1 ========================
2 LLVM Programmer's Manual
3 ========================
9 This is always a work in progress.
16 This document is meant to highlight some of the important classes and interfaces
17 available in the LLVM source-base. This manual is not intended to explain what
18 LLVM is, how it works, and what LLVM code looks like. It assumes that you know
19 the basics of LLVM and are interested in writing transformations or otherwise
20 analyzing or manipulating the code.
22 This document should get you oriented so that you can find your way in the
23 continuously growing source code that makes up the LLVM infrastructure. Note
24 that this manual is not intended to serve as a replacement for reading the
25 source code, so if you think there should be a method in one of these classes to
26 do something, but it's not listed, check the source. Links to the `doxygen
27 <http://llvm.org/doxygen/>`__ sources are provided to make this as easy as
30 The first section of this document describes general information that is useful
31 to know when working in the LLVM infrastructure, and the second describes the
32 Core LLVM classes. In the future this manual will be extended with information
33 describing how to use extension libraries, such as dominator information, CFG
34 traversal routines, and useful utilities like the ``InstVisitor`` (`doxygen
35 <http://llvm.org/doxygen/InstVisitor_8h-source.html>`__) template.
42 This section contains general information that is useful if you are working in
43 the LLVM source-base, but that isn't specific to any particular API.
47 The C++ Standard Template Library
48 ---------------------------------
50 LLVM makes heavy use of the C++ Standard Template Library (STL), perhaps much
51 more than you are used to, or have seen before. Because of this, you might want
52 to do a little background reading in the techniques used and capabilities of the
53 library. There are many good pages that discuss the STL, and several books on
54 the subject that you can get, so it will not be discussed in this document.
56 Here are some useful links:
59 <http://en.cppreference.com/w/>`_ - an excellent
60 reference for the STL and other parts of the standard C++ library.
62 #. `C++ In a Nutshell <http://www.tempest-sw.com/cpp/>`_ - This is an O'Reilly
63 book in the making. It has a decent Standard Library Reference that rivals
64 Dinkumware's, and is unfortunately no longer free since the book has been
67 #. `C++ Frequently Asked Questions <http://www.parashift.com/c++-faq-lite/>`_.
69 #. `SGI's STL Programmer's Guide <http://www.sgi.com/tech/stl/>`_ - Contains a
70 useful `Introduction to the STL
71 <http://www.sgi.com/tech/stl/stl_introduction.html>`_.
73 #. `Bjarne Stroustrup's C++ Page
74 <http://www.research.att.com/%7Ebs/C++.html>`_.
76 #. `Bruce Eckel's Thinking in C++, 2nd ed. Volume 2 Revision 4.0
77 (even better, get the book)
78 <http://www.mindview.net/Books/TICPP/ThinkingInCPP2e.html>`_.
80 You are also encouraged to take a look at the :doc:`LLVM Coding Standards
81 <CodingStandards>` guide which focuses on how to write maintainable code more
82 than where to put your curly braces.
86 Other useful references
87 -----------------------
89 #. `Using static and shared libraries across platforms
90 <http://www.fortran-2000.com/ArnaudRecipes/sharedlib.html>`_
94 Important and useful LLVM APIs
95 ==============================
97 Here we highlight some LLVM APIs that are generally useful and good to know
98 about when writing transformations.
102 The ``isa<>``, ``cast<>`` and ``dyn_cast<>`` templates
103 ------------------------------------------------------
105 The LLVM source-base makes extensive use of a custom form of RTTI. These
106 templates have many similarities to the C++ ``dynamic_cast<>`` operator, but
107 they don't have some drawbacks (primarily stemming from the fact that
108 ``dynamic_cast<>`` only works on classes that have a v-table). Because they are
109 used so often, you must know what they do and how they work. All of these
110 templates are defined in the ``llvm/Support/Casting.h`` (`doxygen
111 <http://llvm.org/doxygen/Casting_8h-source.html>`__) file (note that you very
112 rarely have to include this file directly).
115 The ``isa<>`` operator works exactly like the Java "``instanceof``" operator.
116 It returns true or false depending on whether a reference or pointer points to
117 an instance of the specified class. This can be very useful for constraint
118 checking of various sorts (example below).
121 The ``cast<>`` operator is a "checked cast" operation. It converts a pointer
122 or reference from a base class to a derived class, causing an assertion
123 failure if it is not really an instance of the right type. This should be
124 used in cases where you have some information that makes you believe that
125 something is of the right type. An example of the ``isa<>`` and ``cast<>``
130 static bool isLoopInvariant(const Value *V, const Loop *L) {
131 if (isa<Constant>(V) || isa<Argument>(V) || isa<GlobalValue>(V))
134 // Otherwise, it must be an instruction...
135 return !L->contains(cast<Instruction>(V)->getParent());
138 Note that you should **not** use an ``isa<>`` test followed by a ``cast<>``,
139 for that use the ``dyn_cast<>`` operator.
142 The ``dyn_cast<>`` operator is a "checking cast" operation. It checks to see
143 if the operand is of the specified type, and if so, returns a pointer to it
144 (this operator does not work with references). If the operand is not of the
145 correct type, a null pointer is returned. Thus, this works very much like
146 the ``dynamic_cast<>`` operator in C++, and should be used in the same
147 circumstances. Typically, the ``dyn_cast<>`` operator is used in an ``if``
148 statement or some other flow control statement like this:
152 if (AllocationInst *AI = dyn_cast<AllocationInst>(Val)) {
156 This form of the ``if`` statement effectively combines together a call to
157 ``isa<>`` and a call to ``cast<>`` into one statement, which is very
160 Note that the ``dyn_cast<>`` operator, like C++'s ``dynamic_cast<>`` or Java's
161 ``instanceof`` operator, can be abused. In particular, you should not use big
162 chained ``if/then/else`` blocks to check for lots of different variants of
163 classes. If you find yourself wanting to do this, it is much cleaner and more
164 efficient to use the ``InstVisitor`` class to dispatch over the instruction
168 The ``cast_or_null<>`` operator works just like the ``cast<>`` operator,
169 except that it allows for a null pointer as an argument (which it then
170 propagates). This can sometimes be useful, allowing you to combine several
171 null checks into one.
173 ``dyn_cast_or_null<>``:
174 The ``dyn_cast_or_null<>`` operator works just like the ``dyn_cast<>``
175 operator, except that it allows for a null pointer as an argument (which it
176 then propagates). This can sometimes be useful, allowing you to combine
177 several null checks into one.
179 These five templates can be used with any classes, whether they have a v-table
180 or not. If you want to add support for these templates, see the document
181 :doc:`How to set up LLVM-style RTTI for your class hierarchy
182 <HowToSetUpLLVMStyleRTTI>`
186 Passing strings (the ``StringRef`` and ``Twine`` classes)
187 ---------------------------------------------------------
189 Although LLVM generally does not do much string manipulation, we do have several
190 important APIs which take strings. Two important examples are the Value class
191 -- which has names for instructions, functions, etc. -- and the ``StringMap``
192 class which is used extensively in LLVM and Clang.
194 These are generic classes, and they need to be able to accept strings which may
195 have embedded null characters. Therefore, they cannot simply take a ``const
196 char *``, and taking a ``const std::string&`` requires clients to perform a heap
197 allocation which is usually unnecessary. Instead, many LLVM APIs use a
198 ``StringRef`` or a ``const Twine&`` for passing strings efficiently.
202 The ``StringRef`` class
203 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
205 The ``StringRef`` data type represents a reference to a constant string (a
206 character array and a length) and supports the common operations available on
207 ``std::string``, but does not require heap allocation.
209 It can be implicitly constructed using a C style null-terminated string, an
210 ``std::string``, or explicitly with a character pointer and length. For
211 example, the ``StringRef`` find function is declared as:
215 iterator find(StringRef Key);
217 and clients can call it using any one of:
221 Map.find("foo"); // Lookup "foo"
222 Map.find(std::string("bar")); // Lookup "bar"
223 Map.find(StringRef("\0baz", 4)); // Lookup "\0baz"
225 Similarly, APIs which need to return a string may return a ``StringRef``
226 instance, which can be used directly or converted to an ``std::string`` using
227 the ``str`` member function. See ``llvm/ADT/StringRef.h`` (`doxygen
228 <http://llvm.org/doxygen/classllvm_1_1StringRef_8h-source.html>`__) for more
231 You should rarely use the ``StringRef`` class directly, because it contains
232 pointers to external memory it is not generally safe to store an instance of the
233 class (unless you know that the external storage will not be freed).
234 ``StringRef`` is small and pervasive enough in LLVM that it should always be
240 The ``Twine`` (`doxygen <http://llvm.org/doxygen/classllvm_1_1Twine.html>`__)
241 class is an efficient way for APIs to accept concatenated strings. For example,
242 a common LLVM paradigm is to name one instruction based on the name of another
243 instruction with a suffix, for example:
247 New = CmpInst::Create(..., SO->getName() + ".cmp");
249 The ``Twine`` class is effectively a lightweight `rope
250 <http://en.wikipedia.org/wiki/Rope_(computer_science)>`_ which points to
251 temporary (stack allocated) objects. Twines can be implicitly constructed as
252 the result of the plus operator applied to strings (i.e., a C strings, an
253 ``std::string``, or a ``StringRef``). The twine delays the actual concatenation
254 of strings until it is actually required, at which point it can be efficiently
255 rendered directly into a character array. This avoids unnecessary heap
256 allocation involved in constructing the temporary results of string
257 concatenation. See ``llvm/ADT/Twine.h`` (`doxygen
258 <http://llvm.org/doxygen/Twine_8h_source.html>`__) and :ref:`here <dss_twine>`
259 for more information.
261 As with a ``StringRef``, ``Twine`` objects point to external memory and should
262 almost never be stored or mentioned directly. They are intended solely for use
263 when defining a function which should be able to efficiently accept concatenated
268 Passing functions and other callable objects
269 --------------------------------------------
271 Sometimes you may want a function to be passed a callback object. In order to
272 support lambda expressions and other function objects, you should not use the
273 traditional C approach of taking a function pointer and an opaque cookie:
277 void takeCallback(bool (*Callback)(Function *, void *), void *Cookie);
279 Instead, use one of the following approaches:
284 If you don't mind putting the definition of your function into a header file,
285 make it a function template that is templated on the callable type.
289 template<typename Callable>
290 void takeCallback(Callable Callback) {
294 The ``function_ref`` class template
295 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
298 (`doxygen <http://llvm.org/doxygen/classllvm_1_1function_ref.html>`__) class
299 template represents a reference to a callable object, templated over the type
300 of the callable. This is a good choice for passing a callback to a function,
301 if you don't need to hold onto the callback after the function returns. In this
302 way, ``function_ref`` is to ``std::function`` as ``StringRef`` is to
305 ``function_ref<Ret(Param1, Param2, ...)>`` can be implicitly constructed from
306 any callable object that can be called with arguments of type ``Param1``,
307 ``Param2``, ..., and returns a value that can be converted to type ``Ret``.
312 void visitBasicBlocks(Function *F, function_ref<bool (BasicBlock*)> Callback) {
313 for (BasicBlock &BB : *F)
322 visitBasicBlocks(F, [&](BasicBlock *BB) {
328 Note that a ``function_ref`` object contains pointers to external memory, so it
329 is not generally safe to store an instance of the class (unless you know that
330 the external storage will not be freed). If you need this ability, consider
331 using ``std::function``. ``function_ref`` is small enough that it should always
336 The ``DEBUG()`` macro and ``-debug`` option
337 -------------------------------------------
339 Often when working on your pass you will put a bunch of debugging printouts and
340 other code into your pass. After you get it working, you want to remove it, but
341 you may need it again in the future (to work out new bugs that you run across).
343 Naturally, because of this, you don't want to delete the debug printouts, but
344 you don't want them to always be noisy. A standard compromise is to comment
345 them out, allowing you to enable them if you need them in the future.
347 The ``llvm/Support/Debug.h`` (`doxygen
348 <http://llvm.org/doxygen/Debug_8h-source.html>`__) file provides a macro named
349 ``DEBUG()`` that is a much nicer solution to this problem. Basically, you can
350 put arbitrary code into the argument of the ``DEBUG`` macro, and it is only
351 executed if '``opt``' (or any other tool) is run with the '``-debug``' command
356 DEBUG(errs() << "I am here!\n");
358 Then you can run your pass like this:
362 $ opt < a.bc > /dev/null -mypass
364 $ opt < a.bc > /dev/null -mypass -debug
367 Using the ``DEBUG()`` macro instead of a home-brewed solution allows you to not
368 have to create "yet another" command line option for the debug output for your
369 pass. Note that ``DEBUG()`` macros are disabled for optimized builds, so they
370 do not cause a performance impact at all (for the same reason, they should also
371 not contain side-effects!).
373 One additional nice thing about the ``DEBUG()`` macro is that you can enable or
374 disable it directly in gdb. Just use "``set DebugFlag=0``" or "``set
375 DebugFlag=1``" from the gdb if the program is running. If the program hasn't
376 been started yet, you can always just run it with ``-debug``.
380 Fine grained debug info with ``DEBUG_TYPE`` and the ``-debug-only`` option
381 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
383 Sometimes you may find yourself in a situation where enabling ``-debug`` just
384 turns on **too much** information (such as when working on the code generator).
385 If you want to enable debug information with more fine-grained control, you
386 can define the ``DEBUG_TYPE`` macro and use the ``-debug-only`` option as
392 DEBUG(errs() << "No debug type\n");
393 #define DEBUG_TYPE "foo"
394 DEBUG(errs() << "'foo' debug type\n");
396 #define DEBUG_TYPE "bar"
397 DEBUG(errs() << "'bar' debug type\n"));
399 #define DEBUG_TYPE ""
400 DEBUG(errs() << "No debug type (2)\n");
402 Then you can run your pass like this:
406 $ opt < a.bc > /dev/null -mypass
408 $ opt < a.bc > /dev/null -mypass -debug
413 $ opt < a.bc > /dev/null -mypass -debug-only=foo
415 $ opt < a.bc > /dev/null -mypass -debug-only=bar
418 Of course, in practice, you should only set ``DEBUG_TYPE`` at the top of a file,
419 to specify the debug type for the entire module (if you do this before you
420 ``#include "llvm/Support/Debug.h"``, you don't have to insert the ugly
421 ``#undef``'s). Also, you should use names more meaningful than "foo" and "bar",
422 because there is no system in place to ensure that names do not conflict. If
423 two different modules use the same string, they will all be turned on when the
424 name is specified. This allows, for example, all debug information for
425 instruction scheduling to be enabled with ``-debug-only=InstrSched``, even if
426 the source lives in multiple files.
