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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>
34 <li><a href="#passmanager">Pass Manager Issues</a></li>
38 <li><a href="#using">Using alias analysis results</a>
40 <li><a href="#memdep">Using the <tt>MemoryDependenceAnalysis</tt> Pass</a></li>
41 <li><a href="#ast">Using the <tt>AliasSetTracker</tt> class</a></li>
42 <li><a href="#direct">Using the <tt>AliasAnalysis</tt> interface directly</a></li>
46 <li><a href="#exist">Existing alias analysis implementations and clients</a>
48 <li><a href="#impls">Available <tt>AliasAnalysis</tt> implementations</a></li>
49 <li><a href="#aliasanalysis-xforms">Alias analysis driven transformations</a></li>
50 <li><a href="#aliasanalysis-debug">Clients for debugging and evaluation of
51 implementations</a></li>
54 <li><a href="#memdep">Memory Dependence Analysis</a></li>
57 <div class="doc_author">
58 <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
61 <!-- *********************************************************************** -->
62 <div class="doc_section">
63 <a name="introduction">Introduction</a>
65 <!-- *********************************************************************** -->
67 <div class="doc_text">
69 <p>Alias Analysis (aka Pointer Analysis) is a class of techniques which attempt
70 to determine whether or not two pointers ever can point to the same object in
71 memory. There are many different algorithms for alias analysis and many
72 different ways of classifying them: flow-sensitive vs flow-insensitive,
73 context-sensitive vs context-insensitive, field-sensitive vs field-insensitive,
74 unification-based vs subset-based, etc. Traditionally, alias analyses respond
75 to a query with a <a href="#MustMayNo">Must, May, or No</a> alias response,
76 indicating that two pointers always point to the same object, might point to the
77 same object, or are known to never point to the same object.</p>
80 href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html"><tt>AliasAnalysis</tt></a>
81 class is the primary interface used by clients and implementations of alias
82 analyses in the LLVM system. This class is the common interface between clients
83 of alias analysis information and the implementations providing it, and is
84 designed to support a wide range of implementations and clients (but currently
85 all clients are assumed to be flow-insensitive). In addition to simple alias
86 analysis information, this class exposes Mod/Ref information from those
87 implementations which can provide it, allowing for powerful analyses and
88 transformations to work well together.</p>
90 <p>This document contains information necessary to successfully implement this
91 interface, use it, and to test both sides. It also explains some of the finer
92 points about what exactly results mean. If you feel that something is unclear
93 or should be added, please <a href="mailto:sabre@nondot.org">let me
98 <!-- *********************************************************************** -->
99 <div class="doc_section">
100 <a name="overview"><tt>AliasAnalysis</tt> Class Overview</a>
102 <!-- *********************************************************************** -->
104 <div class="doc_text">
107 href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html"><tt>AliasAnalysis</tt></a>
108 class defines the interface that the various alias analysis implementations
109 should support. This class exports two important enums: <tt>AliasResult</tt>
110 and <tt>ModRefResult</tt> which represent the result of an alias query or a
111 mod/ref query, respectively.</p>
113 <p>The <tt>AliasAnalysis</tt> interface exposes information about memory,
114 represented in several different ways. In particular, memory objects are
115 represented as a starting address and size, and function calls are represented
116 as the actual <tt>call</tt> or <tt>invoke</tt> instructions that performs the
117 call. The <tt>AliasAnalysis</tt> interface also exposes some helper methods
118 which allow you to get mod/ref information for arbitrary instructions.</p>
122 <!-- ======================================================================= -->
123 <div class="doc_subsection">
124 <a name="pointers">Representation of Pointers</a>
127 <div class="doc_text">
129 <p>Most importantly, the <tt>AliasAnalysis</tt> class provides several methods
130 which are used to query whether or not two memory objects alias, whether
131 function calls can modify or read a memory object, etc. For all of these
132 queries, memory objects are represented as a pair of their starting address (a
133 symbolic LLVM <tt>Value*</tt>) and a static size.</p>
135 <p>Representing memory objects as a starting address and a size is critically
136 important for correct Alias Analyses. For example, consider this (silly, but
137 possible) C code:</p>
139 <div class="doc_code">
145 for (i = 0; i != 10; ++i) {
146 C[0] = A[i]; /* One byte store */
147 C[1] = A[9-i]; /* One byte store */
152 <p>In this case, the <tt>basicaa</tt> pass will disambiguate the stores to
153 <tt>C[0]</tt> and <tt>C[1]</tt> because they are accesses to two distinct
