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10 <div class="doc_title">
11 LLVM Alias Analysis Infrastructure
15 <li><a href="#introduction">Introduction</a></li>
17 <li><a href="#overview"><tt>AliasAnalysis</tt> Class Overview</a>
19 <li><a href="#pointers">Representation of Pointers</a></li>
20 <li><a href="#alias">The <tt>alias</tt> method</a></li>
21 <li><a href="#ModRefInfo">The <tt>getModRefInfo</tt> methods</a></li>
22 <li><a href="#OtherItfs">Other useful <tt>AliasAnalysis</tt> methods</a></li>
26 <li><a href="#writingnew">Writing a new <tt>AliasAnalysis</tt> Implementation</a>
28 <li><a href="#passsubclasses">Different Pass styles</a></li>
29 <li><a href="#requiredcalls">Required initialization calls</a></li>
30 <li><a href="#interfaces">Interfaces which may be specified</a></li>
31 <li><a href="#chaining"><tt>AliasAnalysis</tt> chaining behavior</a></li>
32 <li><a href="#updating">Updating analysis results for transformations</a></li>
33 <li><a href="#implefficiency">Efficiency Issues</a></li>
37 <li><a href="#using">Using alias analysis results</a>
39 <li><a href="#loadvn">Using the <tt>-load-vn</tt> Pass</a></li>
40 <li><a href="#ast">Using the <tt>AliasSetTracker</tt> class</a></li>
41 <li><a href="#direct">Using the <tt>AliasAnalysis</tt> interface directly</a></li>
45 <li><a href="#exist">Existing alias analysis implementations and clients</a>
47 <li><a href="#impls">Available <tt>AliasAnalysis</tt> implementations</a></li>
48 <li><a href="#aliasanalysis-xforms">Alias analysis driven transformations</a></li>
49 <li><a href="#aliasanalysis-debug">Clients for debugging and evaluation of
50 implementations</a></li>
55 <div class="doc_author">
56 <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
59 <!-- *********************************************************************** -->
60 <div class="doc_section">
61 <a name="introduction">Introduction</a>
63 <!-- *********************************************************************** -->
65 <div class="doc_text">
67 <p>Alias Analysis (aka Pointer Analysis) is a class of techniques which attempt
68 to determine whether or not two pointers ever can point to the same object in
69 memory. There are many different algorithms for alias analysis and many
70 different ways of classifying them: flow-sensitive vs flow-insensitive,
71 context-sensitive vs context-insensitive, field-sensitive vs field-insensitive,
72 unification-based vs subset-based, etc. Traditionally, alias analyses respond
73 to a query with a <a href="#MustNoMay">Must, May, or No</a> alias response,
74 indicating that two pointers always point to the same object, might point to the
75 same object, or are known to never point to the same object.</p>
78 href="http://llvm.cs.uiuc.edu/doxygen/classllvm_1_1AliasAnalysis.html"><tt>AliasAnalysis</tt></a>
79 class is the primary interface used by clients and implementations of alias
80 analyses in the LLVM system. This class is the common interface between clients
81 of alias analysis information and the implementations providing it, and is
82 designed to support a wide range of implementations and clients (but currently
83 all clients are assumed to be flow-insensitive). In addition to simple alias
84 analysis information, this class exposes Mod/Ref information from those
85 implementations which can provide it, allowing for powerful analyses and
86 transformations to work well together.</p>
88 <p>This document contains information necessary to successfully implement this
89 interface, use it, and to test both sides. It also explains some of the finer
90 points about what exactly results mean. If you feel that something is unclear
91 or should be added, please <a href="mailto:sabre@nondot.org">let me
96 <!-- *********************************************************************** -->
97 <div class="doc_section">
98 <a name="overview"><tt>AliasAnalysis</tt> Class Overview</a>
100 <!-- *********************************************************************** -->
102 <div class="doc_text">
105 href="http://llvm.cs.uiuc.edu/doxygen/classllvm_1_1AliasAnalysis.html"><tt>AliasAnalysis</tt></a>
106 class defines the interface that the various alias analysis implementations
107 should support. This class exports two important enums: <tt>AliasResult</tt>
108 and <tt>ModRefResult</tt> which represent the result of an alias query or a
109 mod/ref query, respectively.</p>
111 <p>The <tt>AliasAnalysis</tt> interface exposes information about memory,
112 represented in several different ways. In particular, memory objects are
113 represented as a starting address and size, and function calls are represented
114 as the actual <tt>call</tt> or <tt>invoke</tt> instructions that performs the