428 The ``DEBUG_WITH_TYPE`` macro is also available for situations where you would
429 like to set ``DEBUG_TYPE``, but only for one specific ``DEBUG`` statement. It
430 takes an additional first parameter, which is the type to use. For example, the
431 preceding example could be written as:
435 DEBUG_WITH_TYPE("", errs() << "No debug type\n");
436 DEBUG_WITH_TYPE("foo", errs() << "'foo' debug type\n");
437 DEBUG_WITH_TYPE("bar", errs() << "'bar' debug type\n"));
438 DEBUG_WITH_TYPE("", errs() << "No debug type (2)\n");
442 The ``Statistic`` class & ``-stats`` option
443 -------------------------------------------
445 The ``llvm/ADT/Statistic.h`` (`doxygen
446 <http://llvm.org/doxygen/Statistic_8h-source.html>`__) file provides a class
447 named ``Statistic`` that is used as a unified way to keep track of what the LLVM
448 compiler is doing and how effective various optimizations are. It is useful to
449 see what optimizations are contributing to making a particular program run
452 Often you may run your pass on some big program, and you're interested to see
453 how many times it makes a certain transformation. Although you can do this with
454 hand inspection, or some ad-hoc method, this is a real pain and not very useful
455 for big programs. Using the ``Statistic`` class makes it very easy to keep
456 track of this information, and the calculated information is presented in a
457 uniform manner with the rest of the passes being executed.
459 There are many examples of ``Statistic`` uses, but the basics of using it are as
462 #. Define your statistic like this:
466 #define DEBUG_TYPE "mypassname" // This goes before any #includes.
467 STATISTIC(NumXForms, "The # of times I did stuff");
469 The ``STATISTIC`` macro defines a static variable, whose name is specified by
470 the first argument. The pass name is taken from the ``DEBUG_TYPE`` macro, and
471 the description is taken from the second argument. The variable defined
472 ("NumXForms" in this case) acts like an unsigned integer.
474 #. Whenever you make a transformation, bump the counter:
478 ++NumXForms; // I did stuff!
480 That's all you have to do. To get '``opt``' to print out the statistics
481 gathered, use the '``-stats``' option:
485 $ opt -stats -mypassname < program.bc > /dev/null
486 ... statistics output ...
488 When running ``opt`` on a C file from the SPEC benchmark suite, it gives a
489 report that looks like this:
493 7646 bitcodewriter - Number of normal instructions
494 725 bitcodewriter - Number of oversized instructions
495 129996 bitcodewriter - Number of bitcode bytes written
496 2817 raise - Number of insts DCEd or constprop'd
497 3213 raise - Number of cast-of-self removed
498 5046 raise - Number of expression trees converted
499 75 raise - Number of other getelementptr's formed
500 138 raise - Number of load/store peepholes
501 42 deadtypeelim - Number of unused typenames removed from symtab
502 392 funcresolve - Number of varargs functions resolved
503 27 globaldce - Number of global variables removed
504 2 adce - Number of basic blocks removed
505 134 cee - Number of branches revectored
506 49 cee - Number of setcc instruction eliminated
507 532 gcse - Number of loads removed
508 2919 gcse - Number of instructions removed
509 86 indvars - Number of canonical indvars added
510 87 indvars - Number of aux indvars removed
511 25 instcombine - Number of dead inst eliminate
512 434 instcombine - Number of insts combined
513 248 licm - Number of load insts hoisted
514 1298 licm - Number of insts hoisted to a loop pre-header
515 3 licm - Number of insts hoisted to multiple loop preds (bad, no loop pre-header)
516 75 mem2reg - Number of alloca's promoted
517 1444 cfgsimplify - Number of blocks simplified
519 Obviously, with so many optimizations, having a unified framework for this stuff
520 is very nice. Making your pass fit well into the framework makes it more
521 maintainable and useful.
525 Viewing graphs while debugging code
526 -----------------------------------
528 Several of the important data structures in LLVM are graphs: for example CFGs
529 made out of LLVM :ref:`BasicBlocks <BasicBlock>`, CFGs made out of LLVM
530 :ref:`MachineBasicBlocks <MachineBasicBlock>`, and :ref:`Instruction Selection
531 DAGs <SelectionDAG>`. In many cases, while debugging various parts of the
532 compiler, it is nice to instantly visualize these graphs.
534 LLVM provides several callbacks that are available in a debug build to do
535 exactly that. If you call the ``Function::viewCFG()`` method, for example, the
536 current LLVM tool will pop up a window containing the CFG for the function where
537 each basic block is a node in the graph, and each node contains the instructions
538 in the block. Similarly, there also exists ``Function::viewCFGOnly()`` (does
539 not include the instructions), the ``MachineFunction::viewCFG()`` and
540 ``MachineFunction::viewCFGOnly()``, and the ``SelectionDAG::viewGraph()``
541 methods. Within GDB, for example, you can usually use something like ``call
542 DAG.viewGraph()`` to pop up a window. Alternatively, you can sprinkle calls to
543 these functions in your code in places you want to debug.
545 Getting this to work requires a small amount of setup. On Unix systems
546 with X11, install the `graphviz <http://www.graphviz.org>`_ toolkit, and make
547 sure 'dot' and 'gv' are in your path. If you are running on Mac OS X, download
548 and install the Mac OS X `Graphviz program
549 <http://www.pixelglow.com/graphviz/>`_ and add
550 ``/Applications/Graphviz.app/Contents/MacOS/`` (or wherever you install it) to
551 your path. The programs need not be present when configuring, building or
552 running LLVM and can simply be installed when needed during an active debug
555 ``SelectionDAG`` has been extended to make it easier to locate *interesting*
556 nodes in large complex graphs. From gdb, if you ``call DAG.setGraphColor(node,
557 "color")``, then the next ``call DAG.viewGraph()`` would highlight the node in
558 the specified color (choices of colors can be found at `colors
559 <http://www.graphviz.org/doc/info/colors.html>`_.) More complex node attributes
560 can be provided with ``call DAG.setGraphAttrs(node, "attributes")`` (choices can
561 be found at `Graph attributes <http://www.graphviz.org/doc/info/attrs.html>`_.)
562 If you want to restart and clear all the current graph attributes, then you can
563 ``call DAG.clearGraphAttrs()``.
565 Note that graph visualization features are compiled out of Release builds to
566 reduce file size. This means that you need a Debug+Asserts or Release+Asserts
567 build to use these features.
571 Picking the Right Data Structure for a Task
572 ===========================================
574 LLVM has a plethora of data structures in the ``llvm/ADT/`` directory, and we
575 commonly use STL data structures. This section describes the trade-offs you
576 should consider when you pick one.
578 The first step is a choose your own adventure: do you want a sequential
579 container, a set-like container, or a map-like container? The most important
580 thing when choosing a container is the algorithmic properties of how you plan to
581 access the container. Based on that, you should use:
584 * a :ref:`map-like <ds_map>` container if you need efficient look-up of a
585 value based on another value. Map-like containers also support efficient
586 queries for containment (whether a key is in the map). Map-like containers
587 generally do not support efficient reverse mapping (values to keys). If you
588 need that, use two maps. Some map-like containers also support efficient
589 iteration through the keys in sorted order. Map-like containers are the most
590 expensive sort, only use them if you need one of these capabilities.
592 * a :ref:`set-like <ds_set>` container if you need to put a bunch of stuff into
593 a container that automatically eliminates duplicates. Some set-like
594 containers support efficient iteration through the elements in sorted order.
595 Set-like containers are more expensive than sequential containers.
597 * a :ref:`sequential <ds_sequential>` container provides the most efficient way
598 to add elements and keeps track of the order they are added to the collection.
599 They permit duplicates and support efficient iteration, but do not support
600 efficient look-up based on a key.
602 * a :ref:`string <ds_string>` container is a specialized sequential container or
603 reference structure that is used for character or byte arrays.
605 * a :ref:`bit <ds_bit>` container provides an efficient way to store and
606 perform set operations on sets of numeric id's, while automatically
607 eliminating duplicates. Bit containers require a maximum of 1 bit for each
608 identifier you want to store.
610 Once the proper category of container is determined, you can fine tune the
611 memory use, constant factors, and cache behaviors of access by intelligently
612 picking a member of the category. Note that constant factors and cache behavior
613 can be a big deal. If you have a vector that usually only contains a few
614 elements (but could contain many), for example, it's much better to use
615 :ref:`SmallVector <dss_smallvector>` than :ref:`vector <dss_vector>`. Doing so
616 avoids (relatively) expensive malloc/free calls, which dwarf the cost of adding
617 the elements to the container.
621 Sequential Containers (std::vector, std::list, etc)
622 ---------------------------------------------------
624 There are a variety of sequential containers available for you, based on your
625 needs. Pick the first in this section that will do what you want.
632 The ``llvm::ArrayRef`` class is the preferred class to use in an interface that
633 accepts a sequential list of elements in memory and just reads from them. By
634 taking an ``ArrayRef``, the API can be passed a fixed size array, an
635 ``std::vector``, an ``llvm::SmallVector`` and anything else that is contiguous
643 Fixed size arrays are very simple and very fast. They are good if you know
644 exactly how many elements you have, or you have a (low) upper bound on how many
649 Heap Allocated Arrays
650 ^^^^^^^^^^^^^^^^^^^^^
652 Heap allocated arrays (``new[]`` + ``delete[]``) are also simple. They are good
653 if the number of elements is variable, if you know how many elements you will
654 need before the array is allocated, and if the array is usually large (if not,
655 consider a :ref:`SmallVector <dss_smallvector>`). The cost of a heap allocated
656 array is the cost of the new/delete (aka malloc/free). Also note that if you
657 are allocating an array of a type with a constructor, the constructor and
658 destructors will be run for every element in the array (re-sizable vectors only
659 construct those elements actually used).
661 .. _dss_tinyptrvector:
663 llvm/ADT/TinyPtrVector.h
664 ^^^^^^^^^^^^^^^^^^^^^^^^
666 ``TinyPtrVector<Type>`` is a highly specialized collection class that is
667 optimized to avoid allocation in the case when a vector has zero or one
668 elements. It has two major restrictions: 1) it can only hold values of pointer
669 type, and 2) it cannot hold a null pointer.
671 Since this container is highly specialized, it is rarely used.
675 llvm/ADT/SmallVector.h
676 ^^^^^^^^^^^^^^^^^^^^^^
678 ``SmallVector<Type, N>`` is a simple class that looks and smells just like
679 ``vector<Type>``: it supports efficient iteration, lays out elements in memory
680 order (so you can do pointer arithmetic between elements), supports efficient
681 push_back/pop_back operations, supports efficient random access to its elements,
684 The advantage of SmallVector is that it allocates space for some number of
685 elements (N) **in the object itself**. Because of this, if the SmallVector is
686 dynamically smaller than N, no malloc is performed. This can be a big win in
687 cases where the malloc/free call is far more expensive than the code that
688 fiddles around with the elements.
690 This is good for vectors that are "usually small" (e.g. the number of
691 predecessors/successors of a block is usually less than 8). On the other hand,
692 this makes the size of the SmallVector itself large, so you don't want to
693 allocate lots of them (doing so will waste a lot of space). As such,
694 SmallVectors are most useful when on the stack.
696 SmallVector also provides a nice portable and efficient replacement for
701 Prefer to use ``SmallVectorImpl<T>`` as a parameter type.
703 In APIs that don't care about the "small size" (most?), prefer to use
704 the ``SmallVectorImpl<T>`` class, which is basically just the "vector
705 header" (and methods) without the elements allocated after it. Note that
706 ``SmallVector<T, N>`` inherits from ``SmallVectorImpl<T>`` so the
707 conversion is implicit and costs nothing. E.g.
711 // BAD: Clients cannot pass e.g. SmallVector<Foo, 4>.
712 hardcodedSmallSize(SmallVector<Foo, 2> &Out);
713 // GOOD: Clients can pass any SmallVector<Foo, N>.
714 allowsAnySmallSize(SmallVectorImpl<Foo> &Out);
717 SmallVector<Foo, 8> Vec;
718 hardcodedSmallSize(Vec); // Error.
719 allowsAnySmallSize(Vec); // Works.
722 Even though it has "``Impl``" in the name, this is so widely used that
723 it really isn't "private to the implementation" anymore. A name like
724 ``SmallVectorHeader`` would be more appropriate.
731 ``std::vector`` is well loved and respected. It is useful when SmallVector
732 isn't: when the size of the vector is often large (thus the small optimization
733 will rarely be a benefit) or if you will be allocating many instances of the
734 vector itself (which would waste space for elements that aren't in the
735 container). vector is also useful when interfacing with code that expects
738 One worthwhile note about std::vector: avoid code like this:
747 Instead, write this as:
757 Doing so will save (at least) one heap allocation and free per iteration of the
765 ``std::deque`` is, in some senses, a generalized version of ``std::vector``.
766 Like ``std::vector``, it provides constant time random access and other similar
767 properties, but it also provides efficient access to the front of the list. It
768 does not guarantee continuity of elements within memory.
770 In exchange for this extra flexibility, ``std::deque`` has significantly higher
771 constant factor costs than ``std::vector``. If possible, use ``std::vector`` or
779 ``std::list`` is an extremely inefficient class that is rarely useful. It
780 performs a heap allocation for every element inserted into it, thus having an
781 extremely high constant factor, particularly for small data types.
782 ``std::list`` also only supports bidirectional iteration, not random access
785 In exchange for this high cost, std::list supports efficient access to both ends
786 of the list (like ``std::deque``, but unlike ``std::vector`` or
787 ``SmallVector``). In addition, the iterator invalidation characteristics of
788 std::list are stronger than that of a vector class: inserting or removing an
789 element into the list does not invalidate iterator or pointers to other elements
797 ``ilist<T>`` implements an 'intrusive' doubly-linked list. It is intrusive,
798 because it requires the element to store and provide access to the prev/next
799 pointers for the list.
801 ``ilist`` has the same drawbacks as ``std::list``, and additionally requires an
802 ``ilist_traits`` implementation for the element type, but it provides some novel
803 characteristics. In particular, it can efficiently store polymorphic objects,
804 the traits class is informed when an element is inserted or removed from the
805 list, and ``ilist``\ s are guaranteed to support a constant-time splice
808 These properties are exactly what we want for things like ``Instruction``\ s and
809 basic blocks, which is why these are implemented with ``ilist``\ s.
811 Related classes of interest are explained in the following subsections:
813 * :ref:`ilist_traits <dss_ilist_traits>`
815 * :ref:`iplist <dss_iplist>`
817 * :ref:`llvm/ADT/ilist_node.h <dss_ilist_node>`
819 * :ref:`Sentinels <dss_ilist_sentinel>`
821 .. _dss_packedvector:
823 llvm/ADT/PackedVector.h
824 ^^^^^^^^^^^^^^^^^^^^^^^
826 Useful for storing a vector of values using only a few number of bits for each
827 value. Apart from the standard operations of a vector-like container, it can
828 also perform an 'or' set operation.
836 FirstCondition = 0x1,
837 SecondCondition = 0x2,
842 PackedVector<State, 2> Vec1;
843 Vec1.push_back(FirstCondition);
845 PackedVector<State, 2> Vec2;
846 Vec2.push_back(SecondCondition);
849 return Vec1[0]; // returns 'Both'.