154 locations one byte apart, and the accesses are each one byte. In this case, the
155 LICM pass can use store motion to remove the stores from the loop. In
156 constrast, the following code:</p>
158 <div class="doc_code">
164 for (i = 0; i != 10; ++i) {
165 ((short*)C)[0] = A[i]; /* Two byte store! */
166 C[1] = A[9-i]; /* One byte store */
171 <p>In this case, the two stores to C do alias each other, because the access to
172 the <tt>&C[0]</tt> element is a two byte access. If size information wasn't
173 available in the query, even the first case would have to conservatively assume
174 that the accesses alias.</p>
178 <!-- ======================================================================= -->
179 <div class="doc_subsection">
180 <a name="alias">The <tt>alias</tt> method</a>
183 <div class="doc_text">
184 The <tt>alias</tt> method is the primary interface used to determine whether or
185 not two memory objects alias each other. It takes two memory objects as input
186 and returns MustAlias, MayAlias, or NoAlias as appropriate.
189 <!-- _______________________________________________________________________ -->
190 <div class="doc_subsubsection">
191 <a name="MustMayNo">Must, May, and No Alias Responses</a>
194 <div class="doc_text">
195 <p>The NoAlias response is used when the two pointers refer to distinct objects,
196 regardless of whether the pointers compare equal. For example, freed pointers
197 don't alias any pointers that were allocated afterwards. As a degenerate case,
198 pointers returned by malloc(0) have no bytes for an object, and are considered
199 NoAlias even when malloc returns the same pointer. The same rule applies to
202 <p>The MayAlias response is used whenever the two pointers might refer to the
203 same object. If the two memory objects overlap, but do not start at the same
204 location, return MayAlias.</p>
206 <p>The MustAlias response may only be returned if the two memory objects are
207 guaranteed to always start at exactly the same location. A MustAlias response
208 implies that the pointers compare equal.</p>
212 <!-- ======================================================================= -->
213 <div class="doc_subsection">
214 <a name="ModRefInfo">The <tt>getModRefInfo</tt> methods</a>
217 <div class="doc_text">
219 <p>The <tt>getModRefInfo</tt> methods return information about whether the
220 execution of an instruction can read or modify a memory location. Mod/Ref
221 information is always conservative: if an instruction <b>might</b> read or write
222 a location, ModRef is returned.</p>
224 <p>The <tt>AliasAnalysis</tt> class also provides a <tt>getModRefInfo</tt>
225 method for testing dependencies between function calls. This method takes two
226 call sites (CS1 & CS2), returns NoModRef if the two calls refer to disjoint
227 memory locations, Ref if CS1 reads memory written by CS2, Mod if CS1 writes to
228 memory read or written by CS2, or ModRef if CS1 might read or write memory
229 accessed by CS2. Note that this relation is not commutative.</p>
234 <!-- ======================================================================= -->
235 <div class="doc_subsection">
236 <a name="OtherItfs">Other useful <tt>AliasAnalysis</tt> methods</a>
239 <div class="doc_text">
242 Several other tidbits of information are often collected by various alias
243 analysis implementations and can be put to good use by various clients.
248 <!-- _______________________________________________________________________ -->
249 <div class="doc_subsubsection">
250 The <tt>pointsToConstantMemory</tt> method
253 <div class="doc_text">
255 <p>The <tt>pointsToConstantMemory</tt> method returns true if and only if the
256 analysis can prove that the pointer only points to unchanging memory locations
257 (functions, constant global variables, and the null pointer). This information
258 can be used to refine mod/ref information: it is impossible for an unchanging
259 memory location to be modified.</p>
263 <!-- _______________________________________________________________________ -->
264 <div class="doc_subsubsection">
265 <a name="simplemodref">The <tt>doesNotAccessMemory</tt> and
266 <tt>onlyReadsMemory</tt> methods</a>
269 <div class="doc_text">
271 <p>These methods are used to provide very simple mod/ref information for
272 function calls. The <tt>doesNotAccessMemory</tt> method returns true for a
273 function if the analysis can prove that the function never reads or writes to
274 memory, or if the function only reads from constant memory. Functions with this
275 property are side-effect free and only depend on their input arguments, allowing
276 them to be eliminated if they form common subexpressions or be hoisted out of
277 loops. Many common functions behave this way (e.g., <tt>sin</tt> and
278 <tt>cos</tt>) but many others do not (e.g., <tt>acos</tt>, which modifies the
279 <tt>errno</tt> variable).</p>
281 <p>The <tt>onlyReadsMemory</tt> method returns true for a function if analysis
282 can prove that (at most) the function only reads from non-volatile memory.
