlobalValue</tt>s are memory objects, they are always referred to
3687 by their <b>address</b>. As such, the <a href="#Type"><tt>Type</tt></a> of a
3688 global is always a pointer to its contents. It is important to remember this
3689 when using the <tt>GetElementPtrInst</tt> instruction because this pointer must
3690 be dereferenced first. For example, if you have a <tt>GlobalVariable</tt> (a
3691 subclass of <tt>GlobalValue)</tt> that is an array of 24 ints, type <tt>[24 x
3692 i32]</tt>, then the <tt>GlobalVariable</tt> is a pointer to that array. Although
3693 the address of the first element of this array and the value of the
3694 <tt>GlobalVariable</tt> are the same, they have different types. The
3695 <tt>GlobalVariable</tt>'s type is <tt>[24 x i32]</tt>. The first element's type
3696 is <tt>i32.</tt> Because of this, accessing a global value requires you to
3697 dereference the pointer with <tt>GetElementPtrInst</tt> first, then its elements
3698 can be accessed. This is explained in the <a href="LangRef.html#globalvars">LLVM
3699 Language Reference Manual</a>.</p>
3701 <!-- _______________________________________________________________________ -->
3703 <a name="m_GlobalValue">
3704 Important Public Members of the <tt>GlobalValue</tt> class
3711 <li><tt>bool hasInternalLinkage() const</tt><br>
3712 <tt>bool hasExternalLinkage() const</tt><br>
3713 <tt>void setInternalLinkage(bool HasInternalLinkage)</tt>
3714 <p> These methods manipulate the linkage characteristics of the <tt>GlobalValue</tt>.</p>
3717 <li><tt><a href="#Module">Module</a> *getParent()</tt>
3718 <p> This returns the <a href="#Module"><tt>Module</tt></a> that the
3719 GlobalValue is currently embedded into.</p></li>
3726 <!-- ======================================================================= -->
3728 <a name="Function">The <tt>Function</tt> class</a>
3734 href="/doxygen/Function_8h-source.html">llvm/Function.h</a>"</tt><br> doxygen
3735 info: <a href="/doxygen/classllvm_1_1Function.html">Function Class</a><br>
3736 Superclasses: <a href="#GlobalValue"><tt>GlobalValue</tt></a>,
3737 <a href="#Constant"><tt>Constant</tt></a>,
3738 <a href="#User"><tt>User</tt></a>,
3739 <a href="#Value"><tt>Value</tt></a></p>
3741 <p>The <tt>Function</tt> class represents a single procedure in LLVM. It is
3742 actually one of the more complex classes in the LLVM hierarchy because it must
3743 keep track of a large amount of data. The <tt>Function</tt> class keeps track
3744 of a list of <a href="#BasicBlock"><tt>BasicBlock</tt></a>s, a list of formal
3745 <a href="#Argument"><tt>Argument</tt></a>s, and a
3746 <a href="#SymbolTable"><tt>SymbolTable</tt></a>.</p>
3748 <p>The list of <a href="#BasicBlock"><tt>BasicBlock</tt></a>s is the most
3749 commonly used part of <tt>Function</tt> objects. The list imposes an implicit
3750 ordering of the blocks in the function, which indicate how the code will be
3751 laid out by the backend. Additionally, the first <a
3752 href="#BasicBlock"><tt>BasicBlock</tt></a> is the implicit entry node for the
3753 <tt>Function</tt>. It is not legal in LLVM to explicitly branch to this initial
3754 block. There are no implicit exit nodes, and in fact there may be multiple exit
3755 nodes from a single <tt>Function</tt>. If the <a
3756 href="#BasicBlock"><tt>BasicBlock</tt></a> list is empty, this indicates that
3757 the <tt>Function</tt> is actually a function declaration: the actual body of the
3758 function hasn't been linked in yet.</p>
3760 <p>In addition to a list of <a href="#BasicBlock"><tt>BasicBlock</tt></a>s, the
3761 <tt>Function</tt> class also keeps track of the list of formal <a
3762 href="#Argument"><tt>Argument</tt></a>s that the function receives. This
3763 container manages the lifetime of the <a href="#Argument"><tt>Argument</tt></a>
3764 nodes, just like the <a href="#BasicBlock"><tt>BasicBlock</tt></a> list does for
3765 the <a href="#BasicBlock"><tt>BasicBlock</tt></a>s.</p>