852 .. _dss_ilist_traits:
857 ``ilist_traits<T>`` is ``ilist<T>``'s customization mechanism. ``iplist<T>``
858 (and consequently ``ilist<T>``) publicly derive from this traits class.
865 ``iplist<T>`` is ``ilist<T>``'s base and as such supports a slightly narrower
866 interface. Notably, inserters from ``T&`` are absent.
868 ``ilist_traits<T>`` is a public base of this class and can be used for a wide
869 variety of customizations.
873 llvm/ADT/ilist_node.h
874 ^^^^^^^^^^^^^^^^^^^^^
876 ``ilist_node<T>`` implements a the forward and backward links that are expected
877 by the ``ilist<T>`` (and analogous containers) in the default manner.
879 ``ilist_node<T>``\ s are meant to be embedded in the node type ``T``, usually
880 ``T`` publicly derives from ``ilist_node<T>``.
882 .. _dss_ilist_sentinel:
887 ``ilist``\ s have another specialty that must be considered. To be a good
888 citizen in the C++ ecosystem, it needs to support the standard container
889 operations, such as ``begin`` and ``end`` iterators, etc. Also, the
890 ``operator--`` must work correctly on the ``end`` iterator in the case of
891 non-empty ``ilist``\ s.
893 The only sensible solution to this problem is to allocate a so-called *sentinel*
894 along with the intrusive list, which serves as the ``end`` iterator, providing
895 the back-link to the last element. However conforming to the C++ convention it
896 is illegal to ``operator++`` beyond the sentinel and it also must not be
899 These constraints allow for some implementation freedom to the ``ilist`` how to
900 allocate and store the sentinel. The corresponding policy is dictated by
901 ``ilist_traits<T>``. By default a ``T`` gets heap-allocated whenever the need
902 for a sentinel arises.
904 While the default policy is sufficient in most cases, it may break down when
905 ``T`` does not provide a default constructor. Also, in the case of many
906 instances of ``ilist``\ s, the memory overhead of the associated sentinels is
907 wasted. To alleviate the situation with numerous and voluminous
908 ``T``-sentinels, sometimes a trick is employed, leading to *ghostly sentinels*.
910 Ghostly sentinels are obtained by specially-crafted ``ilist_traits<T>`` which
911 superpose the sentinel with the ``ilist`` instance in memory. Pointer
912 arithmetic is used to obtain the sentinel, which is relative to the ``ilist``'s
913 ``this`` pointer. The ``ilist`` is augmented by an extra pointer, which serves
914 as the back-link of the sentinel. This is the only field in the ghostly
915 sentinel which can be legally accessed.
919 Other Sequential Container options
920 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
922 Other STL containers are available, such as ``std::string``.
924 There are also various STL adapter classes such as ``std::queue``,
925 ``std::priority_queue``, ``std::stack``, etc. These provide simplified access
926 to an underlying container but don't affect the cost of the container itself.
930 String-like containers
931 ----------------------
933 There are a variety of ways to pass around and use strings in C and C++, and
934 LLVM adds a few new options to choose from. Pick the first option on this list
935 that will do what you need, they are ordered according to their relative cost.
937 Note that is is generally preferred to *not* pass strings around as ``const
938 char*``'s. These have a number of problems, including the fact that they
939 cannot represent embedded nul ("\0") characters, and do not have a length
940 available efficiently. The general replacement for '``const char*``' is
943 For more information on choosing string containers for APIs, please see
944 :ref:`Passing Strings <string_apis>`.
951 The StringRef class is a simple value class that contains a pointer to a
952 character and a length, and is quite related to the :ref:`ArrayRef
953 <dss_arrayref>` class (but specialized for arrays of characters). Because
954 StringRef carries a length with it, it safely handles strings with embedded nul
955 characters in it, getting the length does not require a strlen call, and it even
956 has very convenient APIs for slicing and dicing the character range that it
959 StringRef is ideal for passing simple strings around that are known to be live,
960 either because they are C string literals, std::string, a C array, or a
961 SmallVector. Each of these cases has an efficient implicit conversion to
962 StringRef, which doesn't result in a dynamic strlen being executed.
964 StringRef has a few major limitations which make more powerful string containers
967 #. You cannot directly convert a StringRef to a 'const char*' because there is
968 no way to add a trailing nul (unlike the .c_str() method on various stronger
971 #. StringRef doesn't own or keep alive the underlying string bytes.
972 As such it can easily lead to dangling pointers, and is not suitable for
973 embedding in datastructures in most cases (instead, use an std::string or
974 something like that).
976 #. For the same reason, StringRef cannot be used as the return value of a
977 method if the method "computes" the result string. Instead, use std::string.
979 #. StringRef's do not allow you to mutate the pointed-to string bytes and it
980 doesn't allow you to insert or remove bytes from the range. For editing
981 operations like this, it interoperates with the :ref:`Twine <dss_twine>`
984 Because of its strengths and limitations, it is very common for a function to
985 take a StringRef and for a method on an object to return a StringRef that points
986 into some string that it owns.
993 The Twine class is used as an intermediary datatype for APIs that want to take a
994 string that can be constructed inline with a series of concatenations. Twine
995 works by forming recursive instances of the Twine datatype (a simple value
996 object) on the stack as temporary objects, linking them together into a tree
997 which is then linearized when the Twine is consumed. Twine is only safe to use
998 as the argument to a function, and should always be a const reference, e.g.:
1002 void foo(const Twine &T);
1006 foo(X + "." + Twine(i));
1008 This example forms a string like "blarg.42" by concatenating the values
1009 together, and does not form intermediate strings containing "blarg" or "blarg.".
1011 Because Twine is constructed with temporary objects on the stack, and because
1012 these instances are destroyed at the end of the current statement, it is an
1013 inherently dangerous API. For example, this simple variant contains undefined
1014 behavior and will probably crash:
1018 void foo(const Twine &T);
1022 const Twine &Tmp = X + "." + Twine(i);
1025 ... because the temporaries are destroyed before the call. That said, Twine's
1026 are much more efficient than intermediate std::string temporaries, and they work
1027 really well with StringRef. Just be aware of their limitations.
1029 .. _dss_smallstring:
1031 llvm/ADT/SmallString.h
1032 ^^^^^^^^^^^^^^^^^^^^^^
1034 SmallString is a subclass of :ref:`SmallVector <dss_smallvector>` that adds some
1035 convenience APIs like += that takes StringRef's. SmallString avoids allocating
1036 memory in the case when the preallocated space is enough to hold its data, and
1037 it calls back to general heap allocation when required. Since it owns its data,
1038 it is very safe to use and supports full mutation of the string.
1040 Like SmallVector's, the big downside to SmallString is their sizeof. While they
1041 are optimized for small strings, they themselves are not particularly small.
1042 This means that they work great for temporary scratch buffers on the stack, but
1043 should not generally be put into the heap: it is very rare to see a SmallString
1044 as the member of a frequently-allocated heap data structure or returned
1052 The standard C++ std::string class is a very general class that (like
1053 SmallString) owns its underlying data. sizeof(std::string) is very reasonable
1054 so it can be embedded into heap data structures and returned by-value. On the
1055 other hand, std::string is highly inefficient for inline editing (e.g.
1056 concatenating a bunch of stuff together) and because it is provided by the
1057 standard library, its performance characteristics depend a lot of the host
1058 standard library (e.g. libc++ and MSVC provide a highly optimized string class,
1059 GCC contains a really slow implementation).
1061 The major disadvantage of std::string is that almost every operation that makes
1062 them larger can allocate memory, which is slow. As such, it is better to use
1063 SmallVector or Twine as a scratch buffer, but then use std::string to persist
1068 Set-Like Containers (std::set, SmallSet, SetVector, etc)
1069 --------------------------------------------------------
1071 Set-like containers are useful when you need to canonicalize multiple values
1072 into a single representation. There are several different choices for how to do
1073 this, providing various trade-offs.
1075 .. _dss_sortedvectorset:
1080 If you intend to insert a lot of elements, then do a lot of queries, a great
1081 approach is to use a vector (or other sequential container) with
1082 std::sort+std::unique to remove duplicates. This approach works really well if
1083 your usage pattern has these two distinct phases (insert then query), and can be
1084 coupled with a good choice of :ref:`sequential container <ds_sequential>`.
1086 This combination provides the several nice properties: the result data is
1087 contiguous in memory (good for cache locality), has few allocations, is easy to
1088 address (iterators in the final vector are just indices or pointers), and can be
1089 efficiently queried with a standard binary search (e.g.
1090 ``std::lower_bound``; if you want the whole range of elements comparing
1091 equal, use ``std::equal_range``).
1098 If you have a set-like data structure that is usually small and whose elements
1099 are reasonably small, a ``SmallSet<Type, N>`` is a good choice. This set has
1100 space for N elements in place (thus, if the set is dynamically smaller than N,
1101 no malloc traffic is required) and accesses them with a simple linear search.
1102 When the set grows beyond 'N' elements, it allocates a more expensive
1103 representation that guarantees efficient access (for most types, it falls back
1104 to std::set, but for pointers it uses something far better, :ref:`SmallPtrSet
1107 The magic of this class is that it handles small sets extremely efficiently, but
1108 gracefully handles extremely large sets without loss of efficiency. The
1109 drawback is that the interface is quite small: it supports insertion, queries
1110 and erasing, but does not support iteration.
1112 .. _dss_smallptrset:
1114 llvm/ADT/SmallPtrSet.h
1115 ^^^^^^^^^^^^^^^^^^^^^^
1117 SmallPtrSet has all the advantages of ``SmallSet`` (and a ``SmallSet`` of
1118 pointers is transparently implemented with a ``SmallPtrSet``), but also supports
1119 iterators. If more than 'N' insertions are performed, a single quadratically
1120 probed hash table is allocated and grows as needed, providing extremely
1121 efficient access (constant time insertion/deleting/queries with low constant
1122 factors) and is very stingy with malloc traffic.
1124 Note that, unlike ``std::set``, the iterators of ``SmallPtrSet`` are invalidated
1125 whenever an insertion occurs. Also, the values visited by the iterators are not
1126 visited in sorted order.
1133 DenseSet is a simple quadratically probed hash table. It excels at supporting
1134 small values: it uses a single allocation to hold all of the pairs that are
1135 currently inserted in the set. DenseSet is a great way to unique small values
1136 that are not simple pointers (use :ref:`SmallPtrSet <dss_smallptrset>` for
1137 pointers). Note that DenseSet has the same requirements for the value type that
1138 :ref:`DenseMap <dss_densemap>` has.
1142 llvm/ADT/SparseSet.h
1143 ^^^^^^^^^^^^^^^^^^^^
1145 SparseSet holds a small number of objects identified by unsigned keys of
1146 moderate size. It uses a lot of memory, but provides operations that are almost
1147 as fast as a vector. Typical keys are physical registers, virtual registers, or
1148 numbered basic blocks.
1150 SparseSet is useful for algorithms that need very fast clear/find/insert/erase
1151 and fast iteration over small sets. It is not intended for building composite
1154 .. _dss_sparsemultiset:
1156 llvm/ADT/SparseMultiSet.h
1157 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
1159 SparseMultiSet adds multiset behavior to SparseSet, while retaining SparseSet's
1160 desirable attributes. Like SparseSet, it typically uses a lot of memory, but
1161 provides operations that are almost as fast as a vector. Typical keys are
1162 physical registers, virtual registers, or numbered basic blocks.
1164 SparseMultiSet is useful for algorithms that need very fast
1165 clear/find/insert/erase of the entire collection, and iteration over sets of
1166 elements sharing a key. It is often a more efficient choice than using composite
1167 data structures (e.g. vector-of-vectors, map-of-vectors). It is not intended for
1168 building composite data structures.
1172 llvm/ADT/FoldingSet.h
1173 ^^^^^^^^^^^^^^^^^^^^^
1175 FoldingSet is an aggregate class that is really good at uniquing
1176 expensive-to-create or polymorphic objects. It is a combination of a chained
1177 hash table with intrusive links (uniqued objects are required to inherit from
1178 FoldingSetNode) that uses :ref:`SmallVector <dss_smallvector>` as part of its ID
1181 Consider a case where you want to implement a "getOrCreateFoo" method for a
1182 complex object (for example, a node in the code generator). The client has a
1183 description of **what** it wants to generate (it knows the opcode and all the
1184 operands), but we don't want to 'new' a node, then try inserting it into a set
1185 only to find out it already exists, at which point we would have to delete it
1186 and return the node that already exists.
1188 To support this style of client, FoldingSet perform a query with a
1189 FoldingSetNodeID (which wraps SmallVector) that can be used to describe the
1190 element that we want to query for. The query either returns the element
1191 matching the ID or it returns an opaque ID that indicates where insertion should
1192 take place. Construction of the ID usually does not require heap traffic.
1194 Because FoldingSet uses intrusive links, it can support polymorphic objects in
1195 the set (for example, you can have SDNode instances mixed with LoadSDNodes).
1196 Because the elements are individually allocated, pointers to the elements are
1197 stable: inserting or removing elements does not invalidate any pointers to other
1205 ``std::set`` is a reasonable all-around set class, which is decent at many
1206 things but great at nothing. std::set allocates memory for each element
1207 inserted (thus it is very malloc intensive) and typically stores three pointers
1208 per element in the set (thus adding a large amount of per-element space
1209 overhead). It offers guaranteed log(n) performance, which is not particularly
1210 fast from a complexity standpoint (particularly if the elements of the set are
1211 expensive to compare, like strings), and has extremely high constant factors for
1212 lookup, insertion and removal.
1214 The advantages of std::set are that its iterators are stable (deleting or
1215 inserting an element from the set does not affect iterators or pointers to other
1216 elements) and that iteration over the set is guaranteed to be in sorted order.
1217 If the elements in the set are large, then the relative overhead of the pointers
1218 and malloc traffic is not a big deal, but if the elements of the set are small,
1219 std::set is almost never a good choice.
1223 llvm/ADT/SetVector.h
1224 ^^^^^^^^^^^^^^^^^^^^
1226 LLVM's ``SetVector<Type>`` is an adapter class that combines your choice of a
1227 set-like container along with a :ref:`Sequential Container <ds_sequential>` The
1228 important property that this provides is efficient insertion with uniquing
1229 (duplicate elements are ignored) with iteration support. It implements this by
1230 inserting elements into both a set-like container and the sequential container,
1231 using the set-like container for uniquing and the sequential container for
1234 The difference between SetVector and other sets is that the order of iteration
1235 is guaranteed to match the order of insertion into the SetVector. This property
1236 is really important for things like sets of pointers. Because pointer values
1237 are non-deterministic (e.g. vary across runs of the program on different
1238 machines), iterating over the pointers in the set will not be in a well-defined
1241 The drawback of SetVector is that it requires twice as much space as a normal
1242 set and has the sum of constant factors from the set-like container and the
1243 sequential container that it uses. Use it **only** if you need to iterate over
1244 the elements in a deterministic order. SetVector is also expensive to delete
1245 elements out of (linear time), unless you use its "pop_back" method, which is
1248 ``SetVector`` is an adapter class that defaults to using ``std::vector`` and a
1249 size 16 ``SmallSet`` for the underlying containers, so it is quite expensive.