283 Functions with this property are side-effect free, only depending on their input
284 arguments and the state of memory when they are called. This property allows
285 calls to these functions to be eliminated and moved around, as long as there is
286 no store instruction that changes the contents of memory. Note that all
287 functions that satisfy the <tt>doesNotAccessMemory</tt> method also satisfies
288 <tt>onlyReadsMemory</tt>.</p>
292 <!-- *********************************************************************** -->
293 <div class="doc_section">
294 <a name="writingnew">Writing a new <tt>AliasAnalysis</tt> Implementation</a>
296 <!-- *********************************************************************** -->
298 <div class="doc_text">
300 <p>Writing a new alias analysis implementation for LLVM is quite
301 straight-forward. There are already several implementations that you can use
302 for examples, and the following information should help fill in any details.
303 For a examples, take a look at the <a href="#impls">various alias analysis
304 implementations</a> included with LLVM.</p>
308 <!-- ======================================================================= -->
309 <div class="doc_subsection">
310 <a name="passsubclasses">Different Pass styles</a>
313 <div class="doc_text">
315 <p>The first step to determining what type of <a
316 href="WritingAnLLVMPass.html">LLVM pass</a> you need to use for your Alias
317 Analysis. As is the case with most other analyses and transformations, the
318 answer should be fairly obvious from what type of problem you are trying to
322 <li>If you require interprocedural analysis, it should be a
324 <li>If you are a function-local analysis, subclass <tt>FunctionPass</tt>.</li>
325 <li>If you don't need to look at the program at all, subclass
326 <tt>ImmutablePass</tt>.</li>
329 <p>In addition to the pass that you subclass, you should also inherit from the
330 <tt>AliasAnalysis</tt> interface, of course, and use the
331 <tt>RegisterAnalysisGroup</tt> template to register as an implementation of
332 <tt>AliasAnalysis</tt>.</p>
336 <!-- ======================================================================= -->
337 <div class="doc_subsection">
338 <a name="requiredcalls">Required initialization calls</a>
341 <div class="doc_text">
343 <p>Your subclass of <tt>AliasAnalysis</tt> is required to invoke two methods on
344 the <tt>AliasAnalysis</tt> base class: <tt>getAnalysisUsage</tt> and
345 <tt>InitializeAliasAnalysis</tt>. In particular, your implementation of
346 <tt>getAnalysisUsage</tt> should explicitly call into the
347 <tt>AliasAnalysis::getAnalysisUsage</tt> method in addition to doing any
348 declaring any pass dependencies your pass has. Thus you should have something
351 <div class="doc_code">
353 void getAnalysisUsage(AnalysisUsage &AU) const {
354 AliasAnalysis::getAnalysisUsage(AU);
355 <i>// declare your dependencies here.</i>
360 <p>Additionally, your must invoke the <tt>InitializeAliasAnalysis</tt> method
361 from your analysis run method (<tt>run</tt> for a <tt>Pass</tt>,
362 <tt>runOnFunction</tt> for a <tt>FunctionPass</tt>, or <tt>InitializePass</tt>
363 for an <tt>ImmutablePass</tt>). For example (as part of a <tt>Pass</tt>):</p>
365 <div class="doc_code">
367 bool run(Module &M) {
368 InitializeAliasAnalysis(this);
369 <i>// Perform analysis here...</i>
377 <!-- ======================================================================= -->
378 <div class="doc_subsection">
379 <a name="interfaces">Interfaces which may be specified</a>
382 <div class="doc_text">
385 href="/doxygen/classllvm_1_1AliasAnalysis.html"><tt>AliasAnalysis</tt></a>
386 virtual methods default to providing <a href="#chaining">chaining</a> to another
387 alias analysis implementation, which ends up returning conservatively correct
388 information (returning "May" Alias and "Mod/Ref" for alias and mod/ref queries
389 respectively). Depending on the capabilities of the analysis you are