3767 <p>The <a href="#SymbolTable"><tt>SymbolTable</tt></a> is a very rarely used
3768 LLVM feature that is only used when you have to look up a value by name. Aside
3769 from that, the <a href="#SymbolTable"><tt>SymbolTable</tt></a> is used
3770 internally to make sure that there are not conflicts between the names of <a
3771 href="#Instruction"><tt>Instruction</tt></a>s, <a
3772 href="#BasicBlock"><tt>BasicBlock</tt></a>s, or <a
3773 href="#Argument"><tt>Argument</tt></a>s in the function body.</p>
3775 <p>Note that <tt>Function</tt> is a <a href="#GlobalValue">GlobalValue</a>
3776 and therefore also a <a href="#Constant">Constant</a>. The value of the function
3777 is its address (after linking) which is guaranteed to be constant.</p>
3779 <!-- _______________________________________________________________________ -->
3781 <a name="m_Function">
3782 Important Public Members of the <tt>Function</tt> class
3789 <li><tt>Function(const </tt><tt><a href="#FunctionType">FunctionType</a>
3790 *Ty, LinkageTypes Linkage, const std::string &N = "", Module* Parent = 0)</tt>
3792 <p>Constructor used when you need to create new <tt>Function</tt>s to add
3793 the the program. The constructor must specify the type of the function to
3794 create and what type of linkage the function should have. The <a
3795 href="#FunctionType"><tt>FunctionType</tt></a> argument
3796 specifies the formal arguments and return value for the function. The same
3797 <a href="#FunctionType"><tt>FunctionType</tt></a> value can be used to
3798 create multiple functions. The <tt>Parent</tt> argument specifies the Module
3799 in which the function is defined. If this argument is provided, the function
3800 will automatically be inserted into that module's list of
3803 <li><tt>bool isDeclaration()</tt>
3805 <p>Return whether or not the <tt>Function</tt> has a body defined. If the
3806 function is "external", it does not have a body, and thus must be resolved
3807 by linking with a function defined in a different translation unit.</p></li>
3809 <li><tt>Function::iterator</tt> - Typedef for basic block list iterator<br>
3810 <tt>Function::const_iterator</tt> - Typedef for const_iterator.<br>
3812 <tt>begin()</tt>, <tt>end()</tt>
3813 <tt>size()</tt>, <tt>empty()</tt>
3815 <p>These are forwarding methods that make it easy to access the contents of
3816 a <tt>Function</tt> object's <a href="#BasicBlock"><tt>BasicBlock</tt></a>
3819 <li><tt>Function::BasicBlockListType &getBasicBlockList()</tt>
3821 <p>Returns the list of <a href="#BasicBlock"><tt>BasicBlock</tt></a>s. This
3822 is necessary to use when you need to update the list or perform a complex
3823 action that doesn't have a forwarding method.</p></li>
3825 <li><tt>Function::arg_iterator</tt> - Typedef for the argument list
3827 <tt>Function::const_arg_iterator</tt> - Typedef for const_iterator.<br>
3829 <tt>arg_begin()</tt>, <tt>arg_end()</tt>
3830 <tt>arg_size()</tt>, <tt>arg_empty()</tt>
3832 <p>These are forwarding methods that make it easy to access the contents of
3833 a <tt>Function</tt> object's <a href="#Argument"><tt>Argument</tt></a>
3836 <li><tt>Function::ArgumentListType &getArgumentList()</tt>
3838 <p>Returns the list of <a href="#Argument"><tt>Argument</tt></a>s. This is
3839 necessary to use when you need to update the list or perform a complex
3840 action that doesn't have a forwarding method.</p></li>
3842 <li><tt><a href="#BasicBlock">BasicBlock</a> &getEntryBlock()</tt>
3844 <p>Returns the entry <a href="#BasicBlock"><tt>BasicBlock</tt></a> for the
3845 function. Because the entry block for the function is always the first
3846 block, this returns the first block of the <tt>Function</tt>.</p></li>
3848 <li><tt><a href="#Type">Type</a> *getReturnType()</tt><br>
3849 <tt><a href="#FunctionType">FunctionType</a> *getFunctionType()</tt>
3851 <p>This traverses the <a href="#Type"><tt>Type</tt></a> of the
3852 <tt>Function</tt> and returns the return type of the function, or the <a