1250 However, ``"llvm/ADT/SetVector.h"`` also provides a ``SmallSetVector`` class,
1251 which defaults to using a ``SmallVector`` and ``SmallSet`` of a specified size.
1252 If you use this, and if your sets are dynamically smaller than ``N``, you will
1253 save a lot of heap traffic.
1255 .. _dss_uniquevector:
1257 llvm/ADT/UniqueVector.h
1258 ^^^^^^^^^^^^^^^^^^^^^^^
1260 UniqueVector is similar to :ref:`SetVector <dss_setvector>` but it retains a
1261 unique ID for each element inserted into the set. It internally contains a map
1262 and a vector, and it assigns a unique ID for each value inserted into the set.
1264 UniqueVector is very expensive: its cost is the sum of the cost of maintaining
1265 both the map and vector, it has high complexity, high constant factors, and
1266 produces a lot of malloc traffic. It should be avoided.
1268 .. _dss_immutableset:
1270 llvm/ADT/ImmutableSet.h
1271 ^^^^^^^^^^^^^^^^^^^^^^^
1273 ImmutableSet is an immutable (functional) set implementation based on an AVL
1274 tree. Adding or removing elements is done through a Factory object and results
1275 in the creation of a new ImmutableSet object. If an ImmutableSet already exists
1276 with the given contents, then the existing one is returned; equality is compared
1277 with a FoldingSetNodeID. The time and space complexity of add or remove
1278 operations is logarithmic in the size of the original set.
1280 There is no method for returning an element of the set, you can only check for
1285 Other Set-Like Container Options
1286 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
1288 The STL provides several other options, such as std::multiset and the various
1289 "hash_set" like containers (whether from C++ TR1 or from the SGI library). We
1290 never use hash_set and unordered_set because they are generally very expensive
1291 (each insertion requires a malloc) and very non-portable.
1293 std::multiset is useful if you're not interested in elimination of duplicates,
1294 but has all the drawbacks of std::set. A sorted vector (where you don't delete
1295 duplicate entries) or some other approach is almost always better.
1299 Map-Like Containers (std::map, DenseMap, etc)
1300 ---------------------------------------------
1302 Map-like containers are useful when you want to associate data to a key. As
1303 usual, there are a lot of different ways to do this. :)
1305 .. _dss_sortedvectormap:
1310 If your usage pattern follows a strict insert-then-query approach, you can
1311 trivially use the same approach as :ref:`sorted vectors for set-like containers
1312 <dss_sortedvectorset>`. The only difference is that your query function (which
1313 uses std::lower_bound to get efficient log(n) lookup) should only compare the
1314 key, not both the key and value. This yields the same advantages as sorted
1319 llvm/ADT/StringMap.h
1320 ^^^^^^^^^^^^^^^^^^^^
1322 Strings are commonly used as keys in maps, and they are difficult to support
1323 efficiently: they are variable length, inefficient to hash and compare when
1324 long, expensive to copy, etc. StringMap is a specialized container designed to
1325 cope with these issues. It supports mapping an arbitrary range of bytes to an
1326 arbitrary other object.
1328 The StringMap implementation uses a quadratically-probed hash table, where the
1329 buckets store a pointer to the heap allocated entries (and some other stuff).
1330 The entries in the map must be heap allocated because the strings are variable
1331 length. The string data (key) and the element object (value) are stored in the
1332 same allocation with the string data immediately after the element object.
1333 This container guarantees the "``(char*)(&Value+1)``" points to the key string
1336 The StringMap is very fast for several reasons: quadratic probing is very cache
1337 efficient for lookups, the hash value of strings in buckets is not recomputed
1338 when looking up an element, StringMap rarely has to touch the memory for
1339 unrelated objects when looking up a value (even when hash collisions happen),
1340 hash table growth does not recompute the hash values for strings already in the
1341 table, and each pair in the map is store in a single allocation (the string data
1342 is stored in the same allocation as the Value of a pair).
1344 StringMap also provides query methods that take byte ranges, so it only ever
1345 copies a string if a value is inserted into the table.
1347 StringMap iteratation order, however, is not guaranteed to be deterministic, so
1348 any uses which require that should instead use a std::map.
1352 llvm/ADT/IndexedMap.h
1353 ^^^^^^^^^^^^^^^^^^^^^
1355 IndexedMap is a specialized container for mapping small dense integers (or
1356 values that can be mapped to small dense integers) to some other type. It is
1357 internally implemented as a vector with a mapping function that maps the keys
1358 to the dense integer range.
1360 This is useful for cases like virtual registers in the LLVM code generator: they
1361 have a dense mapping that is offset by a compile-time constant (the first
1362 virtual register ID).
1369 DenseMap is a simple quadratically probed hash table. It excels at supporting
1370 small keys and values: it uses a single allocation to hold all of the pairs
1371 that are currently inserted in the map. DenseMap is a great way to map
1372 pointers to pointers, or map other small types to each other.
1374 There are several aspects of DenseMap that you should be aware of, however.
1375 The iterators in a DenseMap are invalidated whenever an insertion occurs,
1376 unlike map. Also, because DenseMap allocates space for a large number of
1377 key/value pairs (it starts with 64 by default), it will waste a lot of space if
1378 your keys or values are large. Finally, you must implement a partial
1379 specialization of DenseMapInfo for the key that you want, if it isn't already
1380 supported. This is required to tell DenseMap about two special marker values
1381 (which can never be inserted into the map) that it needs internally.
1383 DenseMap's find_as() method supports lookup operations using an alternate key
1384 type. This is useful in cases where the normal key type is expensive to
1385 construct, but cheap to compare against. The DenseMapInfo is responsible for
1386 defining the appropriate comparison and hashing methods for each alternate key
1394 ValueMap is a wrapper around a :ref:`DenseMap <dss_densemap>` mapping
1395 ``Value*``\ s (or subclasses) to another type. When a Value is deleted or
1396 RAUW'ed, ValueMap will update itself so the new version of the key is mapped to
1397 the same value, just as if the key were a WeakVH. You can configure exactly how
1398 this happens, and what else happens on these two events, by passing a ``Config``
1399 parameter to the ValueMap template.
1401 .. _dss_intervalmap:
1403 llvm/ADT/IntervalMap.h
1404 ^^^^^^^^^^^^^^^^^^^^^^
1406 IntervalMap is a compact map for small keys and values. It maps key intervals
1407 instead of single keys, and it will automatically coalesce adjacent intervals.
1408 When then map only contains a few intervals, they are stored in the map object
1409 itself to avoid allocations.
1411 The IntervalMap iterators are quite big, so they should not be passed around as
1412 STL iterators. The heavyweight iterators allow a smaller data structure.
1419 std::map has similar characteristics to :ref:`std::set <dss_set>`: it uses a
1420 single allocation per pair inserted into the map, it offers log(n) lookup with
1421 an extremely large constant factor, imposes a space penalty of 3 pointers per
1422 pair in the map, etc.
1424 std::map is most useful when your keys or values are very large, if you need to
1425 iterate over the collection in sorted order, or if you need stable iterators
1426 into the map (i.e. they don't get invalidated if an insertion or deletion of
1427 another element takes place).
1431 llvm/ADT/MapVector.h
1432 ^^^^^^^^^^^^^^^^^^^^
1434 ``MapVector<KeyT,ValueT>`` provides a subset of the DenseMap interface. The
1435 main difference is that the iteration order is guaranteed to be the insertion
1436 order, making it an easy (but somewhat expensive) solution for non-deterministic
1437 iteration over maps of pointers.
1439 It is implemented by mapping from key to an index in a vector of key,value
1440 pairs. This provides fast lookup and iteration, but has two main drawbacks:
1441 the key is stored twice and removing elements takes linear time. If it is
1442 necessary to remove elements, it's best to remove them in bulk using
1445 .. _dss_inteqclasses:
1447 llvm/ADT/IntEqClasses.h
1448 ^^^^^^^^^^^^^^^^^^^^^^^
1450 IntEqClasses provides a compact representation of equivalence classes of small
1451 integers. Initially, each integer in the range 0..n-1 has its own equivalence
1452 class. Classes can be joined by passing two class representatives to the
1453 join(a, b) method. Two integers are in the same class when findLeader() returns
1454 the same representative.
1456 Once all equivalence classes are formed, the map can be compressed so each
1457 integer 0..n-1 maps to an equivalence class number in the range 0..m-1, where m
1458 is the total number of equivalence classes. The map must be uncompressed before
1459 it can be edited again.
1461 .. _dss_immutablemap:
1463 llvm/ADT/ImmutableMap.h
1464 ^^^^^^^^^^^^^^^^^^^^^^^
1466 ImmutableMap is an immutable (functional) map implementation based on an AVL
1467 tree. Adding or removing elements is done through a Factory object and results
1468 in the creation of a new ImmutableMap object. If an ImmutableMap already exists
1469 with the given key set, then the existing one is returned; equality is compared
1470 with a FoldingSetNodeID. The time and space complexity of add or remove
1471 operations is logarithmic in the size of the original map.
1475 Other Map-Like Container Options
1476 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
1478 The STL provides several other options, such as std::multimap and the various
1479 "hash_map" like containers (whether from C++ TR1 or from the SGI library). We
1480 never use hash_set and unordered_set because they are generally very expensive
1481 (each insertion requires a malloc) and very non-portable.
1483 std::multimap is useful if you want to map a key to multiple values, but has all
1484 the drawbacks of std::map. A sorted vector or some other approach is almost
1489 Bit storage containers (BitVector, SparseBitVector)
1490 ---------------------------------------------------
1492 Unlike the other containers, there are only two bit storage containers, and
1493 choosing when to use each is relatively straightforward.
1495 One additional option is ``std::vector<bool>``: we discourage its use for two
1496 reasons 1) the implementation in many common compilers (e.g. commonly
1497 available versions of GCC) is extremely inefficient and 2) the C++ standards
1498 committee is likely to deprecate this container and/or change it significantly
1499 somehow. In any case, please don't use it.
1506 The BitVector container provides a dynamic size set of bits for manipulation.
1507 It supports individual bit setting/testing, as well as set operations. The set
1508 operations take time O(size of bitvector), but operations are performed one word
1509 at a time, instead of one bit at a time. This makes the BitVector very fast for
1510 set operations compared to other containers. Use the BitVector when you expect
1511 the number of set bits to be high (i.e. a dense set).
1513 .. _dss_smallbitvector:
1518 The SmallBitVector container provides the same interface as BitVector, but it is
1519 optimized for the case where only a small number of bits, less than 25 or so,
1520 are needed. It also transparently supports larger bit counts, but slightly less
1521 efficiently than a plain BitVector, so SmallBitVector should only be used when
1522 larger counts are rare.
1524 At this time, SmallBitVector does not support set operations (and, or, xor), and
1525 its operator[] does not provide an assignable lvalue.
1527 .. _dss_sparsebitvector:
1532 The SparseBitVector container is much like BitVector, with one major difference:
1533 Only the bits that are set, are stored. This makes the SparseBitVector much
1534 more space efficient than BitVector when the set is sparse, as well as making
1535 set operations O(number of set bits) instead of O(size of universe). The
1536 downside to the SparseBitVector is that setting and testing of random bits is
1537 O(N), and on large SparseBitVectors, this can be slower than BitVector. In our
1538 implementation, setting or testing bits in sorted order (either forwards or
1539 reverse) is O(1) worst case. Testing and setting bits within 128 bits (depends
1540 on size) of the current bit is also O(1). As a general statement,
1541 testing/setting bits in a SparseBitVector is O(distance away from last set bit).
1545 Helpful Hints for Common Operations
1546 ===================================
1548 This section describes how to perform some very simple transformations of LLVM
1549 code. This is meant to give examples of common idioms used, showing the
1550 practical side of LLVM transformations.
1552 Because this is a "how-to" section, you should also read about the main classes
1553 that you will be working with. The :ref:`Core LLVM Class Hierarchy Reference
1554 <coreclasses>` contains details and descriptions of the main classes that you
1559 Basic Inspection and Traversal Routines
1560 ---------------------------------------
1562 The LLVM compiler infrastructure have many different data structures that may be
1563 traversed. Following the example of the C++ standard template library, the
1564 techniques used to traverse these various data structures are all basically the
1565 same. For a enumerable sequence of values, the ``XXXbegin()`` function (or
1566 method) returns an iterator to the start of the sequence, the ``XXXend()``
1567 function returns an iterator pointing to one past the last valid element of the
1568 sequence, and there is some ``XXXiterator`` data type that is common between the
1571 Because the pattern for iteration is common across many different aspects of the
1572 program representation, the standard template library algorithms may be used on
1573 them, and it is easier to remember how to iterate. First we show a few common
1574 examples of the data structures that need to be traversed. Other data
1575 structures are traversed in very similar ways.
1577 .. _iterate_function:
1579 Iterating over the ``BasicBlock`` in a ``Function``
1580 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
1582 It's quite common to have a ``Function`` instance that you'd like to transform
1583 in some way; in particular, you'd like to manipulate its ``BasicBlock``\ s. To
1584 facilitate this, you'll need to iterate over all of the ``BasicBlock``\ s that
1585 constitute the ``Function``. The following is an example that prints the name
1586 of a ``BasicBlock`` and the number of ``Instruction``\ s it contains:
1590 // func is a pointer to a Function instance
1591 for (Function::iterator i = func->begin(), e = func->end(); i != e; ++i)
1592 // Print out the name of the basic block if it has one, and then the
1593 // number of instructions that it contains
1594 errs() << "Basic block (name=" << i->getName() << ") has "
1595 << i->size() << " instructions.\n";
1597 Note that i can be used as if it were a pointer for the purposes of invoking
1598 member functions of the ``Instruction`` class. This is because the indirection
1599 operator is overloaded for the iterator classes. In the above code, the
1600 expression ``i->size()`` is exactly equivalent to ``(*i).size()`` just like
1603 .. _iterate_basicblock:
1605 Iterating over the ``Instruction`` in a ``BasicBlock``
1606 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
1608 Just like when dealing with ``BasicBlock``\ s in ``Function``\ s, it's easy to
1609 iterate over the individual instructions that make up ``BasicBlock``\ s. Here's
1610 a code snippet that prints out each instruction in a ``BasicBlock``:
1614 // blk is a pointer to a BasicBlock instance
1615 for (BasicBlock::iterator i = blk->begin(), e = blk->end(); i != e; ++i)
1616 // The next statement works since operator<<(ostream&,...)