390 implementing, you just override the interfaces you can improve.</p>
396 <!-- ======================================================================= -->
397 <div class="doc_subsection">
398 <a name="chaining"><tt>AliasAnalysis</tt> chaining behavior</a>
401 <div class="doc_text">
403 <p>With only two special exceptions (the <tt><a
404 href="#basic-aa">basicaa</a></tt> and <a href="#no-aa"><tt>no-aa</tt></a>
405 passes) every alias analysis pass chains to another alias analysis
406 implementation (for example, the user can specify "<tt>-basicaa -ds-aa
407 -licm</tt>" to get the maximum benefit from both alias
408 analyses). The alias analysis class automatically takes care of most of this
409 for methods that you don't override. For methods that you do override, in code
410 paths that return a conservative MayAlias or Mod/Ref result, simply return
411 whatever the superclass computes. For example:</p>
413 <div class="doc_code">
415 AliasAnalysis::AliasResult alias(const Value *V1, unsigned V1Size,
416 const Value *V2, unsigned V2Size) {
421 <i>// Couldn't determine a must or no-alias result.</i>
422 return AliasAnalysis::alias(V1, V1Size, V2, V2Size);
427 <p>In addition to analysis queries, you must make sure to unconditionally pass
428 LLVM <a href="#updating">update notification</a> methods to the superclass as
429 well if you override them, which allows all alias analyses in a change to be
435 <!-- ======================================================================= -->
436 <div class="doc_subsection">
437 <a name="updating">Updating analysis results for transformations</a>
440 <div class="doc_text">
442 Alias analysis information is initially computed for a static snapshot of the
443 program, but clients will use this information to make transformations to the
444 code. All but the most trivial forms of alias analysis will need to have their
445 analysis results updated to reflect the changes made by these transformations.
449 The <tt>AliasAnalysis</tt> interface exposes two methods which are used to
450 communicate program changes from the clients to the analysis implementations.
451 Various alias analysis implementations should use these methods to ensure that
452 their internal data structures are kept up-to-date as the program changes (for
453 example, when an instruction is deleted), and clients of alias analysis must be
454 sure to call these interfaces appropriately.
458 <!-- _______________________________________________________________________ -->
459 <div class="doc_subsubsection">The <tt>deleteValue</tt> method</div>
461 <div class="doc_text">
462 The <tt>deleteValue</tt> method is called by transformations when they remove an
463 instruction or any other value from the program (including values that do not
464 use pointers). Typically alias analyses keep data structures that have entries
465 for each value in the program. When this method is called, they should remove
466 any entries for the specified value, if they exist.
469 <!-- _______________________________________________________________________ -->
470 <div class="doc_subsubsection">The <tt>copyValue</tt> method</div>
472 <div class="doc_text">
473 The <tt>copyValue</tt> method is used when a new value is introduced into the
474 program. There is no way to introduce a value into the program that did not
475 exist before (this doesn't make sense for a safe compiler transformation), so
476 this is the only way to introduce a new value. This method indicates that the
477 new value has exactly the same properties as the value being copied.
480 <!-- _______________________________________________________________________ -->
481 <div class="doc_subsubsection">The <tt>replaceWithNewValue</tt> method</div>
483 <div class="doc_text">
484 This method is a simple helper method that is provided to make clients easier to
485 use. It is implemented by copying the old analysis information to the new
486 value, then deleting the old value. This method cannot be overridden by alias
487 analysis implementations.