115 call. The <tt>AliasAnalysis</tt> interface also exposes some helper methods
116 which allow you to get mod/ref information for arbitrary instructions.</p>
120 <!-- ======================================================================= -->
121 <div class="doc_subsection">
122 <a name="pointers">Representation of Pointers</a>
125 <div class="doc_text">
127 <p>Most importantly, the <tt>AliasAnalysis</tt> class provides several methods
128 which are used to query whether or not two memory objects alias, whether
129 function calls can modify or read a memory object, etc. For all of these
130 queries, memory objects are represented as a pair of their starting address (a
131 symbolic LLVM <tt>Value*</tt>) and a static size.</p>
133 <p>Representing memory objects as a starting address and a size is critically
134 important for correct Alias Analyses. For example, consider this (silly, but
135 possible) C code:</p>
142 for (i = 0; i != 10; ++i) {
143 C[0] = A[i]; /* One byte store */
144 C[1] = A[9-i]; /* One byte store */
148 <p>In this case, the <tt>basicaa</tt> pass will disambiguate the stores to
149 <tt>C[0]</tt> and <tt>C[1]</tt> because they are accesses to two distinct
150 locations one byte apart, and the accesses are each one byte. In this case, the
151 LICM pass can use store motion to remove the stores from the loop. In
152 constrast, the following code:</p>
159 for (i = 0; i != 10; ++i) {
160 ((short*)C)[0] = A[i]; /* Two byte store! */
161 C[1] = A[9-i]; /* One byte store */
165 <p>In this case, the two stores to C do alias each other, because the access to
166 the <tt>&C[0]</tt> element is a two byte access. If size information wasn't
167 available in the query, even the first case would have to conservatively assume
168 that the accesses alias.</p>
172 <!-- ======================================================================= -->
173 <div class="doc_subsection">
174 <a name="alias">The <tt>alias</tt> method</a>
177 <div class="doc_text">
178 The <tt>alias</tt> method is the primary interface used to determine whether or
179 not two memory objects alias each other. It takes two memory objects as input
180 and returns MustAlias, MayAlias, or NoAlias as appropriate.
183 <!-- _______________________________________________________________________ -->
184 <div class="doc_subsubsection">
185 <a name="MustMayNo">Must, May, and No Alias Responses</a>
188 <div class="doc_text">
190 <p>An Alias Analysis implementation can return one of three responses:
191 MustAlias, MayAlias, and NoAlias. The No and May alias results are obvious: if
192 the two pointers can never equal each other, return NoAlias, if they might,
195 <p>The MustAlias response is trickier though. In LLVM, the Must Alias response
196 may only be returned if the two memory objects are guaranteed to always start at
197 exactly the same location. If two memory objects overlap, but do not start at
198 the same location, return MayAlias.</p>
202 <!-- ======================================================================= -->
203 <div class="doc_subsection">
204 <a name="ModRefInfo">The <tt>getModRefInfo</tt> methods</a>
207 <div class="doc_text">
209 <p>The <tt>getModRefInfo</tt> methods return information about whether the
210 execution of an instruction can read or modify a memory location. Mod/Ref
211 information is always conservative: if an instruction <b>might</b> read or write
212 a location, ModRef is returned.</p>
214 <p>The <tt>AliasAnalysis</tt> class also provides a <tt>getModRefInfo</tt>
215 method for testing dependencies between function calls. This method takes two
216 call sites (CS1 & CS2), returns NoModRef if the two calls refer to disjoint
217 memory locations, Ref if CS1 reads memory written by CS2, Mod if CS1 writes to
218 memory read or written by CS2, or ModRef if CS1 might read or write memory
219 accessed by CS2. Note that this relation is not commutative. Clients that use
220 this method should be predicated on the <tt>hasNoModRefInfoForCalls()</tt>
221 method, which indicates whether or not an analysis can provide mod/ref
222 information for function call pairs (most can not). If this predicate is false,
223 the client shouldn't waste analysis time querying the <tt>getModRefInfo</tt>
224 method many times.</p>
229 <!-- ======================================================================= -->
230 <div class="doc_subsection">
231 <a name="OtherItfs">Other useful <tt>AliasAnalysis</tt> methods</a>
234 <div class="doc_text">
237 Several other tidbits of information are often collected by various alias
238 analysis implementations and can be put to good use by various clients.