1617 // is overloaded for Instruction&
1618 errs() << *i << "\n";
1621 However, this isn't really the best way to print out the contents of a
1622 ``BasicBlock``! Since the ostream operators are overloaded for virtually
1623 anything you'll care about, you could have just invoked the print routine on the
1624 basic block itself: ``errs() << *blk << "\n";``.
1626 .. _iterate_insiter:
1628 Iterating over the ``Instruction`` in a ``Function``
1629 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
1631 If you're finding that you commonly iterate over a ``Function``'s
1632 ``BasicBlock``\ s and then that ``BasicBlock``'s ``Instruction``\ s,
1633 ``InstIterator`` should be used instead. You'll need to include
1634 ``llvm/IR/InstIterator.h`` (`doxygen
1635 <http://llvm.org/doxygen/InstIterator_8h.html>`__) and then instantiate
1636 ``InstIterator``\ s explicitly in your code. Here's a small example that shows
1637 how to dump all instructions in a function to the standard error stream:
1641 #include "llvm/IR/InstIterator.h"
1643 // F is a pointer to a Function instance
1644 for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I)
1645 errs() << *I << "\n";
1647 Easy, isn't it? You can also use ``InstIterator``\ s to fill a work list with
1648 its initial contents. For example, if you wanted to initialize a work list to
1649 contain all instructions in a ``Function`` F, all you would need to do is
1654 std::set<Instruction*> worklist;
1655 // or better yet, SmallPtrSet<Instruction*, 64> worklist;
1657 for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I)
1658 worklist.insert(&*I);
1660 The STL set ``worklist`` would now contain all instructions in the ``Function``
1663 .. _iterate_convert:
1665 Turning an iterator into a class pointer (and vice-versa)
1666 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
1668 Sometimes, it'll be useful to grab a reference (or pointer) to a class instance
1669 when all you've got at hand is an iterator. Well, extracting a reference or a
1670 pointer from an iterator is very straight-forward. Assuming that ``i`` is a
1671 ``BasicBlock::iterator`` and ``j`` is a ``BasicBlock::const_iterator``:
1675 Instruction& inst = *i; // Grab reference to instruction reference
1676 Instruction* pinst = &*i; // Grab pointer to instruction reference
1677 const Instruction& inst = *j;
1679 However, the iterators you'll be working with in the LLVM framework are special:
1680 they will automatically convert to a ptr-to-instance type whenever they need to.
1681 Instead of derferencing the iterator and then taking the address of the result,
1682 you can simply assign the iterator to the proper pointer type and you get the
1683 dereference and address-of operation as a result of the assignment (behind the
1684 scenes, this is a result of overloading casting mechanisms). Thus the last line
1685 of the last example,
1689 Instruction *pinst = &*i;
1691 is semantically equivalent to
1695 Instruction *pinst = i;
1697 It's also possible to turn a class pointer into the corresponding iterator, and
1698 this is a constant time operation (very efficient). The following code snippet
1699 illustrates use of the conversion constructors provided by LLVM iterators. By
1700 using these, you can explicitly grab the iterator of something without actually
1701 obtaining it via iteration over some structure:
1705 void printNextInstruction(Instruction* inst) {
1706 BasicBlock::iterator it(inst);
1707 ++it; // After this line, it refers to the instruction after *inst
1708 if (it != inst->getParent()->end()) errs() << *it << "\n";
1711 Unfortunately, these implicit conversions come at a cost; they prevent these
1712 iterators from conforming to standard iterator conventions, and thus from being
1713 usable with standard algorithms and containers. For example, they prevent the
1714 following code, where ``B`` is a ``BasicBlock``, from compiling:
1718 llvm::SmallVector<llvm::Instruction *, 16>(B->begin(), B->end());
1720 Because of this, these implicit conversions may be removed some day, and
1721 ``operator*`` changed to return a pointer instead of a reference.
1723 .. _iterate_complex:
1725 Finding call sites: a slightly more complex example
1726 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
1728 Say that you're writing a FunctionPass and would like to count all the locations
1729 in the entire module (that is, across every ``Function``) where a certain
1730 function (i.e., some ``Function *``) is already in scope. As you'll learn
1731 later, you may want to use an ``InstVisitor`` to accomplish this in a much more
1732 straight-forward manner, but this example will allow us to explore how you'd do
1733 it if you didn't have ``InstVisitor`` around. In pseudo-code, this is what we
1736 .. code-block:: none
1738 initialize callCounter to zero
1739 for each Function f in the Module
1740 for each BasicBlock b in f
1741 for each Instruction i in b
1742 if (i is a CallInst and calls the given function)
1743 increment callCounter
1745 And the actual code is (remember, because we're writing a ``FunctionPass``, our
1746 ``FunctionPass``-derived class simply has to override the ``runOnFunction``
1751 Function* targetFunc = ...;
1753 class OurFunctionPass : public FunctionPass {
1755 OurFunctionPass(): callCounter(0) { }
1757 virtual runOnFunction(Function& F) {
1758 for (Function::iterator b = F.begin(), be = F.end(); b != be; ++b) {
1759 for (BasicBlock::iterator i = b->begin(), ie = b->end(); i != ie; ++i) {
1760 if (CallInst* callInst = dyn_cast<CallInst>(&*i)) {
1761 // We know we've encountered a call instruction, so we
1762 // need to determine if it's a call to the
1763 // function pointed to by m_func or not.
1764 if (callInst->getCalledFunction() == targetFunc)
1772 unsigned callCounter;
1775 .. _calls_and_invokes:
1777 Treating calls and invokes the same way
1778 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
1780 You may have noticed that the previous example was a bit oversimplified in that
1781 it did not deal with call sites generated by 'invoke' instructions. In this,
1782 and in other situations, you may find that you want to treat ``CallInst``\ s and
1783 ``InvokeInst``\ s the same way, even though their most-specific common base
1784 class is ``Instruction``, which includes lots of less closely-related things.
1785 For these cases, LLVM provides a handy wrapper class called ``CallSite``
1786 (`doxygen <http://llvm.org/doxygen/classllvm_1_1CallSite.html>`__) It is
1787 essentially a wrapper around an ``Instruction`` pointer, with some methods that
1788 provide functionality common to ``CallInst``\ s and ``InvokeInst``\ s.
1790 This class has "value semantics": it should be passed by value, not by reference
1791 and it should not be dynamically allocated or deallocated using ``operator new``
1792 or ``operator delete``. It is efficiently copyable, assignable and
1793 constructable, with costs equivalents to that of a bare pointer. If you look at
1794 its definition, it has only a single pointer member.
1798 Iterating over def-use & use-def chains
1799 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
1801 Frequently, we might have an instance of the ``Value`` class (`doxygen
1802 <http://llvm.org/doxygen/classllvm_1_1Value.html>`__) and we want to determine
1803 which ``User`` s use the ``Value``. The list of all ``User``\ s of a particular
1804 ``Value`` is called a *def-use* chain. For example, let's say we have a
1805 ``Function*`` named ``F`` to a particular function ``foo``. Finding all of the
1806 instructions that *use* ``foo`` is as simple as iterating over the *def-use*
1813 for (User *U : GV->users()) {
1814 if (Instruction *Inst = dyn_cast<Instruction>(U)) {
1815 errs() << "F is used in instruction:\n";
1816 errs() << *Inst << "\n";
1819 Alternatively, it's common to have an instance of the ``User`` Class (`doxygen
1820 <http://llvm.org/doxygen/classllvm_1_1User.html>`__) and need to know what
1821 ``Value``\ s are used by it. The list of all ``Value``\ s used by a ``User`` is
1822 known as a *use-def* chain. Instances of class ``Instruction`` are common
1823 ``User`` s, so we might want to iterate over all of the values that a particular
1824 instruction uses (that is, the operands of the particular ``Instruction``):
1828 Instruction *pi = ...;
1830 for (Use &U : pi->operands()) {
1835 Declaring objects as ``const`` is an important tool of enforcing mutation free
1836 algorithms (such as analyses, etc.). For this purpose above iterators come in
1837 constant flavors as ``Value::const_use_iterator`` and
1838 ``Value::const_op_iterator``. They automatically arise when calling
1839 ``use/op_begin()`` on ``const Value*``\ s or ``const User*``\ s respectively.
1840 Upon dereferencing, they return ``const Use*``\ s. Otherwise the above patterns
1845 Iterating over predecessors & successors of blocks
1846 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
1848 Iterating over the predecessors and successors of a block is quite easy with the
1849 routines defined in ``"llvm/Support/CFG.h"``. Just use code like this to
1850 iterate over all predecessors of BB:
1854 #include "llvm/Support/CFG.h"
1855 BasicBlock *BB = ...;
1857 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
1858 BasicBlock *Pred = *PI;
1862 Similarly, to iterate over successors use ``succ_iterator/succ_begin/succ_end``.
1866 Making simple changes
1867 ---------------------
1869 There are some primitive transformation operations present in the LLVM
1870 infrastructure that are worth knowing about. When performing transformations,
1871 it's fairly common to manipulate the contents of basic blocks. This section
1872 describes some of the common methods for doing so and gives example code.
1874 .. _schanges_creating:
1876 Creating and inserting new ``Instruction``\ s
1877 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
1879 *Instantiating Instructions*
1881 Creation of ``Instruction``\ s is straight-forward: simply call the constructor
1882 for the kind of instruction to instantiate and provide the necessary parameters.
1883 For example, an ``AllocaInst`` only *requires* a (const-ptr-to) ``Type``. Thus:
1887 AllocaInst* ai = new AllocaInst(Type::Int32Ty);
1889 will create an ``AllocaInst`` instance that represents the allocation of one
1890 integer in the current stack frame, at run time. Each ``Instruction`` subclass
1891 is likely to have varying default parameters which change the semantics of the
1892 instruction, so refer to the `doxygen documentation for the subclass of
1893 Instruction <http://llvm.org/doxygen/classllvm_1_1Instruction.html>`_ that
1894 you're interested in instantiating.
1898 It is very useful to name the values of instructions when you're able to, as
1899 this facilitates the debugging of your transformations. If you end up looking
1900 at generated LLVM machine code, you definitely want to have logical names
1901 associated with the results of instructions! By supplying a value for the
1902 ``Name`` (default) parameter of the ``Instruction`` constructor, you associate a
1903 logical name with the result of the instruction's execution at run time. For
1904 example, say that I'm writing a transformation that dynamically allocates space
1905 for an integer on the stack, and that integer is going to be used as some kind
1906 of index by some other code. To accomplish this, I place an ``AllocaInst`` at
1907 the first point in the first ``BasicBlock`` of some ``Function``, and I'm
1908 intending to use it within the same ``Function``. I might do:
1912 AllocaInst* pa = new AllocaInst(Type::Int32Ty, 0, "indexLoc");
1914 where ``indexLoc`` is now the logical name of the instruction's execution value,
1915 which is a pointer to an integer on the run time stack.
1917 *Inserting instructions*
1919 There are essentially three ways to insert an ``Instruction`` into an existing
1920 sequence of instructions that form a ``BasicBlock``:
1922 * Insertion into an explicit instruction list
1924 Given a ``BasicBlock* pb``, an ``Instruction* pi`` within that ``BasicBlock``,
1925 and a newly-created instruction we wish to insert before ``*pi``, we do the
1930 BasicBlock *pb = ...;
1931 Instruction *pi = ...;
1932 Instruction *newInst = new Instruction(...);
1934 pb->getInstList().insert(pi, newInst); // Inserts newInst before pi in pb
1936 Appending to the end of a ``BasicBlock`` is so common that the ``Instruction``
1937 class and ``Instruction``-derived classes provide constructors which take a
1938 pointer to a ``BasicBlock`` to be appended to. For example code that looked
1943 BasicBlock *pb = ...;
1944 Instruction *newInst = new Instruction(...);
1946 pb->getInstList().push_back(newInst); // Appends newInst to pb
1952 BasicBlock *pb = ...;
1953 Instruction *newInst = new Instruction(..., pb);
1955 which is much cleaner, especially if you are creating long instruction
1958 * Insertion into an implicit instruction list
1960 ``Instruction`` instances that are already in ``BasicBlock``\ s are implicitly
1961 associated with an existing instruction list: the instruction list of the
1962 enclosing basic block. Thus, we could have accomplished the same thing as the
1963 above code without being given a ``BasicBlock`` by doing:
1967 Instruction *pi = ...;
1968 Instruction *newInst = new Instruction(...);
1970 pi->getParent()->getInstList().insert(pi, newInst);
1972 In fact, this sequence of steps occurs so frequently that the ``Instruction``
1973 class and ``Instruction``-derived classes provide constructors which take (as
1974 a default parameter) a pointer to an ``Instruction`` which the newly-created
1975 ``Instruction`` should precede. That is, ``Instruction`` constructors are
1976 capable of inserting the newly-created instance into the ``BasicBlock`` of a
1977 provided instruction, immediately before that instruction. Using an
1978 ``Instruction`` constructor with a ``insertBefore`` (default) parameter, the
1983 Instruction* pi = ...;
1984 Instruction* newInst = new Instruction(..., pi);
1986 which is much cleaner, especially if you're creating a lot of instructions and
1987 adding them to ``BasicBlock``\ s.
1989 * Insertion using an instance of ``IRBuilder``
1991 Inserting several ``Instruction``\ s can be quite laborious using the previous
1992 methods. The ``IRBuilder`` is a convenience class that can be used to add
1993 several instructions to the end of a ``BasicBlock`` or before a particular
1994 ``Instruction``. It also supports constant folding and renaming named
1995 registers (see ``IRBuilder``'s template arguments).
1997 The example below demonstrates a very simple use of the ``IRBuilder`` where
1998 three instructions are inserted before the instruction ``pi``. The first two
1999 instructions are Call instructions and third instruction multiplies the return
2000 value of the two calls.
2004 Instruction *pi = ...;
2005 IRBuilder<> Builder(pi);
2006 CallInst* callOne = Builder.CreateCall(...);
2007 CallInst* callTwo = Builder.CreateCall(...);
2008 Value* result = Builder.CreateMul(callOne, callTwo);
2010 The example below is similar to the above example except that the created
2011 ``IRBuilder`` inserts instructions at the end of the ``BasicBlock`` ``pb``.
2015 BasicBlock *pb = ...;
2016 IRBuilder<> Builder(pb);
2017 CallInst* callOne = Builder.CreateCall(...);
2018 CallInst* callTwo = Builder.CreateCall(...);
2019 Value* result = Builder.CreateMul(callOne, callTwo);
2021 See :doc:`tutorial/LangImpl3` for a practical use of the ``IRBuilder``.
2024 .. _schanges_deleting:
2026 Deleting Instructions
2027 ^^^^^^^^^^^^^^^^^^^^^
2029 Deleting an instruction from an existing sequence of instructions that form a
2030 BasicBlock_ is very straight-forward: just call the instruction's
2031 ``eraseFromParent()`` method. For example:
2035 Instruction *I = .. ;
2036 I->eraseFromParent();
2038 This unlinks the instruction from its containing basic block and deletes it. If
2039 you'd just like to unlink the instruction from its containing basic block but
2040 not delete it, you can use the ``removeFromParent()`` method.