490 <!-- ======================================================================= -->
491 <div class="doc_subsection">
492 <a name="implefficiency">Efficiency Issues</a>
495 <div class="doc_text">
497 <p>From the LLVM perspective, the only thing you need to do to provide an
498 efficient alias analysis is to make sure that alias analysis <b>queries</b> are
499 serviced quickly. The actual calculation of the alias analysis results (the
500 "run" method) is only performed once, but many (perhaps duplicate) queries may
501 be performed. Because of this, try to move as much computation to the run
502 method as possible (within reason).</p>
506 <!-- ======================================================================= -->
507 <div class="doc_subsection">
508 <a name="passmanager">Pass Manager Issues</a>
511 <div class="doc_text">
513 <p>PassManager support for alternative AliasAnalysis implementation
516 <p>There is no way to override the default alias analysis. It would
517 be very useful to be able to do something like "opt -my-aa -O2" and
518 have it use -my-aa for all passes which need AliasAnalysis, but there
519 is currently no support for that, short of changing the source code
520 and recompiling. Similarly, there is also no way of setting a chain
521 of analyses as the default.</p>
523 <p>There is no way for transform passes to declare that they preserve
524 <tt>AliasAnalysis</tt> implementations. The <tt>AliasAnalysis</tt>
525 interface includes <tt>deleteValue</tt> and <tt>copyValue</tt> methods
526 which are intended to allow a pass to keep an AliasAnalysis consistent,
527 however there's no way for a pass to declare in its
528 <tt>getAnalysisUsage</tt> that it does so. Some passes attempt to use
529 <tt>AU.addPreserved<AliasAnalysis></tt>, however this doesn't
530 actually have any effect.</tt>
532 <p><tt>AliasAnalysisCounter</tt> (<tt>-count-aa</tt>) and <tt>AliasDebugger</tt>
533 (<tt>-debug-aa</tt>) are implemented as <tt>ModulePass</tt> classes, so if your
534 alias analysis uses <tt>FunctionPass</tt>, it won't be able to use
535 these utilities. If you try to use them, the pass manager will
536 silently route alias analysis queries directly to
537 <tt>BasicAliasAnalysis</tt> instead.</p>
539 <p>Similarly, the <tt>opt -p</tt> option introduces <tt>ModulePass</tt>
540 passes between each pass, which prevents the use of <tt>FunctionPass</tt>
541 alias analysis passes.</p>
545 <!-- *********************************************************************** -->
546 <div class="doc_section">
547 <a name="using">Using alias analysis results</a>
549 <!-- *********************************************************************** -->
551 <div class="doc_text">
553 <p>There are several different ways to use alias analysis results. In order of
554 preference, these are...</p>
558 <!-- ======================================================================= -->
559 <div class="doc_subsection">
560 <a name="memdep">Using the <tt>MemoryDependenceAnalysis</tt> Pass</a>
563 <div class="doc_text">
565 <p>The <tt>memdep</tt> pass uses alias analysis to provide high-level dependence
566 information about memory-using instructions. This will tell you which store
567 feeds into a load, for example. It uses caching and other techniques to be
568 efficient, and is used by Dead Store Elimination, GVN, and memcpy optimizations.
573 <!-- ======================================================================= -->
574 <div class="doc_subsection">
575 <a name="ast">Using the <tt>AliasSetTracker</tt> class</a>
578 <div class="doc_text">
580 <p>Many transformations need information about alias <b>sets</b> that are active
581 in some scope, rather than information about pairwise aliasing. The <tt><a
582 href="/doxygen/classllvm_1_1AliasSetTracker.html">AliasSetTracker</a></tt> class
583 is used to efficiently build these Alias Sets from the pairwise alias analysis
584 information provided by the <tt>AliasAnalysis</tt> interface.</p>
586 <p>First you initialize the AliasSetTracker by using the "<tt>add</tt>" methods
587 to add information about various potentially aliasing instructions in the scope
588 you are interested in. Once all of the alias sets are completed, your pass
589 should simply iterate through the constructed alias sets, using the
590 <tt>AliasSetTracker</tt> <tt>begin()</tt>/<tt>end()</tt> methods.</p>
592 <p>The <tt>AliasSet</tt>s formed by the <tt>AliasSetTracker</tt> are guaranteed
593 to be disjoint, calculate mod/ref information and volatility for the set, and
594 keep track of whether or not all of the pointers in the set are Must aliases.