243 <!-- _______________________________________________________________________ -->
244 <div class="doc_subsubsection">
245 The <tt>getMustAliases</tt> method
248 <div class="doc_text">
250 <p>The <tt>getMustAliases</tt> method returns all values that are known to
251 always must alias a pointer. This information can be provided in some cases for
252 important objects like the null pointer and global values. Knowing that a
253 pointer always points to a particular function allows indirect calls to be
254 turned into direct calls, for example.</p>
258 <!-- _______________________________________________________________________ -->
259 <div class="doc_subsubsection">
260 The <tt>pointsToConstantMemory</tt> method
263 <div class="doc_text">
265 <p>The <tt>pointsToConstantMemory</tt> method returns true if and only if the
266 analysis can prove that the pointer only points to unchanging memory locations
267 (functions, constant global variables, and the null pointer). This information
268 can be used to refine mod/ref information: it is impossible for an unchanging
269 memory location to be modified.</p>
274 <!-- _______________________________________________________________________ -->
275 <div class="doc_subsubsection">
276 <a name="simplemodref">The <tt>doesNotAccessMemory</tt> and
277 <tt>onlyReadsMemory</tt> methods</a>
280 <div class="doc_text">
282 <p>These methods are used to provide very simple mod/ref information for
283 function calls. The <tt>doesNotAccessMemory</tt> method returns true for a
284 function if the analysis can prove that the function never reads or writes to
285 memory, or if the function only reads from constant memory. Functions with this
286 property are side-effect free and only depend on their input arguments, allowing
287 them to be eliminated if they form common subexpressions or be hoisted out of
288 loops. Many common functions behave this way (e.g., <tt>sin</tt> and
289 <tt>cos</tt>) but many others do not (e.g., <tt>acos</tt>, which modifies the
290 <tt>errno</tt> variable).</p>
292 <p>The <tt>onlyReadsMemory</tt> method returns true for a function if analysis
293 can prove that (at most) the function only reads from non-volatile memory.
294 Functions with this property are side-effect free, only depending on their input
295 arguments and the state of memory when they are called. This property allows
296 calls to these functions to be eliminated and moved around, as long as there is
297 no store instruction that changes the contents of memory. Note that all
298 functions that satisfy the <tt>doesNotAccessMemory</tt> method also satisfies
299 <tt>onlyReadsMemory</tt>.</p>
305 <!-- *********************************************************************** -->
306 <div class="doc_section">
307 <a name="writingnew">Writing a new <tt>AliasAnalysis</tt> Implementation</a>
309 <!-- *********************************************************************** -->
311 <div class="doc_text">
313 <p>Writing a new alias analysis implementation for LLVM is quite
314 straight-forward. There are already several implementations that you can use
315 for examples, and the following information should help fill in any details.