2042 .. _schanges_replacing:
2044 Replacing an Instruction with another Value
2045 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
2047 Replacing individual instructions
2048 """""""""""""""""""""""""""""""""
2050 Including "`llvm/Transforms/Utils/BasicBlockUtils.h
2051 <http://llvm.org/doxygen/BasicBlockUtils_8h-source.html>`_" permits use of two
2052 very useful replace functions: ``ReplaceInstWithValue`` and
2053 ``ReplaceInstWithInst``.
2055 .. _schanges_deleting_sub:
2057 Deleting Instructions
2058 """""""""""""""""""""
2060 * ``ReplaceInstWithValue``
2062 This function replaces all uses of a given instruction with a value, and then
2063 removes the original instruction. The following example illustrates the
2064 replacement of the result of a particular ``AllocaInst`` that allocates memory
2065 for a single integer with a null pointer to an integer.
2069 AllocaInst* instToReplace = ...;
2070 BasicBlock::iterator ii(instToReplace);
2072 ReplaceInstWithValue(instToReplace->getParent()->getInstList(), ii,
2073 Constant::getNullValue(PointerType::getUnqual(Type::Int32Ty)));
2075 * ``ReplaceInstWithInst``
2077 This function replaces a particular instruction with another instruction,
2078 inserting the new instruction into the basic block at the location where the
2079 old instruction was, and replacing any uses of the old instruction with the
2080 new instruction. The following example illustrates the replacement of one
2081 ``AllocaInst`` with another.
2085 AllocaInst* instToReplace = ...;
2086 BasicBlock::iterator ii(instToReplace);
2088 ReplaceInstWithInst(instToReplace->getParent()->getInstList(), ii,
2089 new AllocaInst(Type::Int32Ty, 0, "ptrToReplacedInt"));
2092 Replacing multiple uses of Users and Values
2093 """""""""""""""""""""""""""""""""""""""""""
2095 You can use ``Value::replaceAllUsesWith`` and ``User::replaceUsesOfWith`` to
2096 change more than one use at a time. See the doxygen documentation for the
2097 `Value Class <http://llvm.org/doxygen/classllvm_1_1Value.html>`_ and `User Class
2098 <http://llvm.org/doxygen/classllvm_1_1User.html>`_, respectively, for more
2101 .. _schanges_deletingGV:
2103 Deleting GlobalVariables
2104 ^^^^^^^^^^^^^^^^^^^^^^^^
2106 Deleting a global variable from a module is just as easy as deleting an
2107 Instruction. First, you must have a pointer to the global variable that you
2108 wish to delete. You use this pointer to erase it from its parent, the module.
2113 GlobalVariable *GV = .. ;
2115 GV->eraseFromParent();
2123 In generating IR, you may need some complex types. If you know these types
2124 statically, you can use ``TypeBuilder<...>::get()``, defined in
2125 ``llvm/Support/TypeBuilder.h``, to retrieve them. ``TypeBuilder`` has two forms
2126 depending on whether you're building types for cross-compilation or native
2127 library use. ``TypeBuilder<T, true>`` requires that ``T`` be independent of the
2128 host environment, meaning that it's built out of types from the ``llvm::types``
2129 (`doxygen <http://llvm.org/doxygen/namespacellvm_1_1types.html>`__) namespace
2130 and pointers, functions, arrays, etc. built of those. ``TypeBuilder<T, false>``
2131 additionally allows native C types whose size may depend on the host compiler.
2136 FunctionType *ft = TypeBuilder<types::i<8>(types::i<32>*), true>::get();
2138 is easier to read and write than the equivalent
2142 std::vector<const Type*> params;
2143 params.push_back(PointerType::getUnqual(Type::Int32Ty));
2144 FunctionType *ft = FunctionType::get(Type::Int8Ty, params, false);
2146 See the `class comment
2147 <http://llvm.org/doxygen/TypeBuilder_8h-source.html#l00001>`_ for more details.
2154 This section describes the interaction of the LLVM APIs with multithreading,
2155 both on the part of client applications, and in the JIT, in the hosted
2158 Note that LLVM's support for multithreading is still relatively young. Up
2159 through version 2.5, the execution of threaded hosted applications was
2160 supported, but not threaded client access to the APIs. While this use case is
2161 now supported, clients *must* adhere to the guidelines specified below to ensure
2162 proper operation in multithreaded mode.
2164 Note that, on Unix-like platforms, LLVM requires the presence of GCC's atomic
2165 intrinsics in order to support threaded operation. If you need a
2166 multhreading-capable LLVM on a platform without a suitably modern system
2167 compiler, consider compiling LLVM and LLVM-GCC in single-threaded mode, and
2168 using the resultant compiler to build a copy of LLVM with multithreading
2173 Ending Execution with ``llvm_shutdown()``
2174 -----------------------------------------
2176 When you are done using the LLVM APIs, you should call ``llvm_shutdown()`` to
2177 deallocate memory used for internal structures.
2181 Lazy Initialization with ``ManagedStatic``
2182 ------------------------------------------
2184 ``ManagedStatic`` is a utility class in LLVM used to implement static
2185 initialization of static resources, such as the global type tables. In a
2186 single-threaded environment, it implements a simple lazy initialization scheme.
2187 When LLVM is compiled with support for multi-threading, however, it uses
2188 double-checked locking to implement thread-safe lazy initialization.
2192 Achieving Isolation with ``LLVMContext``
2193 ----------------------------------------
2195 ``LLVMContext`` is an opaque class in the LLVM API which clients can use to
2196 operate multiple, isolated instances of LLVM concurrently within the same
2197 address space. For instance, in a hypothetical compile-server, the compilation
2198 of an individual translation unit is conceptually independent from all the
2199 others, and it would be desirable to be able to compile incoming translation
2200 units concurrently on independent server threads. Fortunately, ``LLVMContext``
2201 exists to enable just this kind of scenario!
2203 Conceptually, ``LLVMContext`` provides isolation. Every LLVM entity
2204 (``Module``\ s, ``Value``\ s, ``Type``\ s, ``Constant``\ s, etc.) in LLVM's
2205 in-memory IR belongs to an ``LLVMContext``. Entities in different contexts
2206 *cannot* interact with each other: ``Module``\ s in different contexts cannot be
2207 linked together, ``Function``\ s cannot be added to ``Module``\ s in different
2208 contexts, etc. What this means is that is is safe to compile on multiple
2209 threads simultaneously, as long as no two threads operate on entities within the
2212 In practice, very few places in the API require the explicit specification of a
2213 ``LLVMContext``, other than the ``Type`` creation/lookup APIs. Because every
2214 ``Type`` carries a reference to its owning context, most other entities can
2215 determine what context they belong to by looking at their own ``Type``. If you
2216 are adding new entities to LLVM IR, please try to maintain this interface
2219 For clients that do *not* require the benefits of isolation, LLVM provides a
2220 convenience API ``getGlobalContext()``. This returns a global, lazily
2221 initialized ``LLVMContext`` that may be used in situations where isolation is
2229 LLVM's "eager" JIT compiler is safe to use in threaded programs. Multiple
2230 threads can call ``ExecutionEngine::getPointerToFunction()`` or
2231 ``ExecutionEngine::runFunction()`` concurrently, and multiple threads can run
2232 code output by the JIT concurrently. The user must still ensure that only one
2233 thread accesses IR in a given ``LLVMContext`` while another thread might be
2234 modifying it. One way to do that is to always hold the JIT lock while accessing
2235 IR outside the JIT (the JIT *modifies* the IR by adding ``CallbackVH``\ s).
2236 Another way is to only call ``getPointerToFunction()`` from the
2237 ``LLVMContext``'s thread.
2239 When the JIT is configured to compile lazily (using
2240 ``ExecutionEngine::DisableLazyCompilation(false)``), there is currently a `race
2241 condition <http://llvm.org/bugs/show_bug.cgi?id=5184>`_ in updating call sites
2242 after a function is lazily-jitted. It's still possible to use the lazy JIT in a
2243 threaded program if you ensure that only one thread at a time can call any
2244 particular lazy stub and that the JIT lock guards any IR access, but we suggest
2245 using only the eager JIT in threaded programs.
2252 This section describes some of the advanced or obscure API's that most clients
2253 do not need to be aware of. These API's tend manage the inner workings of the
2254 LLVM system, and only need to be accessed in unusual circumstances.
2258 The ``ValueSymbolTable`` class
2259 ------------------------------
2261 The ``ValueSymbolTable`` (`doxygen
2262 <http://llvm.org/doxygen/classllvm_1_1ValueSymbolTable.html>`__) class provides
2263 a symbol table that the :ref:`Function <c_Function>` and Module_ classes use for
2264 naming value definitions. The symbol table can provide a name for any Value_.
2266 Note that the ``SymbolTable`` class should not be directly accessed by most
2267 clients. It should only be used when iteration over the symbol table names
2268 themselves are required, which is very special purpose. Note that not all LLVM
2269 Value_\ s have names, and those without names (i.e. they have an empty name) do
2270 not exist in the symbol table.
2272 Symbol tables support iteration over the values in the symbol table with
2273 ``begin/end/iterator`` and supports querying to see if a specific name is in the
2274 symbol table (with ``lookup``). The ``ValueSymbolTable`` class exposes no
2275 public mutator methods, instead, simply call ``setName`` on a value, which will
2276 autoinsert it into the appropriate symbol table.
2280 The ``User`` and owned ``Use`` classes' memory layout
2281 -----------------------------------------------------
2283 The ``User`` (`doxygen <http://llvm.org/doxygen/classllvm_1_1User.html>`__)
2284 class provides a basis for expressing the ownership of ``User`` towards other
2285 `Value instance <http://llvm.org/doxygen/classllvm_1_1Value.html>`_\ s. The
2286 ``Use`` (`doxygen <http://llvm.org/doxygen/classllvm_1_1Use.html>`__) helper
2287 class is employed to do the bookkeeping and to facilitate *O(1)* addition and
2292 Interaction and relationship between ``User`` and ``Use`` objects
2293 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
2295 A subclass of ``User`` can choose between incorporating its ``Use`` objects or
2296 refer to them out-of-line by means of a pointer. A mixed variant (some ``Use``
2297 s inline others hung off) is impractical and breaks the invariant that the
2298 ``Use`` objects belonging to the same ``User`` form a contiguous array.
2300 We have 2 different layouts in the ``User`` (sub)classes:
2304 The ``Use`` object(s) are inside (resp. at fixed offset) of the ``User``
2305 object and there are a fixed number of them.
2309 The ``Use`` object(s) are referenced by a pointer to an array from the
2310 ``User`` object and there may be a variable number of them.
2312 As of v2.4 each layout still possesses a direct pointer to the start of the
2313 array of ``Use``\ s. Though not mandatory for layout a), we stick to this
2314 redundancy for the sake of simplicity. The ``User`` object also stores the
2315 number of ``Use`` objects it has. (Theoretically this information can also be
2316 calculated given the scheme presented below.)
2318 Special forms of allocation operators (``operator new``) enforce the following
2321 * Layout a) is modelled by prepending the ``User`` object by the ``Use[]``
2324 .. code-block:: none
2326 ...---.---.---.---.-------...
2327 | P | P | P | P | User
2328 '''---'---'---'---'-------'''
2330 * Layout b) is modelled by pointing at the ``Use[]`` array.
2332 .. code-block:: none
2343 *(In the above figures* '``P``' *stands for the* ``Use**`` *that is stored in
2344 each* ``Use`` *object in the member* ``Use::Prev`` *)*
2348 The waymarking algorithm
2349 ^^^^^^^^^^^^^^^^^^^^^^^^
2351 Since the ``Use`` objects are deprived of the direct (back)pointer to their
2352 ``User`` objects, there must be a fast and exact method to recover it. This is
2353 accomplished by the following scheme:
2355 A bit-encoding in the 2 LSBits (least significant bits) of the ``Use::Prev``
2356 allows to find the start of the ``User`` object:
2358 * ``00`` --- binary digit 0
2360 * ``01`` --- binary digit 1
2362 * ``10`` --- stop and calculate (``s``)
2364 * ``11`` --- full stop (``S``)
2366 Given a ``Use*``, all we have to do is to walk till we get a stop and we either
2367 have a ``User`` immediately behind or we have to walk to the next stop picking
2368 up digits and calculating the offset:
2370 .. code-block:: none
2372 .---.---.---.---.---.---.---.---.---.---.---.---.---.---.---.---.----------------
2373 | 1 | s | 1 | 0 | 1 | 0 | s | 1 | 1 | 0 | s | 1 | 1 | s | 1 | S | User (or User*)
2374 '---'---'---'---'---'---'---'---'---'---'---'---'---'---'---'---'----------------
2375 |+15 |+10 |+6 |+3 |+1
2378 | | | ______________________>
2379 | | ______________________________________>
2380 | __________________________________________________________>
2382 Only the significant number of bits need to be stored between the stops, so that
2383 the *worst case is 20 memory accesses* when there are 1000 ``Use`` objects
2384 associated with a ``User``.
2388 Reference implementation
2389 ^^^^^^^^^^^^^^^^^^^^^^^^
2391 The following literate Haskell fragment demonstrates the concept:
2393 .. code-block:: haskell
2395 > import Test.QuickCheck
2397 > digits :: Int -> [Char] -> [Char]
2398 > digits 0 acc = '0' : acc
2399 > digits 1 acc = '1' : acc
2400 > digits n acc = digits (n `div` 2) $ digits (n `mod` 2) acc
2402 > dist :: Int -> [Char] -> [Char]
2405 > dist 1 acc = let r = dist 0 acc in 's' : digits (length r) r
2406 > dist n acc = dist (n - 1) $ dist 1 acc
2408 > takeLast n ss = reverse $ take n $ reverse ss
2410 > test = takeLast 40 $ dist 20 []
2413 Printing <test> gives: ``"1s100000s11010s10100s1111s1010s110s11s1S"``
2415 The reverse algorithm computes the length of the string just by examining a
2418 .. code-block:: haskell
2420 > pref :: [Char] -> Int
2422 > pref ('s':'1':rest) = decode 2 1 rest
2423 > pref (_:rest) = 1 + pref rest
2425 > decode walk acc ('0':rest) = decode (walk + 1) (acc * 2) rest
2426 > decode walk acc ('1':rest) = decode (walk + 1) (acc * 2 + 1) rest
2427 > decode walk acc _ = walk + acc
2430 Now, as expected, printing <pref test> gives ``40``.
2432 We can *quickCheck* this with following property:
2434 .. code-block:: haskell
2436 > testcase = dist 2000 []
2437 > testcaseLength = length testcase
2439 > identityProp n = n > 0 && n <= testcaseLength ==> length arr == pref arr
2440 > where arr = takeLast n testcase
2443 As expected <quickCheck identityProp> gives:
2447 *Main> quickCheck identityProp
2448 OK, passed 100 tests.
2450 Let's be a bit more exhaustive:
2452 .. code-block:: haskell
2455 > deepCheck p = check (defaultConfig { configMaxTest = 500 }) p
2458 And here is the result of <deepCheck identityProp>:
2462 *Main> deepCheck identityProp
2463 OK, passed 500 tests.
2467 Tagging considerations
2468 ^^^^^^^^^^^^^^^^^^^^^^
2470 To maintain the invariant that the 2 LSBits of each ``Use**`` in ``Use`` never
2471 change after being set up, setters of ``Use::Prev`` must re-tag the new
2472 ``Use**`` on every modification. Accordingly getters must strip the tag bits.