595 The AliasSetTracker also makes sure that sets are properly folded due to call
596 instructions, and can provide a list of pointers in each set.</p>
598 <p>As an example user of this, the <a href="/doxygen/structLICM.html">Loop
599 Invariant Code Motion</a> pass uses <tt>AliasSetTracker</tt>s to calculate alias
600 sets for each loop nest. If an <tt>AliasSet</tt> in a loop is not modified,
601 then all load instructions from that set may be hoisted out of the loop. If any
602 alias sets are stored to <b>and</b> are must alias sets, then the stores may be
603 sunk to outside of the loop, promoting the memory location to a register for the
604 duration of the loop nest. Both of these transformations only apply if the
605 pointer argument is loop-invariant.</p>
609 <!-- _______________________________________________________________________ -->
610 <div class="doc_subsubsection">
611 The AliasSetTracker implementation
614 <div class="doc_text">
616 <p>The AliasSetTracker class is implemented to be as efficient as possible. It
617 uses the union-find algorithm to efficiently merge AliasSets when a pointer is
618 inserted into the AliasSetTracker that aliases multiple sets. The primary data
619 structure is a hash table mapping pointers to the AliasSet they are in.</p>
621 <p>The AliasSetTracker class must maintain a list of all of the LLVM Value*'s
622 that are in each AliasSet. Since the hash table already has entries for each
623 LLVM Value* of interest, the AliasesSets thread the linked list through these
624 hash-table nodes to avoid having to allocate memory unnecessarily, and to make
625 merging alias sets extremely efficient (the linked list merge is constant time).
628 <p>You shouldn't need to understand these details if you are just a client of
629 the AliasSetTracker, but if you look at the code, hopefully this brief
630 description will help make sense of why things are designed the way they
635 <!-- ======================================================================= -->
636 <div class="doc_subsection">
637 <a name="direct">Using the <tt>AliasAnalysis</tt> interface directly</a>
640 <div class="doc_text">
642 <p>If neither of these utility class are what your pass needs, you should use
643 the interfaces exposed by the <tt>AliasAnalysis</tt> class directly. Try to use
644 the higher-level methods when possible (e.g., use mod/ref information instead of
645 the <a href="#alias"><tt>alias</tt></a> method directly if possible) to get the
646 best precision and efficiency.</p>
650 <!-- *********************************************************************** -->
651 <div class="doc_section">
652 <a name="exist">Existing alias analysis implementations and clients</a>
654 <!-- *********************************************************************** -->
656 <div class="doc_text">
658 <p>If you're going to be working with the LLVM alias analysis infrastructure,
659 you should know what clients and implementations of alias analysis are
660 available. In particular, if you are implementing an alias analysis, you should
661 be aware of the <a href="#aliasanalysis-debug">the clients</a> that are useful
662 for monitoring and evaluating different implementations.</p>
666 <!-- ======================================================================= -->
667 <div class="doc_subsection">
668 <a name="impls">Available <tt>AliasAnalysis</tt> implementations</a>
671 <div class="doc_text">
673 <p>This section lists the various implementations of the <tt>AliasAnalysis</tt>
674 interface. With the exception of the <a href="#no-aa"><tt>-no-aa</tt></a> and
675 <a href="#basic-aa"><tt>-basicaa</tt></a> implementations, all of these <a
676 href="#chaining">chain</a> to other alias analysis implementations.</p>
680 <!-- _______________________________________________________________________ -->
681 <div class="doc_subsubsection">
682 <a name="no-aa">The <tt>-no-aa</tt> pass</a>
685 <div class="doc_text">
687 <p>The <tt>-no-aa</tt> pass is just like what it sounds: an alias analysis that
688 never returns any useful information. This pass can be useful if you think that
689 alias analysis is doing something wrong and are trying to narrow down a
694 <!-- _______________________________________________________________________ -->
695 <div class="doc_subsubsection">
696 <a name="basic-aa">The <tt>-basicaa</tt> pass</a>
699 <div class="doc_text">
701 <p>The <tt>-basicaa</tt> pass is the default LLVM alias analysis. It is an
702 aggressive local analysis that "knows" many important facts:</p>
705 <li>Distinct globals, stack allocations, and heap allocations can never
707 <li>Globals, stack allocations, and heap allocations never alias the null
709 <li>Different fields of a structure do not alias.</li>
710 <li>Indexes into arrays with statically differing subscripts cannot alias.</li>
711 <li>Many common standard C library functions <a
712 href="#simplemodref">never access memory or only read memory</a>.</li>
713 <li>Pointers that obviously point to constant globals
714 "<tt>pointToConstantMemory</tt>".</li>
715 <li>Function calls can not modify or references stack allocations if they never
716 escape from the function that allocates them (a common case for automatic
722 <!-- _______________________________________________________________________ -->
723 <div class="doc_subsubsection">
724 <a name="globalsmodref">The <tt>-globalsmodref-aa</tt> pass</a>
727 <div class="doc_text">
729 <p>This pass implements a simple context-sensitive mod/ref and alias analysis
730 for internal global variables that don't "have their address taken". If a
731 global does not have its address taken, the pass knows that no pointers alias
732 the global. This pass also keeps track of functions that it knows never access
733 memory or never read memory. This allows certain optimizations (e.g. GVN) to
734 eliminate call instructions entirely.