316 For a examples, take a look at the <a href="#impls">various alias analysis
317 implementations</a> included with LLVM.</p>
321 <!-- ======================================================================= -->
322 <div class="doc_subsection">
323 <a name="passsubclasses">Different Pass styles</a>
326 <div class="doc_text">
328 <p>The first step to determining what type of <a
329 href="WritingAnLLVMPass.html">LLVM pass</a> you need to use for your Alias
330 Analysis. As is the case with most other analyses and transformations, the
331 answer should be fairly obvious from what type of problem you are trying to
335 <li>If you require interprocedural analysis, it should be a
337 <li>If you are a function-local analysis, subclass <tt>FunctionPass</tt>.</li>
338 <li>If you don't need to look at the program at all, subclass
339 <tt>ImmutablePass</tt>.</li>
342 <p>In addition to the pass that you subclass, you should also inherit from the
343 <tt>AliasAnalysis</tt> interface, of course, and use the
344 <tt>RegisterAnalysisGroup</tt> template to register as an implementation of
345 <tt>AliasAnalysis</tt>.</p>
349 <!-- ======================================================================= -->
350 <div class="doc_subsection">
351 <a name="requiredcalls">Required initialization calls</a>
354 <div class="doc_text">
356 <p>Your subclass of <tt>AliasAnalysis</tt> is required to invoke two methods on
357 the <tt>AliasAnalysis</tt> base class: <tt>getAnalysisUsage</tt> and
358 <tt>InitializeAliasAnalysis</tt>. In particular, your implementation of
359 <tt>getAnalysisUsage</tt> should explicitly call into the
360 <tt>AliasAnalysis::getAnalysisUsage</tt> method in addition to doing any
361 declaring any pass dependencies your pass has. Thus you should have something
365 void getAnalysisUsage(AnalysisUsage &AU) const {
366 AliasAnalysis::getAnalysisUsage(AU);
367 <i>// declare your dependencies here.</i>
371 <p>Additionally, your must invoke the <tt>InitializeAliasAnalysis</tt> method
372 from your analysis run method (<tt>run</tt> for a <tt>Pass</tt>,
373 <tt>runOnFunction</tt> for a <tt>FunctionPass</tt>, or <tt>InitializePass</tt>
374 for an <tt>ImmutablePass</tt>). For example (as part of a <tt>Pass</tt>):</p>
377 bool run(Module &M) {
378 InitializeAliasAnalysis(this);
379 <i>// Perform analysis here...</i>
386 <!-- ======================================================================= -->
387 <div class="doc_subsection">
388 <a name="interfaces">Interfaces which may be specified</a>
391 <div class="doc_text">
394 href="/doxygen/classllvm_1_1AliasAnalysis.html"><tt>AliasAnalysis</tt></a>
395 virtual methods default to providing <a href="#chaining">chaining</a> to another
396 alias analysis implementation, which ends up returning conservatively correct
397 information (returning "May" Alias and "Mod/Ref" for alias and mod/ref queries
398 respectively). Depending on the capabilities of the analysis you are
399 implementing, you just override the interfaces you can improve.</p>
405 <!-- ======================================================================= -->
406 <div class="doc_subsection">
407 <a name="chaining"><tt>AliasAnalysis</tt> chaining behavior</a>
410 <div class="doc_text">
412 <p>With only two special exceptions (the <tt><a
413 href="#basic-aa">basicaa</a></tt> and <a href="#no-aa"><tt>no-aa</tt></a>
414 passes) every alias analysis pass chains to another alias analysis
415 implementation (for example, the user can specify "<tt>-basicaa -ds-aa
416 -anders-aa -licm</tt>" to get the maximum benefit from the three alias
417 analyses). The alias analysis class automatically takes care of most of this
418 for methods that you don't override. For methods that you do override, in code
419 paths that return a conservative MayAlias or Mod/Ref result, simply return
420 whatever the superclass computes. For example:</p>
423 AliasAnalysis::AliasResult alias(const Value *V1, unsigned V1Size,
424 const Value *V2, unsigned V2Size) {
429 <i>// Couldn't determine a must or no-alias result.</i>
430 return AliasAnalysis::alias(V1, V1Size, V2, V2Size);
434 <p>In addition to analysis queries, you must make sure to unconditionally pass
435 LLVM <a href="#updating">update notification</a> methods to the superclass as
436 well if you override them, which allows all alias analyses in a change to be
442 <!-- ======================================================================= -->
443 <div class="doc_subsection">
444 <a name="updating">Updating analysis results for transformations</a>
447 <div class="doc_text">
449 Alias analysis information is initially computed for a static snapshot of the
450 program, but clients will use this information to make transformations to the
451 code. All but the most trivial forms of alias analysis will need to have their
452 analysis results updated to reflect the changes made by these transformations.
456 The <tt>AliasAnalysis</tt> interface exposes two methods which are used to
457 communicate program changes from the clients to the analysis implementations.
458 Various alias analysis implementations should use these methods to ensure that
459 their internal data structures are kept up-to-date as the program changes (for
460 example, when an instruction is deleted), and clients of alias analysis must be
461 sure to call these interfaces appropriately.