2474 For layout b) instead of the ``User`` we find a pointer (``User*`` with LSBit
2475 set). Following this pointer brings us to the ``User``. A portable trick
2476 ensures that the first bytes of ``User`` (if interpreted as a pointer) never has
2477 the LSBit set. (Portability is relying on the fact that all known compilers
2478 place the ``vptr`` in the first word of the instances.)
2482 The Core LLVM Class Hierarchy Reference
2483 =======================================
2485 ``#include "llvm/IR/Type.h"``
2487 header source: `Type.h <http://llvm.org/doxygen/Type_8h-source.html>`_
2489 doxygen info: `Type Clases <http://llvm.org/doxygen/classllvm_1_1Type.html>`_
2491 The Core LLVM classes are the primary means of representing the program being
2492 inspected or transformed. The core LLVM classes are defined in header files in
2493 the ``include/llvm/`` directory, and implemented in the ``lib/VMCore``
2498 The Type class and Derived Types
2499 --------------------------------
2501 ``Type`` is a superclass of all type classes. Every ``Value`` has a ``Type``.
2502 ``Type`` cannot be instantiated directly but only through its subclasses.
2503 Certain primitive types (``VoidType``, ``LabelType``, ``FloatType`` and
2504 ``DoubleType``) have hidden subclasses. They are hidden because they offer no
2505 useful functionality beyond what the ``Type`` class offers except to distinguish
2506 themselves from other subclasses of ``Type``.
2508 All other types are subclasses of ``DerivedType``. Types can be named, but this
2509 is not a requirement. There exists exactly one instance of a given shape at any
2510 one time. This allows type equality to be performed with address equality of
2511 the Type Instance. That is, given two ``Type*`` values, the types are identical
2512 if the pointers are identical.
2516 Important Public Methods
2517 ^^^^^^^^^^^^^^^^^^^^^^^^
2519 * ``bool isIntegerTy() const``: Returns true for any integer type.
2521 * ``bool isFloatingPointTy()``: Return true if this is one of the five
2522 floating point types.
2524 * ``bool isSized()``: Return true if the type has known size. Things
2525 that don't have a size are abstract types, labels and void.
2529 Important Derived Types
2530 ^^^^^^^^^^^^^^^^^^^^^^^
2533 Subclass of DerivedType that represents integer types of any bit width. Any
2534 bit width between ``IntegerType::MIN_INT_BITS`` (1) and
2535 ``IntegerType::MAX_INT_BITS`` (~8 million) can be represented.
2537 * ``static const IntegerType* get(unsigned NumBits)``: get an integer
2538 type of a specific bit width.
2540 * ``unsigned getBitWidth() const``: Get the bit width of an integer type.
2543 This is subclassed by ArrayType, PointerType and VectorType.
2545 * ``const Type * getElementType() const``: Returns the type of each
2546 of the elements in the sequential type.
2549 This is a subclass of SequentialType and defines the interface for array
2552 * ``unsigned getNumElements() const``: Returns the number of elements
2556 Subclass of SequentialType for pointer types.
2559 Subclass of SequentialType for vector types. A vector type is similar to an
2560 ArrayType but is distinguished because it is a first class type whereas
2561 ArrayType is not. Vector types are used for vector operations and are usually
2562 small vectors of of an integer or floating point type.
2565 Subclass of DerivedTypes for struct types.
2570 Subclass of DerivedTypes for function types.
2572 * ``bool isVarArg() const``: Returns true if it's a vararg function.
2574 * ``const Type * getReturnType() const``: Returns the return type of the
2577 * ``const Type * getParamType (unsigned i)``: Returns the type of the ith
2580 * ``const unsigned getNumParams() const``: Returns the number of formal
2585 The ``Module`` class
2586 --------------------
2588 ``#include "llvm/IR/Module.h"``
2590 header source: `Module.h <http://llvm.org/doxygen/Module_8h-source.html>`_
2592 doxygen info: `Module Class <http://llvm.org/doxygen/classllvm_1_1Module.html>`_
2594 The ``Module`` class represents the top level structure present in LLVM
2595 programs. An LLVM module is effectively either a translation unit of the
2596 original program or a combination of several translation units merged by the
2597 linker. The ``Module`` class keeps track of a list of :ref:`Function
2598 <c_Function>`\ s, a list of GlobalVariable_\ s, and a SymbolTable_.
2599 Additionally, it contains a few helpful member functions that try to make common
2604 Important Public Members of the ``Module`` class
2605 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
2607 * ``Module::Module(std::string name = "")``
2609 Constructing a Module_ is easy. You can optionally provide a name for it
2610 (probably based on the name of the translation unit).
2612 * | ``Module::iterator`` - Typedef for function list iterator
2613 | ``Module::const_iterator`` - Typedef for const_iterator.
2614 | ``begin()``, ``end()``, ``size()``, ``empty()``
2616 These are forwarding methods that make it easy to access the contents of a
2617 ``Module`` object's :ref:`Function <c_Function>` list.
2619 * ``Module::FunctionListType &getFunctionList()``
2621 Returns the list of :ref:`Function <c_Function>`\ s. This is necessary to use
2622 when you need to update the list or perform a complex action that doesn't have
2623 a forwarding method.
2627 * | ``Module::global_iterator`` - Typedef for global variable list iterator
2628 | ``Module::const_global_iterator`` - Typedef for const_iterator.
2629 | ``global_begin()``, ``global_end()``, ``global_size()``, ``global_empty()``
2631 These are forwarding methods that make it easy to access the contents of a
2632 ``Module`` object's GlobalVariable_ list.
2634 * ``Module::GlobalListType &getGlobalList()``
2636 Returns the list of GlobalVariable_\ s. This is necessary to use when you
2637 need to update the list or perform a complex action that doesn't have a
2642 * ``SymbolTable *getSymbolTable()``
2644 Return a reference to the SymbolTable_ for this ``Module``.
2648 * ``Function *getFunction(StringRef Name) const``
2650 Look up the specified function in the ``Module`` SymbolTable_. If it does not
2651 exist, return ``null``.
2653 * ``Function *getOrInsertFunction(const std::string &Name, const FunctionType
2656 Look up the specified function in the ``Module`` SymbolTable_. If it does not
2657 exist, add an external declaration for the function and return it.
2659 * ``std::string getTypeName(const Type *Ty)``
2661 If there is at least one entry in the SymbolTable_ for the specified Type_,
2662 return it. Otherwise return the empty string.
2664 * ``bool addTypeName(const std::string &Name, const Type *Ty)``
2666 Insert an entry in the SymbolTable_ mapping ``Name`` to ``Ty``. If there is
2667 already an entry for this name, true is returned and the SymbolTable_ is not
2675 ``#include "llvm/IR/Value.h"``
2677 header source: `Value.h <http://llvm.org/doxygen/Value_8h-source.html>`_
2679 doxygen info: `Value Class <http://llvm.org/doxygen/classllvm_1_1Value.html>`_
2681 The ``Value`` class is the most important class in the LLVM Source base. It
2682 represents a typed value that may be used (among other things) as an operand to
2683 an instruction. There are many different types of ``Value``\ s, such as
2684 Constant_\ s, Argument_\ s. Even Instruction_\ s and :ref:`Function
2685 <c_Function>`\ s are ``Value``\ s.
2687 A particular ``Value`` may be used many times in the LLVM representation for a
2688 program. For example, an incoming argument to a function (represented with an
2689 instance of the Argument_ class) is "used" by every instruction in the function
2690 that references the argument. To keep track of this relationship, the ``Value``
2691 class keeps a list of all of the ``User``\ s that is using it (the User_ class
2692 is a base class for all nodes in the LLVM graph that can refer to ``Value``\ s).
2693 This use list is how LLVM represents def-use information in the program, and is
2694 accessible through the ``use_*`` methods, shown below.
2696 Because LLVM is a typed representation, every LLVM ``Value`` is typed, and this
2697 Type_ is available through the ``getType()`` method. In addition, all LLVM
2698 values can be named. The "name" of the ``Value`` is a symbolic string printed
2701 .. code-block:: llvm
2707 The name of this instruction is "foo". **NOTE** that the name of any value may
2708 be missing (an empty string), so names should **ONLY** be used for debugging
2709 (making the source code easier to read, debugging printouts), they should not be
2710 used to keep track of values or map between them. For this purpose, use a
2711 ``std::map`` of pointers to the ``Value`` itself instead.
2713 One important aspect of LLVM is that there is no distinction between an SSA
2714 variable and the operation that produces it. Because of this, any reference to
2715 the value produced by an instruction (or the value available as an incoming
2716 argument, for example) is represented as a direct pointer to the instance of the
2717 class that represents this value. Although this may take some getting used to,
2718 it simplifies the representation and makes it easier to manipulate.
2722 Important Public Members of the ``Value`` class
2723 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
2725 * | ``Value::use_iterator`` - Typedef for iterator over the use-list
2726 | ``Value::const_use_iterator`` - Typedef for const_iterator over the
2728 | ``unsigned use_size()`` - Returns the number of users of the value.
2729 | ``bool use_empty()`` - Returns true if there are no users.
2730 | ``use_iterator use_begin()`` - Get an iterator to the start of the
2732 | ``use_iterator use_end()`` - Get an iterator to the end of the use-list.
2733 | ``User *use_back()`` - Returns the last element in the list.
2735 These methods are the interface to access the def-use information in LLVM.
2736 As with all other iterators in LLVM, the naming conventions follow the
2737 conventions defined by the STL_.
2739 * ``Type *getType() const``
2740 This method returns the Type of the Value.
2742 * | ``bool hasName() const``
2743 | ``std::string getName() const``
2744 | ``void setName(const std::string &Name)``
2746 This family of methods is used to access and assign a name to a ``Value``, be
2747 aware of the :ref:`precaution above <nameWarning>`.
2749 * ``void replaceAllUsesWith(Value *V)``
2751 This method traverses the use list of a ``Value`` changing all User_\ s of the
2752 current value to refer to "``V``" instead. For example, if you detect that an
2753 instruction always produces a constant value (for example through constant
2754 folding), you can replace all uses of the instruction with the constant like
2759 Inst->replaceAllUsesWith(ConstVal);
2766 ``#include "llvm/IR/User.h"``
2768 header source: `User.h <http://llvm.org/doxygen/User_8h-source.html>`_
2770 doxygen info: `User Class <http://llvm.org/doxygen/classllvm_1_1User.html>`_
2774 The ``User`` class is the common base class of all LLVM nodes that may refer to
2775 ``Value``\ s. It exposes a list of "Operands" that are all of the ``Value``\ s
2776 that the User is referring to. The ``User`` class itself is a subclass of
2779 The operands of a ``User`` point directly to the LLVM ``Value`` that it refers
2780 to. Because LLVM uses Static Single Assignment (SSA) form, there can only be
2781 one definition referred to, allowing this direct connection. This connection
2782 provides the use-def information in LLVM.
2786 Important Public Members of the ``User`` class
2787 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
2789 The ``User`` class exposes the operand list in two ways: through an index access
2790 interface and through an iterator based interface.
2792 * | ``Value *getOperand(unsigned i)``
2793 | ``unsigned getNumOperands()``
2795 These two methods expose the operands of the ``User`` in a convenient form for
2798 * | ``User::op_iterator`` - Typedef for iterator over the operand list
2799 | ``op_iterator op_begin()`` - Get an iterator to the start of the operand
2801 | ``op_iterator op_end()`` - Get an iterator to the end of the operand list.
2803 Together, these methods make up the iterator based interface to the operands
2809 The ``Instruction`` class
2810 -------------------------
2812 ``#include "llvm/IR/Instruction.h"``
2814 header source: `Instruction.h
2815 <http://llvm.org/doxygen/Instruction_8h-source.html>`_
2817 doxygen info: `Instruction Class
2818 <http://llvm.org/doxygen/classllvm_1_1Instruction.html>`_
2820 Superclasses: User_, Value_
2822 The ``Instruction`` class is the common base class for all LLVM instructions.
2823 It provides only a few methods, but is a very commonly used class. The primary
2824 data tracked by the ``Instruction`` class itself is the opcode (instruction
2825 type) and the parent BasicBlock_ the ``Instruction`` is embedded into. To
2826 represent a specific type of instruction, one of many subclasses of
2827 ``Instruction`` are used.
2829 Because the ``Instruction`` class subclasses the User_ class, its operands can
2830 be accessed in the same way as for other ``User``\ s (with the
2831 ``getOperand()``/``getNumOperands()`` and ``op_begin()``/``op_end()`` methods).
2832 An important file for the ``Instruction`` class is the ``llvm/Instruction.def``
2833 file. This file contains some meta-data about the various different types of
2834 instructions in LLVM. It describes the enum values that are used as opcodes
2835 (for example ``Instruction::Add`` and ``Instruction::ICmp``), as well as the
2836 concrete sub-classes of ``Instruction`` that implement the instruction (for
2837 example BinaryOperator_ and CmpInst_). Unfortunately, the use of macros in this
2838 file confuses doxygen, so these enum values don't show up correctly in the
2839 `doxygen output <http://llvm.org/doxygen/classllvm_1_1Instruction.html>`_.
2843 Important Subclasses of the ``Instruction`` class
2844 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
2848 * ``BinaryOperator``
2850 This subclasses represents all two operand instructions whose operands must be
2851 the same type, except for the comparison instructions.
2856 This subclass is the parent of the 12 casting instructions. It provides
2857 common operations on cast instructions.
2863 This subclass respresents the two comparison instructions,
2864 `ICmpInst <LangRef.html#i_icmp>`_ (integer opreands), and
2865 `FCmpInst <LangRef.html#i_fcmp>`_ (floating point operands).
2869 * ``TerminatorInst``
2871 This subclass is the parent of all terminator instructions (those which can
2876 Important Public Members of the ``Instruction`` class
2877 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
2879 * ``BasicBlock *getParent()``
2881 Returns the BasicBlock_ that this
2882 ``Instruction`` is embedded into.
2884 * ``bool mayWriteToMemory()``
2886 Returns true if the instruction writes to memory, i.e. it is a ``call``,
2887 ``free``, ``invoke``, or ``store``.
2889 * ``unsigned getOpcode()``
2891 Returns the opcode for the ``Instruction``.
2893 * ``Instruction *clone() const``
2895 Returns another instance of the specified instruction, identical in all ways
2896 to the original except that the instruction has no parent (i.e. it's not
2897 embedded into a BasicBlock_), and it has no name.