737 <p>The real power of this pass is that it provides context-sensitive mod/ref
738 information for call instructions. This allows the optimizer to know that
739 calls to a function do not clobber or read the value of the global, allowing
740 loads and stores to be eliminated.</p>
742 <p>Note that this pass is somewhat limited in its scope (only support
743 non-address taken globals), but is very quick analysis.</p>
746 <!-- _______________________________________________________________________ -->
747 <div class="doc_subsubsection">
748 <a name="steens-aa">The <tt>-steens-aa</tt> pass</a>
751 <div class="doc_text">
753 <p>The <tt>-steens-aa</tt> pass implements a variation on the well-known
754 "Steensgaard's algorithm" for interprocedural alias analysis. Steensgaard's
755 algorithm is a unification-based, flow-insensitive, context-insensitive, and
756 field-insensitive alias analysis that is also very scalable (effectively linear
759 <p>The LLVM <tt>-steens-aa</tt> pass implements a "speculatively
760 field-<b>sensitive</b>" version of Steensgaard's algorithm using the Data
761 Structure Analysis framework. This gives it substantially more precision than
762 the standard algorithm while maintaining excellent analysis scalability.</p>
764 <p>Note that <tt>-steens-aa</tt> is available in the optional "poolalloc"
765 module, it is not part of the LLVM core.</p>
769 <!-- _______________________________________________________________________ -->
770 <div class="doc_subsubsection">
771 <a name="ds-aa">The <tt>-ds-aa</tt> pass</a>
774 <div class="doc_text">
776 <p>The <tt>-ds-aa</tt> pass implements the full Data Structure Analysis
777 algorithm. Data Structure Analysis is a modular unification-based,
778 flow-insensitive, context-<b>sensitive</b>, and speculatively
779 field-<b>sensitive</b> alias analysis that is also quite scalable, usually at
782 <p>This algorithm is capable of responding to a full variety of alias analysis
783 queries, and can provide context-sensitive mod/ref information as well. The
784 only major facility not implemented so far is support for must-alias
787 <p>Note that <tt>-ds-aa</tt> is available in the optional "poolalloc"
788 module, it is not part of the LLVM core.</p>
793 <!-- ======================================================================= -->
794 <div class="doc_subsection">
795 <a name="aliasanalysis-xforms">Alias analysis driven transformations</a>
798 <div class="doc_text">
799 LLVM includes several alias-analysis driven transformations which can be used
800 with any of the implementations above.