465 <!-- _______________________________________________________________________ -->
466 <div class="doc_subsubsection">The <tt>deleteValue</tt> method</div>
468 <div class="doc_text">
469 The <tt>deleteValue</tt> method is called by transformations when they remove an
470 instruction or any other value from the program (including values that do not
471 use pointers). Typically alias analyses keep data structures that have entries
472 for each value in the program. When this method is called, they should remove
473 any entries for the specified value, if they exist.
477 <!-- _______________________________________________________________________ -->
478 <div class="doc_subsubsection">The <tt>copyValue</tt> method</div>
480 <div class="doc_text">
481 The <tt>copyValue</tt> method is used when a new value is introduced into the
482 program. There is no way to introduce a value into the program that did not
483 exist before (this doesn't make sense for a safe compiler transformation), so
484 this is the only way to introduce a new value. This method indicates that the
485 new value has exactly the same properties as the value being copied.
489 <!-- _______________________________________________________________________ -->
490 <div class="doc_subsubsection">The <tt>replaceWithNewValue</tt> method</div>
492 <div class="doc_text">
493 This method is a simple helper method that is provided to make clients easier to
494 use. It is implemented by copying the old analysis information to the new
495 value, then deleting the old value. This method cannot be overridden by alias
496 analysis implementations.
499 <!-- ======================================================================= -->
500 <div class="doc_subsection">
501 <a name="implefficiency">Efficiency Issues</a>
504 <div class="doc_text">
506 <p>From the LLVM perspective, the only thing you need to do to provide an
507 efficient alias analysis is to make sure that alias analysis <b>queries</b> are
508 serviced quickly. The actual calculation of the alias analysis results (the
509 "run" method) is only performed once, but many (perhaps duplicate) queries may
510 be performed. Because of this, try to move as much computation to the run
511 method as possible (within reason).</p>
515 <!-- *********************************************************************** -->
516 <div class="doc_section">
517 <a name="using">Using alias analysis results</a>
519 <!-- *********************************************************************** -->
521 <div class="doc_text">
523 <p>There are several different ways to use alias analysis results. In order of
524 preference, these are...</p>
528 <!-- ======================================================================= -->
529 <div class="doc_subsection">
530 <a name="loadvn">Using the <tt>-load-vn</tt> Pass</a>
533 <div class="doc_text">
535 <p>The <tt>load-vn</tt> pass uses alias analysis to provide value numbering
536 information for <tt>load</tt> instructions and pointer values. If your analysis
537 or transformation can be modeled in a form that uses value numbering
538 information, you don't have to do anything special to handle load instructions:
539 just use the <tt>load-vn</tt> pass, which uses alias analysis.</p>
543 <!-- ======================================================================= -->
544 <div class="doc_subsection">
545 <a name="ast">Using the <tt>AliasSetTracker</tt> class</a>
548 <div class="doc_text">
550 <p>Many transformations need information about alias <b>sets</b> that are active
551 in some scope, rather than information about pairwise aliasing. The <tt><a
552 href="/doxygen/classllvm_1_1AliasSetTracker.html">AliasSetTracker</a></tt> class is used
553 to efficiently build these Alias Sets from the pairwise alias analysis
554 information provided by the <tt>AliasAnalysis</tt> interface.</p>
556 <p>First you initialize the AliasSetTracker by using the "<tt>add</tt>" methods
557 to add information about various potentially aliasing instructions in the scope
558 you are interested in. Once all of the alias sets are completed, your pass
559 should simply iterate through the constructed alias sets, using the
560 <tt>AliasSetTracker</tt> <tt>begin()</tt>/<tt>end()</tt> methods.</p>
562 <p>The <tt>AliasSet</tt>s formed by the <tt>AliasSetTracker</tt> are guaranteed
563 to be disjoint, calculate mod/ref information and volatility for the set, and
564 keep track of whether or not all of the pointers in the set are Must aliases.