3853 href="#FunctionType"><tt>FunctionType</tt></a> of the actual
3856 <li><tt><a href="#SymbolTable">SymbolTable</a> *getSymbolTable()</tt>
3858 <p> Return a pointer to the <a href="#SymbolTable"><tt>SymbolTable</tt></a>
3859 for this <tt>Function</tt>.</p></li>
3866 <!-- ======================================================================= -->
3868 <a name="GlobalVariable">The <tt>GlobalVariable</tt> class</a>
3874 href="/doxygen/GlobalVariable_8h-source.html">llvm/GlobalVariable.h</a>"</tt>
3876 doxygen info: <a href="/doxygen/classllvm_1_1GlobalVariable.html">GlobalVariable
3878 Superclasses: <a href="#GlobalValue"><tt>GlobalValue</tt></a>,
3879 <a href="#Constant"><tt>Constant</tt></a>,
3880 <a href="#User"><tt>User</tt></a>,
3881 <a href="#Value"><tt>Value</tt></a></p>
3883 <p>Global variables are represented with the (surprise surprise)
3884 <tt>GlobalVariable</tt> class. Like functions, <tt>GlobalVariable</tt>s are also
3885 subclasses of <a href="#GlobalValue"><tt>GlobalValue</tt></a>, and as such are
3886 always referenced by their address (global values must live in memory, so their
3887 "name" refers to their constant address). See
3888 <a href="#GlobalValue"><tt>GlobalValue</tt></a> for more on this. Global
3889 variables may have an initial value (which must be a
3890 <a href="#Constant"><tt>Constant</tt></a>), and if they have an initializer,
3891 they may be marked as "constant" themselves (indicating that their contents
3892 never change at runtime).</p>
3894 <!-- _______________________________________________________________________ -->
3896 <a name="m_GlobalVariable">
3897 Important Public Members of the <tt>GlobalVariable</tt> class
3904 <li><tt>GlobalVariable(const </tt><tt><a href="#Type">Type</a> *Ty, bool
3905 isConstant, LinkageTypes& Linkage, <a href="#Constant">Constant</a>
3906 *Initializer = 0, const std::string &Name = "", Module* Parent = 0)</tt>
3908 <p>Create a new global variable of the specified type. If
3909 <tt>isConstant</tt> is true then the global variable will be marked as
3910 unchanging for the program. The Linkage parameter specifies the type of
3911 linkage (internal, external, weak, linkonce, appending) for the variable.
3912 If the linkage is InternalLinkage, WeakAnyLinkage, WeakODRLinkage,
3913 LinkOnceAnyLinkage or LinkOnceODRLinkage, then the resultant
3914 global variable will have internal linkage. AppendingLinkage concatenates
3915 together all instances (in different translation units) of the variable
3916 into a single variable but is only applicable to arrays. See
3917 the <a href="LangRef.html#modulestructure">LLVM Language Reference</a> for
3918 further details on linkage types. Optionally an initializer, a name, and the
3919 module to put the variable into may be specified for the global variable as
3922 <li><tt>bool isConstant() const</tt>
3924 <p>Returns true if this is a global variable that is known not to
3925 be modified at runtime.</p></li>
3927 <li><tt>bool hasInitializer()</tt>
3929 <p>Returns true if this <tt>GlobalVariable</tt> has an intializer.</p></li>
3931 <li><tt><a href="#Constant">Constant</a> *getInitializer()</tt>
3933 <p>Returns the initial value for a <tt>GlobalVariable</tt>. It is not legal
3934 to call this method if there is no initializer.</p></li>
3941 <!-- ======================================================================= -->
3943 <a name="BasicBlock">The <tt>BasicBlock</tt> class</a>
3949 href="/doxygen/BasicBlock_8h-source.html">llvm/BasicBlock.h</a>"</tt><br>
3950 doxygen info: <a href="/doxygen/classllvm_1_1BasicBlock.html">BasicBlock
3952 Superclass: <a href="#Value"><tt>Value</tt></a></p>
3954 <p>This class represents a single entry single exit section of the code,
3955 commonly known as a basic block by the compiler community. The
3956 <tt>BasicBlock</tt> class maintains a list of <a
3957 href="#Instruction"><tt>Instruction</tt></a>s, which form the body of the block.