2901 The ``Constant`` class and subclasses
2902 -------------------------------------
2904 Constant represents a base class for different types of constants. It is
2905 subclassed by ConstantInt, ConstantArray, etc. for representing the various
2906 types of Constants. GlobalValue_ is also a subclass, which represents the
2907 address of a global variable or function.
2911 Important Subclasses of Constant
2912 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
2914 * ConstantInt : This subclass of Constant represents an integer constant of
2917 * ``const APInt& getValue() const``: Returns the underlying
2918 value of this constant, an APInt value.
2920 * ``int64_t getSExtValue() const``: Converts the underlying APInt value to an
2921 int64_t via sign extension. If the value (not the bit width) of the APInt
2922 is too large to fit in an int64_t, an assertion will result. For this
2923 reason, use of this method is discouraged.
2925 * ``uint64_t getZExtValue() const``: Converts the underlying APInt value
2926 to a uint64_t via zero extension. IF the value (not the bit width) of the
2927 APInt is too large to fit in a uint64_t, an assertion will result. For this
2928 reason, use of this method is discouraged.
2930 * ``static ConstantInt* get(const APInt& Val)``: Returns the ConstantInt
2931 object that represents the value provided by ``Val``. The type is implied
2932 as the IntegerType that corresponds to the bit width of ``Val``.
2934 * ``static ConstantInt* get(const Type *Ty, uint64_t Val)``: Returns the
2935 ConstantInt object that represents the value provided by ``Val`` for integer
2938 * ConstantFP : This class represents a floating point constant.
2940 * ``double getValue() const``: Returns the underlying value of this constant.
2942 * ConstantArray : This represents a constant array.
2944 * ``const std::vector<Use> &getValues() const``: Returns a vector of
2945 component constants that makeup this array.
2947 * ConstantStruct : This represents a constant struct.
2949 * ``const std::vector<Use> &getValues() const``: Returns a vector of
2950 component constants that makeup this array.
2952 * GlobalValue : This represents either a global variable or a function. In
2953 either case, the value is a constant fixed address (after linking).
2957 The ``GlobalValue`` class
2958 -------------------------
2960 ``#include "llvm/IR/GlobalValue.h"``
2962 header source: `GlobalValue.h
2963 <http://llvm.org/doxygen/GlobalValue_8h-source.html>`_
2965 doxygen info: `GlobalValue Class
2966 <http://llvm.org/doxygen/classllvm_1_1GlobalValue.html>`_
2968 Superclasses: Constant_, User_, Value_
2970 Global values ( GlobalVariable_\ s or :ref:`Function <c_Function>`\ s) are the
2971 only LLVM values that are visible in the bodies of all :ref:`Function
2972 <c_Function>`\ s. Because they are visible at global scope, they are also
2973 subject to linking with other globals defined in different translation units.
2974 To control the linking process, ``GlobalValue``\ s know their linkage rules.
2975 Specifically, ``GlobalValue``\ s know whether they have internal or external
2976 linkage, as defined by the ``LinkageTypes`` enumeration.
2978 If a ``GlobalValue`` has internal linkage (equivalent to being ``static`` in C),
2979 it is not visible to code outside the current translation unit, and does not
2980 participate in linking. If it has external linkage, it is visible to external
2981 code, and does participate in linking. In addition to linkage information,
2982 ``GlobalValue``\ s keep track of which Module_ they are currently part of.
2984 Because ``GlobalValue``\ s are memory objects, they are always referred to by
2985 their **address**. As such, the Type_ of a global is always a pointer to its
2986 contents. It is important to remember this when using the ``GetElementPtrInst``
2987 instruction because this pointer must be dereferenced first. For example, if
2988 you have a ``GlobalVariable`` (a subclass of ``GlobalValue)`` that is an array
2989 of 24 ints, type ``[24 x i32]``, then the ``GlobalVariable`` is a pointer to
2990 that array. Although the address of the first element of this array and the
2991 value of the ``GlobalVariable`` are the same, they have different types. The
2992 ``GlobalVariable``'s type is ``[24 x i32]``. The first element's type is
2993 ``i32.`` Because of this, accessing a global value requires you to dereference
2994 the pointer with ``GetElementPtrInst`` first, then its elements can be accessed.
2995 This is explained in the `LLVM Language Reference Manual
2996 <LangRef.html#globalvars>`_.
3000 Important Public Members of the ``GlobalValue`` class
3001 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
3003 * | ``bool hasInternalLinkage() const``
3004 | ``bool hasExternalLinkage() const``
3005 | ``void setInternalLinkage(bool HasInternalLinkage)``
3007 These methods manipulate the linkage characteristics of the ``GlobalValue``.
3009 * ``Module *getParent()``
3011 This returns the Module_ that the
3012 GlobalValue is currently embedded into.
3016 The ``Function`` class
3017 ----------------------
3019 ``#include "llvm/IR/Function.h"``
3021 header source: `Function.h <http://llvm.org/doxygen/Function_8h-source.html>`_
3023 doxygen info: `Function Class
3024 <http://llvm.org/doxygen/classllvm_1_1Function.html>`_
3026 Superclasses: GlobalValue_, Constant_, User_, Value_
3028 The ``Function`` class represents a single procedure in LLVM. It is actually
3029 one of the more complex classes in the LLVM hierarchy because it must keep track
3030 of a large amount of data. The ``Function`` class keeps track of a list of
3031 BasicBlock_\ s, a list of formal Argument_\ s, and a SymbolTable_.
3033 The list of BasicBlock_\ s is the most commonly used part of ``Function``
3034 objects. The list imposes an implicit ordering of the blocks in the function,
3035 which indicate how the code will be laid out by the backend. Additionally, the
3036 first BasicBlock_ is the implicit entry node for the ``Function``. It is not
3037 legal in LLVM to explicitly branch to this initial block. There are no implicit
3038 exit nodes, and in fact there may be multiple exit nodes from a single
3039 ``Function``. If the BasicBlock_ list is empty, this indicates that the
3040 ``Function`` is actually a function declaration: the actual body of the function
3041 hasn't been linked in yet.
3043 In addition to a list of BasicBlock_\ s, the ``Function`` class also keeps track
3044 of the list of formal Argument_\ s that the function receives. This container
3045 manages the lifetime of the Argument_ nodes, just like the BasicBlock_ list does
3046 for the BasicBlock_\ s.
3048 The SymbolTable_ is a very rarely used LLVM feature that is only used when you
3049 have to look up a value by name. Aside from that, the SymbolTable_ is used
3050 internally to make sure that there are not conflicts between the names of
3051 Instruction_\ s, BasicBlock_\ s, or Argument_\ s in the function body.
3053 Note that ``Function`` is a GlobalValue_ and therefore also a Constant_. The
3054 value of the function is its address (after linking) which is guaranteed to be
3059 Important Public Members of the ``Function``
3060 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
3062 * ``Function(const FunctionType *Ty, LinkageTypes Linkage,
3063 const std::string &N = "", Module* Parent = 0)``
3065 Constructor used when you need to create new ``Function``\ s to add the
3066 program. The constructor must specify the type of the function to create and
3067 what type of linkage the function should have. The FunctionType_ argument
3068 specifies the formal arguments and return value for the function. The same
3069 FunctionType_ value can be used to create multiple functions. The ``Parent``
3070 argument specifies the Module in which the function is defined. If this
3071 argument is provided, the function will automatically be inserted into that
3072 module's list of functions.
3074 * ``bool isDeclaration()``
3076 Return whether or not the ``Function`` has a body defined. If the function is
3077 "external", it does not have a body, and thus must be resolved by linking with
3078 a function defined in a different translation unit.
3080 * | ``Function::iterator`` - Typedef for basic block list iterator
3081 | ``Function::const_iterator`` - Typedef for const_iterator.
3082 | ``begin()``, ``end()``, ``size()``, ``empty()``
3084 These are forwarding methods that make it easy to access the contents of a
3085 ``Function`` object's BasicBlock_ list.
3087 * ``Function::BasicBlockListType &getBasicBlockList()``
3089 Returns the list of BasicBlock_\ s. This is necessary to use when you need to
3090 update the list or perform a complex action that doesn't have a forwarding
3093 * | ``Function::arg_iterator`` - Typedef for the argument list iterator
3094 | ``Function::const_arg_iterator`` - Typedef for const_iterator.
3095 | ``arg_begin()``, ``arg_end()``, ``arg_size()``, ``arg_empty()``
3097 These are forwarding methods that make it easy to access the contents of a
3098 ``Function`` object's Argument_ list.
3100 * ``Function::ArgumentListType &getArgumentList()``
3102 Returns the list of Argument_. This is necessary to use when you need to
3103 update the list or perform a complex action that doesn't have a forwarding
3106 * ``BasicBlock &getEntryBlock()``
3108 Returns the entry ``BasicBlock`` for the function. Because the entry block
3109 for the function is always the first block, this returns the first block of
3112 * | ``Type *getReturnType()``
3113 | ``FunctionType *getFunctionType()``
3115 This traverses the Type_ of the ``Function`` and returns the return type of
3116 the function, or the FunctionType_ of the actual function.
3118 * ``SymbolTable *getSymbolTable()``
3120 Return a pointer to the SymbolTable_ for this ``Function``.
3124 The ``GlobalVariable`` class
3125 ----------------------------
3127 ``#include "llvm/IR/GlobalVariable.h"``
3129 header source: `GlobalVariable.h
3130 <http://llvm.org/doxygen/GlobalVariable_8h-source.html>`_
3132 doxygen info: `GlobalVariable Class
3133 <http://llvm.org/doxygen/classllvm_1_1GlobalVariable.html>`_
3135 Superclasses: GlobalValue_, Constant_, User_, Value_
3137 Global variables are represented with the (surprise surprise) ``GlobalVariable``
3138 class. Like functions, ``GlobalVariable``\ s are also subclasses of
3139 GlobalValue_, and as such are always referenced by their address (global values
3140 must live in memory, so their "name" refers to their constant address). See
3141 GlobalValue_ for more on this. Global variables may have an initial value
3142 (which must be a Constant_), and if they have an initializer, they may be marked
3143 as "constant" themselves (indicating that their contents never change at
3146 .. _m_GlobalVariable:
3148 Important Public Members of the ``GlobalVariable`` class
3149 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
3151 * ``GlobalVariable(const Type *Ty, bool isConstant, LinkageTypes &Linkage,
3152 Constant *Initializer = 0, const std::string &Name = "", Module* Parent = 0)``
3154 Create a new global variable of the specified type. If ``isConstant`` is true
3155 then the global variable will be marked as unchanging for the program. The
3156 Linkage parameter specifies the type of linkage (internal, external, weak,
3157 linkonce, appending) for the variable. If the linkage is InternalLinkage,
3158 WeakAnyLinkage, WeakODRLinkage, LinkOnceAnyLinkage or LinkOnceODRLinkage, then
3159 the resultant global variable will have internal linkage. AppendingLinkage
3160 concatenates together all instances (in different translation units) of the
3161 variable into a single variable but is only applicable to arrays. See the
3162 `LLVM Language Reference <LangRef.html#modulestructure>`_ for further details
3163 on linkage types. Optionally an initializer, a name, and the module to put
3164 the variable into may be specified for the global variable as well.
3166 * ``bool isConstant() const``
3168 Returns true if this is a global variable that is known not to be modified at
3171 * ``bool hasInitializer()``
3173 Returns true if this ``GlobalVariable`` has an intializer.
3175 * ``Constant *getInitializer()``
3177 Returns the initial value for a ``GlobalVariable``. It is not legal to call
3178 this method if there is no initializer.
3182 The ``BasicBlock`` class
3183 ------------------------
3185 ``#include "llvm/IR/BasicBlock.h"``
3187 header source: `BasicBlock.h
3188 <http://llvm.org/doxygen/BasicBlock_8h-source.html>`_
3190 doxygen info: `BasicBlock Class
3191 <http://llvm.org/doxygen/classllvm_1_1BasicBlock.html>`_
3195 This class represents a single entry single exit section of the code, commonly
3196 known as a basic block by the compiler community. The ``BasicBlock`` class
3197 maintains a list of Instruction_\ s, which form the body of the block. Matching
3198 the language definition, the last element of this list of instructions is always
3199 a terminator instruction (a subclass of the TerminatorInst_ class).
3201 In addition to tracking the list of instructions that make up the block, the
3202 ``BasicBlock`` class also keeps track of the :ref:`Function <c_Function>` that
3203 it is embedded into.
3205 Note that ``BasicBlock``\ s themselves are Value_\ s, because they are
3206 referenced by instructions like branches and can go in the switch tables.
3207 ``BasicBlock``\ s have type ``label``.
3211 Important Public Members of the ``BasicBlock`` class
3212 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
3214 * ``BasicBlock(const std::string &Name = "", Function *Parent = 0)``
3216 The ``BasicBlock`` constructor is used to create new basic blocks for
3217 insertion into a function. The constructor optionally takes a name for the
3218 new block, and a :ref:`Function <c_Function>` to insert it into. If the
3219 ``Parent`` parameter is specified, the new ``BasicBlock`` is automatically
3220 inserted at the end of the specified :ref:`Function <c_Function>`, if not
3221 specified, the BasicBlock must be manually inserted into the :ref:`Function
3224 * | ``BasicBlock::iterator`` - Typedef for instruction list iterator
3225 | ``BasicBlock::const_iterator`` - Typedef for const_iterator.
3226 | ``begin()``, ``end()``, ``front()``, ``back()``,
3227 ``size()``, ``empty()``
3228 STL-style functions for accessing the instruction list.
3230 These methods and typedefs are forwarding functions that have the same
3231 semantics as the standard library methods of the same names. These methods
3232 expose the underlying instruction list of a basic block in a way that is easy
3233 to manipulate. To get the full complement of container operations (including
3234 operations to update the list), you must use the ``getInstList()`` method.
3236 * ``BasicBlock::InstListType &getInstList()``
3238 This method is used to get access to the underlying container that actually
3239 holds the Instructions. This method must be used when there isn't a
3240 forwarding function in the ``BasicBlock`` class for the operation that you
3241 would like to perform. Because there are no forwarding functions for
3242 "updating" operations, you need to use this if you want to update the contents
3243 of a ``BasicBlock``.
3245 * ``Function *getParent()``
3247 Returns a pointer to :ref:`Function <c_Function>` the block is embedded into,
3248 or a null pointer if it is homeless.
3250 * ``TerminatorInst *getTerminator()``
3252 Returns a pointer to the terminator instruction that appears at the end of the
3253 ``BasicBlock``. If there is no terminator instruction, or if the last
3254 instruction in the block is not a terminator, then a null pointer is returned.
3258 The ``Argument`` class
3259 ----------------------
3261 This subclass of Value defines the interface for incoming formal arguments to a
3262 function. A Function maintains a list of its formal arguments. An argument has
3263 a pointer to the parent Function.