565 The AliasSetTracker also makes sure that sets are properly folded due to call
566 instructions, and can provide a list of pointers in each set.</p>
568 <p>As an example user of this, the <a href="/doxygen/structLICM.html">Loop
569 Invariant Code Motion</a> pass uses <tt>AliasSetTracker</tt>s to calculate alias
570 sets for each loop nest. If an <tt>AliasSet</tt> in a loop is not modified,
571 then all load instructions from that set may be hoisted out of the loop. If any
572 alias sets are stored to <b>and</b> are must alias sets, then the stores may be
573 sunk to outside of the loop, promoting the memory location to a register for the
574 duration of the loop nest. Both of these transformations only apply if the
575 pointer argument is loop-invariant.</p>
579 <!-- _______________________________________________________________________ -->
580 <div class="doc_subsubsection">
581 The AliasSetTracker implementation
584 <div class="doc_text">
586 <p>The AliasSetTracker class is implemented to be as efficient as possible. It
587 uses the union-find algorithm to efficiently merge AliasSets when a pointer is
588 inserted into the AliasSetTracker that aliases multiple sets. The primary data
589 structure is a hash table mapping pointers to the AliasSet they are in.</p>
591 <p>The AliasSetTracker class must maintain a list of all of the LLVM Value*'s
592 that are in each AliasSet. Since the hash table already has entries for each
593 LLVM Value* of interest, the AliasesSets thread the linked list through these
594 hash-table nodes to avoid having to allocate memory unnecessarily, and to make
595 merging alias sets extremely efficient (the linked list merge is constant time).
598 <p>You shouldn't need to understand these details if you are just a client of
599 the AliasSetTracker, but if you look at the code, hopefully this brief
600 description will help make sense of why things are designed the way they
606 <!-- ======================================================================= -->
607 <div class="doc_subsection">
608 <a name="direct">Using the <tt>AliasAnalysis</tt> interface directly</a>
611 <div class="doc_text">
613 <p>If neither of these utility class are what your pass needs, you should use
614 the interfaces exposed by the <tt>AliasAnalysis</tt> class directly. Try to use
615 the higher-level methods when possible (e.g., use mod/ref information instead of
616 the <a href="#alias"><tt>alias</tt></a> method directly if possible) to get the
617 best precision and efficiency.</p>
621 <!-- *********************************************************************** -->
622 <div class="doc_section">
623 <a name="exist">Existing alias analysis implementations and clients</a>
625 <!-- *********************************************************************** -->
627 <div class="doc_text">
629 <p>If you're going to be working with the LLVM alias analysis infrastructure,
630 you should know what clients and implementations of alias analysis are
631 available. In particular, if you are implementing an alias analysis, you should
632 be aware of the <a href="#aliasanalysis-debug">the clients</a> that are useful
633 for monitoring and evaluating different implementations.</p>
637 <!-- ======================================================================= -->
638 <div class="doc_subsection">
639 <a name="impls">Available <tt>AliasAnalysis</tt> implementations</a>
642 <div class="doc_text">
644 <p>This section lists the various implementations of the <tt>AliasAnalysis</tt>
645 interface. With the exception of the <a href="#no-aa"><tt>-no-aa</tt></a> and
646 <a href="#basic-aa"><tt>-basicaa</tt></a> implementations, all of these <a
647 href="#chaining">chain</a> to other alias analysis implementations.</p>
651 <!-- _______________________________________________________________________ -->
652 <div class="doc_subsubsection">
653 <a name="no-aa">The <tt>-no-aa</tt> pass</a>
656 <div class="doc_text">
658 <p>The <tt>-no-aa</tt> pass is just like what it sounds: an alias analysis that
659 never returns any useful information. This pass can be useful if you think that
660 alias analysis is doing something wrong and are trying to narrow down a
666 <!-- _______________________________________________________________________ -->
667 <div class="doc_subsubsection">
668 <a name="basic-aa">The <tt>-basicaa</tt> pass</a>
671 <div class="doc_text">