3958 Matching the language definition, the last element of this list of instructions
3959 is always a terminator instruction (a subclass of the <a
3960 href="#TerminatorInst"><tt>TerminatorInst</tt></a> class).</p>
3962 <p>In addition to tracking the list of instructions that make up the block, the
3963 <tt>BasicBlock</tt> class also keeps track of the <a
3964 href="#Function"><tt>Function</tt></a> that it is embedded into.</p>
3966 <p>Note that <tt>BasicBlock</tt>s themselves are <a
3967 href="#Value"><tt>Value</tt></a>s, because they are referenced by instructions
3968 like branches and can go in the switch tables. <tt>BasicBlock</tt>s have type
3971 <!-- _______________________________________________________________________ -->
3973 <a name="m_BasicBlock">
3974 Important Public Members of the <tt>BasicBlock</tt> class
3981 <li><tt>BasicBlock(const std::string &Name = "", </tt><tt><a
3982 href="#Function">Function</a> *Parent = 0)</tt>
3984 <p>The <tt>BasicBlock</tt> constructor is used to create new basic blocks for
3985 insertion into a function. The constructor optionally takes a name for the new
3986 block, and a <a href="#Function"><tt>Function</tt></a> to insert it into. If
3987 the <tt>Parent</tt> parameter is specified, the new <tt>BasicBlock</tt> is
3988 automatically inserted at the end of the specified <a
3989 href="#Function"><tt>Function</tt></a>, if not specified, the BasicBlock must be
3990 manually inserted into the <a href="#Function"><tt>Function</tt></a>.</p></li>
3992 <li><tt>BasicBlock::iterator</tt> - Typedef for instruction list iterator<br>
3993 <tt>BasicBlock::const_iterator</tt> - Typedef for const_iterator.<br>
3994 <tt>begin()</tt>, <tt>end()</tt>, <tt>front()</tt>, <tt>back()</tt>,
3995 <tt>size()</tt>, <tt>empty()</tt>
3996 STL-style functions for accessing the instruction list.
3998 <p>These methods and typedefs are forwarding functions that have the same
3999 semantics as the standard library methods of the same names. These methods
4000 expose the underlying instruction list of a basic block in a way that is easy to
4001 manipulate. To get the full complement of container operations (including
4002 operations to update the list), you must use the <tt>getInstList()</tt>
4005 <li><tt>BasicBlock::InstListType &getInstList()</tt>
4007 <p>This method is used to get access to the underlying container that actually
4008 holds the Instructions. This method must be used when there isn't a forwarding
4009 function in the <tt>BasicBlock</tt> class for the operation that you would like
4010 to perform. Because there are no forwarding functions for "updating"
4011 operations, you need to use this if you want to update the contents of a
4012 <tt>BasicBlock</tt>.</p></li>
4014 <li><tt><a href="#Function">Function</a> *getParent()</tt>
4016 <p> Returns a pointer to <a href="#Function"><tt>Function</tt></a> the block is
4017 embedded into, or a null pointer if it is homeless.</p></li>
4019 <li><tt><a href="#TerminatorInst">TerminatorInst</a> *getTerminator()</tt>
4021 <p> Returns a pointer to the terminator instruction that appears at the end of
4022 the <tt>BasicBlock</tt>. If there is no terminator instruction, or if the last
4023 instruction in the block is not a terminator, then a null pointer is
4032 <!-- ======================================================================= -->
4034 <a name="Argument">The <tt>Argument</tt> class</a>
4039 <p>This subclass of Value defines the interface for incoming formal
4040 arguments to a function. A Function maintains a list of its formal
4041 arguments. An argument has a pointer to the parent Function.</p>
4047 <!-- *********************************************************************** -->
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4055 <a href="mailto:dhurjati@cs.uiuc.edu">Dinakar Dhurjati</a> and
4056 <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
4057 <a href="http://llvm.org/">The LLVM Compiler Infrastructure</a><br>
4058 Last modified: $Date$