803 <!-- _______________________________________________________________________ -->
804 <div class="doc_subsubsection">
805 <a name="adce">The <tt>-adce</tt> pass</a>
808 <div class="doc_text">
810 <p>The <tt>-adce</tt> pass, which implements Aggressive Dead Code Elimination
811 uses the <tt>AliasAnalysis</tt> interface to delete calls to functions that do
812 not have side-effects and are not used.</p>
817 <!-- _______________________________________________________________________ -->
818 <div class="doc_subsubsection">
819 <a name="licm">The <tt>-licm</tt> pass</a>
822 <div class="doc_text">
824 <p>The <tt>-licm</tt> pass implements various Loop Invariant Code Motion related
825 transformations. It uses the <tt>AliasAnalysis</tt> interface for several
826 different transformations:</p>
829 <li>It uses mod/ref information to hoist or sink load instructions out of loops
830 if there are no instructions in the loop that modifies the memory loaded.</li>
832 <li>It uses mod/ref information to hoist function calls out of loops that do not
833 write to memory and are loop-invariant.</li>
835 <li>If uses alias information to promote memory objects that are loaded and
836 stored to in loops to live in a register instead. It can do this if there are
837 no may aliases to the loaded/stored memory location.</li>
842 <!-- _______________________________________________________________________ -->
843 <div class="doc_subsubsection">
844 <a name="argpromotion">The <tt>-argpromotion</tt> pass</a>
847 <div class="doc_text">
849 The <tt>-argpromotion</tt> pass promotes by-reference arguments to be passed in
850 by-value instead. In particular, if pointer arguments are only loaded from it
851 passes in the value loaded instead of the address to the function. This pass
852 uses alias information to make sure that the value loaded from the argument
853 pointer is not modified between the entry of the function and any load of the
857 <!-- _______________________________________________________________________ -->
858 <div class="doc_subsubsection">
859 <a name="gvn">The <tt>-gvn</tt>, <tt>-memcpyopt</tt>, and <tt>-dse</tt>
863 <div class="doc_text">
865 <p>These passes use AliasAnalysis information to reason about loads and stores.
870 <!-- ======================================================================= -->
871 <div class="doc_subsection">
872 <a name="aliasanalysis-debug">Clients for debugging and evaluation of
876 <div class="doc_text">
878 <p>These passes are useful for evaluating the various alias analysis
879 implementations. You can use them with commands like '<tt>opt -ds-aa
880 -aa-eval foo.bc -disable-output -stats</tt>'.</p>
884 <!-- _______________________________________________________________________ -->
885 <div class="doc_subsubsection">
886 <a name="print-alias-sets">The <tt>-print-alias-sets</tt> pass</a>
889 <div class="doc_text">
891 <p>The <tt>-print-alias-sets</tt> pass is exposed as part of the
892 <tt>opt</tt> tool to print out the Alias Sets formed by the <a
893 href="#ast"><tt>AliasSetTracker</tt></a> class. This is useful if you're using
894 the <tt>AliasSetTracker</tt> class. To use it, use something like:</p>
896 <div class="doc_code">
898 % opt -ds-aa -print-alias-sets -disable-output
905 <!-- _______________________________________________________________________ -->
906 <div class="doc_subsubsection">
907 <a name="count-aa">The <tt>-count-aa</tt> pass</a>
910 <div class="doc_text">
912 <p>The <tt>-count-aa</tt> pass is useful to see how many queries a particular
913 pass is making and what responses are returned by the alias analysis. As an
916 <div class="doc_code">
918 % opt -basicaa -count-aa -ds-aa -count-aa -licm
922 <p>will print out how many queries (and what responses are returned) by the
923 <tt>-licm</tt> pass (of the <tt>-ds-aa</tt> pass) and how many queries are made
924 of the <tt>-basicaa</tt> pass by the <tt>-ds-aa</tt> pass. This can be useful
925 when debugging a transformation or an alias analysis implementation.</p>
929 <!-- _______________________________________________________________________ -->
930 <div class="doc_subsubsection">
931 <a name="aa-eval">The <tt>-aa-eval</tt> pass</a>
934 <div class="doc_text">
936 <p>The <tt>-aa-eval</tt> pass simply iterates through all pairs of pointers in a
937 function and asks an alias analysis whether or not the pointers alias. This
938 gives an indication of the precision of the alias analysis. Statistics are
939 printed indicating the percent of no/may/must aliases found (a more precise
940 algorithm will have a lower number of may aliases).</p>
944 <!-- *********************************************************************** -->
945 <div class="doc_section">
946 <a name="memdep">Memory Dependence Analysis</a>
948 <!-- *********************************************************************** -->
950 <div class="doc_text">
952 <p>If you're just looking to be a client of alias analysis information, consider
953 using the Memory Dependence Analysis interface instead. MemDep is a lazy,
954 caching layer on top of alias analysis that is able to answer the question of
955 what preceding memory operations a given instruction depends on, either at an
956 intra- or inter-block level. Because of its laziness and caching
957 policy, using MemDep can be a significant performance win over accessing alias
958 analysis directly.</p>
962 <!-- *********************************************************************** -->
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971 <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
972 <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
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