673 <p>The <tt>-basicaa</tt> pass is the default LLVM alias analysis. It is an
674 aggressive local analysis that "knows" many important facts:</p>
677 <li>Distinct globals, stack allocations, and heap allocations can never
679 <li>Globals, stack allocations, and heap allocations never alias the null
681 <li>Different fields of a structure do not alias.</li>
682 <li>Indexes into arrays with statically differing subscripts cannot alias.</li>
683 <li>Many common standard C library functions <a
684 href="#simplemodref">never access memory or only read memory</a>.</li>
685 <li>Pointers that obviously point to constant globals
686 "<tt>pointToConstantMemory</tt>".</li>
687 <li>Function calls can not modify or references stack allocations if they never
688 escape from the function that allocates them (a common case for automatic
694 <!-- _______________________________________________________________________ -->
695 <div class="doc_subsubsection">
696 <a name="globalsmodref">The <tt>-globalsmodref-aa</tt> pass</a>
699 <div class="doc_text">
701 <p>This pass implements a simple context-sensitive mod/ref and alias analysis
702 for internal global variables that don't "have their address taken". If a
703 global does not have its address taken, the pass knows that no pointers alias
707 <p>The real power of this pass is that it provides context-sensitive mod/ref
708 information for call instructions. This allows the optimizer to know that
709 calls to a function do not clobber or read the value of the global, allowing
710 loads and stores to be eliminated.</p>
712 <p>Note that this pass is somewhat limited in its scope (only support
713 non-address taken globals), but is very quick analysis.</p>
716 <!-- _______________________________________________________________________ -->
717 <div class="doc_subsubsection">
718 <a name="anders-aa">The <tt>-anders-aa</tt> pass</a>
721 <div class="doc_text">
723 <p>The <tt>-anders-aa</tt> pass implements the well-known "Andersen's algorithm"
724 for interprocedural alias analysis. This algorithm is a subset-based,
725 flow-insensitive, context-insensitive, and field-insensitive alias analysis that
726 is widely believed to be fairly precise. Unfortunately, this algorithm is also
727 O(N<sup>3</sup>). The LLVM implementation currently does not implement any of
728 the refinements (such as "online cycle elimination" or "offline variable
729 substitution") to improve its efficiency, so it can be quite slow in common
735 <!-- _______________________________________________________________________ -->
736 <div class="doc_subsubsection">
737 <a name="steens-aa">The <tt>-steens-aa</tt> pass</a>
740 <div class="doc_text">
742 <p>The <tt>-steens-aa</tt> pass implements a variation on the well-known
743 "Steensgaard's algorithm" for interprocedural alias analysis. Steensgaard's
744 algorithm is a unification-based, flow-insensitive, context-insensitive, and
745 field-insensitive alias analysis that is also very scalable (effectively linear
748 <p>The LLVM <tt>-steens-aa</tt> pass implements a "speculatively
749 field-<b>sensitive</b>" version of Steensgaard's algorithm using the Data
750 Structure Analysis framework. This gives it substantially more precision than
751 the standard algorithm while maintaining excellent analysis scalability.</p>
755 <!-- _______________________________________________________________________ -->
756 <div class="doc_subsubsection">
757 <a name="ds-aa">The <tt>-ds-aa</tt> pass</a>
760 <div class="doc_text">
762 <p>The <tt>-ds-aa</tt> pass implements the full Data Structure Analysis
763 algorithm. Data Structure Analysis is a modular unification-based,
764 flow-insensitive, context-<b>sensitive</b>, and speculatively
765 field-<b>sensitive</b> alias analysis that is also quite scalable, usually at
768 <p>This algorithm is capable of responding to a full variety of alias analysis
769 queries, and can provide context-sensitive mod/ref information as well. The
770 only major facility not implemented so far is support for must-alias
776 <!-- ======================================================================= -->
777 <div class="doc_subsection">
778 <a name="aliasanalysis-xforms">Alias analysis driven transformations</a>
781 <div class="doc_text">
782 LLVM includes several alias-analysis driven transformations which can be used
783 with any of the implementations above.
786 <!-- _______________________________________________________________________ -->
787 <div class="doc_subsubsection">
788 <a name="adce">The <tt>-adce</tt> pass</a>
791 <div class="doc_text">
793 <p>The <tt>-adce</tt> pass, which implements Aggressive Dead Code Elimination
794 uses the <tt>AliasAnalysis</tt> interface to delete calls to functions that do
795 not have side-effects and are not used.</p>
800 <!-- _______________________________________________________________________ -->
801 <div class="doc_subsubsection">
802 <a name="licm">The <tt>-licm</tt> pass</a>
805 <div class="doc_text">
807 <p>The <tt>-licm</tt> pass implements various Loop Invariant Code Motion related
808 transformations. It uses the <tt>AliasAnalysis</tt> interface for several
809 different transformations:</p>
812 <li>It uses mod/ref information to hoist or sink load instructions out of loops
813 if there are no instructions in the loop that modifies the memory loaded.</li>
815 <li>It uses mod/ref information to hoist function calls out of loops that do not
816 write to memory and are loop-invariant.</li>
818 <li>If uses alias information to promote memory objects that are loaded and
819 stored to in loops to live in a register instead. It can do this if there are
820 no may aliases to the loaded/stored memory location.</li>
825 <!-- _______________________________________________________________________ -->
826 <div class="doc_subsubsection">
827 <a name="argpromotion">The <tt>-argpromotion</tt> pass</a>
830 <div class="doc_text">
832 The <tt>-argpromotion</tt> pass promotes by-reference arguments to be passed in
833 by-value instead. In particular, if pointer arguments are only loaded from it
834 passes in the value loaded instead of the address to the function. This pass
835 uses alias information to make sure that the value loaded from the argument
836 pointer is not modified between the entry of the function and any load of the
840 <!-- _______________________________________________________________________ -->
841 <div class="doc_subsubsection">
842 <a name="gcseloadvn">The <tt>-load-vn</tt> & <tt>-gcse</tt> passes</a>
845 <div class="doc_text">
847 The <tt>-load-vn</tt> pass uses alias analysis to "<a href="#loadvn">value
848 number</a>" loads and pointers values, which is used by the GCSE pass to
849 eliminate instructions. The <tt>-load-vn</tt> pass relies on alias information
850 and must-alias information. This combination of passes can make the following
854 <li>Redundant load instructions are eliminated.</li>
855 <li>Load instructions that follow a store to the same location are replaced with
856 the stored value ("store forwarding").</li>
857 <li>Pointers values (e.g. formal arguments) that must-alias simpler expressions
858 (e.g. global variables or the null pointer) are replaced. Note that this
859 implements transformations like "virtual method resolution", turning indirect
860 calls into direct calls.</li>
868 <!-- ======================================================================= -->
869 <div class="doc_subsection">
870 <a name="aliasanalysis-debug">Clients for debugging and evaluation of implementations</a>
873 These passes are useful for evaluating the various alias analysis
874 implementations. You can use them with commands like '<tt>opt -anders-aa -ds-aa
875 -aa-eval foo.bc -disable-output -stats</tt>'.
878 <!-- _______________________________________________________________________ -->
879 <div class="doc_subsubsection">
880 <a name="print-alias-sets">The <tt>-print-alias-sets</tt> pass</a>
883 <div class="doc_text">
885 <p>The <tt>-print-alias-sets</tt> pass is exposed as part of the
886 <tt>analyze</tt> tool to print out the Alias Sets formed by the <a
887 href="#ast"><tt>AliasSetTracker</tt></a> class. This is useful if you're using
888 the <tt>AliasSetTracker</tt> class.</p>
893 <!-- _______________________________________________________________________ -->
894 <div class="doc_subsubsection">
895 <a name="count-aa">The <tt>-count-aa</tt> pass</a>
898 <div class="doc_text">
900 <p>The <tt>-count-aa</tt> pass is useful to see how many queries a particular
901 pass is making and what responses are returned by the alias analysis. An
902 example usage is:</p>
905 $ opt -basicaa -count-aa -ds-aa -count-aa -licm
908 <p>Which will print out how many queries (and what responses are returned) by
909 the <tt>-licm</tt> pass (of the <tt>-ds-aa</tt> pass) and how many queries are
910 made of the <tt>-basicaa</tt> pass by the <tt>-ds-aa</tt> pass. This can be
911 useful when debugging a transformation or an alias analysis implementation.</p>
915 <!-- _______________________________________________________________________ -->
916 <div class="doc_subsubsection">
917 <a name="aa-eval">The <tt>-aa-eval</tt> pass</a>
920 <div class="doc_text">
922 <p>The <tt>-aa-eval</tt> pass simply iterates through all pairs of pointers in a
923 function and asks an alias analysis whether or not the pointers alias. This
924 gives an indication of the precision of the alias analysis. Statistics are
925 printed indicating the percent of no/may/must aliases found (a more precise
926 algorithm will have a lower number of may aliases).</p>
930 <!-- *********************************************************************** -->
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939 <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
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