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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 may be used when there is never an immediate dependence
196 between any memory reference <i>based</i> on one pointer and any memory
197 reference <i>based</i> the other. The most obvious example is when the two
198 pointers point to non-overlapping memory ranges. Another is when the two
199 pointers are only ever used for reading memory. Another is when the memory is
200 freed and reallocated between accesses through one pointer and accesses through
201 the other -- in this case, there is a dependence, but it's mediated by the free
202 and reallocation.</p>
204 <p>As an exception to this is with the
205 <a href="LangRef.html#noalias"><tt>noalias</tt></a> keyword. AliasAnalysis
206 implementations may choose to ignore the "irrelevant" dependencies, provided
207 their clients do not need to be aware of these dependencies for correctness.</p>
209 <p>The MayAlias response is used whenever the two pointers might refer to the
210 same object. If the two memory objects overlap, but do not start at the same
211 location, return MayAlias.</p>
213 <p>The MustAlias response may only be returned if the two memory objects are
214 guaranteed to always start at exactly the same location. A MustAlias response
215 implies that the pointers compare equal.</p>
219 <!-- ======================================================================= -->
220 <div class="doc_subsection">
221 <a name="ModRefInfo">The <tt>getModRefInfo</tt> methods</a>
224 <div class="doc_text">
226 <p>The <tt>getModRefInfo</tt> methods return information about whether the
227 execution of an instruction can read or modify a memory location. Mod/Ref
228 information is always conservative: if an instruction <b>might</b> read or write
229 a location, ModRef is returned.</p>
231 <p>The <tt>AliasAnalysis</tt> class also provides a <tt>getModRefInfo</tt>
232 method for testing dependencies between function calls. This method takes two
233 call sites (CS1 & CS2), returns NoModRef if the two calls refer to disjoint
234 memory locations, Ref if CS1 reads memory written by CS2, Mod if CS1 writes to
235 memory read or written by CS2, or ModRef if CS1 might read or write memory
236 accessed by CS2. Note that this relation is not commutative.</p>
241 <!-- ======================================================================= -->
242 <div class="doc_subsection">
243 <a name="OtherItfs">Other useful <tt>AliasAnalysis</tt> methods</a>
246 <div class="doc_text">
249 Several other tidbits of information are often collected by various alias
250 analysis implementations and can be put to good use by various clients.
255 <!-- _______________________________________________________________________ -->
256 <div class="doc_subsubsection">
257 The <tt>pointsToConstantMemory</tt> method
260 <div class="doc_text">
262 <p>The <tt>pointsToConstantMemory</tt> method returns true if and only if the
263 analysis can prove that the pointer only points to unchanging memory locations
264 (functions, constant global variables, and the null pointer). This information
265 can be used to refine mod/ref information: it is impossible for an unchanging
266 memory location to be modified.</p>
270 <!-- _______________________________________________________________________ -->
271 <div class="doc_subsubsection">
272 <a name="simplemodref">The <tt>doesNotAccessMemory</tt> and
273 <tt>onlyReadsMemory</tt> methods</a>
276 <div class="doc_text">
278 <p>These methods are used to provide very simple mod/ref information for
279 function calls. The <tt>doesNotAccessMemory</tt> method returns true for a
280 function if the analysis can prove that the function never reads or writes to
281 memory, or if the function only reads from constant memory. Functions with this
282 property are side-effect free and only depend on their input arguments, allowing
283 them to be eliminated if they form common subexpressions or be hoisted out of
284 loops. Many common functions behave this way (e.g., <tt>sin</tt> and
285 <tt>cos</tt>) but many others do not (e.g., <tt>acos</tt>, which modifies the
286 <tt>errno</tt> variable).</p>
288 <p>The <tt>onlyReadsMemory</tt> method returns true for a function if analysis
289 can prove that (at most) the function only reads from non-volatile memory.
290 Functions with this property are side-effect free, only depending on their input
291 arguments and the state of memory when they are called. This property allows
292 calls to these functions to be eliminated and moved around, as long as there is
293 no store instruction that changes the contents of memory. Note that all
294 functions that satisfy the <tt>doesNotAccessMemory</tt> method also satisfies
295 <tt>onlyReadsMemory</tt>.</p>
299 <!-- *********************************************************************** -->
300 <div class="doc_section">
301 <a name="writingnew">Writing a new <tt>AliasAnalysis</tt> Implementation</a>
303 <!-- *********************************************************************** -->
305 <div class="doc_text">
307 <p>Writing a new alias analysis implementation for LLVM is quite
308 straight-forward. There are already several implementations that you can use
309 for examples, and the following information should help fill in any details.
310 For a examples, take a look at the <a href="#impls">various alias analysis
311 implementations</a> included with LLVM.</p>
315 <!-- ======================================================================= -->
316 <div class="doc_subsection">
317 <a name="passsubclasses">Different Pass styles</a>
320 <div class="doc_text">
322 <p>The first step to determining what type of <a
323 href="WritingAnLLVMPass.html">LLVM pass</a> you need to use for your Alias
324 Analysis. As is the case with most other analyses and transformations, the
325 answer should be fairly obvious from what type of problem you are trying to
329 <li>If you require interprocedural analysis, it should be a
331 <li>If you are a function-local analysis, subclass <tt>FunctionPass</tt>.</li>
332 <li>If you don't need to look at the program at all, subclass
333 <tt>ImmutablePass</tt>.</li>
336 <p>In addition to the pass that you subclass, you should also inherit from the
337 <tt>AliasAnalysis</tt> interface, of course, and use the
338 <tt>RegisterAnalysisGroup</tt> template to register as an implementation of
339 <tt>AliasAnalysis</tt>.</p>
343 <!-- ======================================================================= -->
344 <div class="doc_subsection">
345 <a name="requiredcalls">Required initialization calls</a>
348 <div class="doc_text">
350 <p>Your subclass of <tt>AliasAnalysis</tt> is required to invoke two methods on
351 the <tt>AliasAnalysis</tt> base class: <tt>getAnalysisUsage</tt> and
352 <tt>InitializeAliasAnalysis</tt>. In particular, your implementation of
353 <tt>getAnalysisUsage</tt> should explicitly call into the
354 <tt>AliasAnalysis::getAnalysisUsage</tt> method in addition to doing any
355 declaring any pass dependencies your pass has. Thus you should have something
358 <div class="doc_code">
360 void getAnalysisUsage(AnalysisUsage &AU) const {
361 AliasAnalysis::getAnalysisUsage(AU);
362 <i>// declare your dependencies here.</i>
367 <p>Additionally, your must invoke the <tt>InitializeAliasAnalysis</tt> method
368 from your analysis run method (<tt>run</tt> for a <tt>Pass</tt>,
369 <tt>runOnFunction</tt> for a <tt>FunctionPass</tt>, or <tt>InitializePass</tt>
370 for an <tt>ImmutablePass</tt>). For example (as part of a <tt>Pass</tt>):</p>
372 <div class="doc_code">
374 bool run(Module &M) {
375 InitializeAliasAnalysis(this);
376 <i>// Perform analysis here...</i>
384 <!-- ======================================================================= -->
385 <div class="doc_subsection">
386 <a name="interfaces">Interfaces which may be specified</a>
389 <div class="doc_text">
392 href="/doxygen/classllvm_1_1AliasAnalysis.html"><tt>AliasAnalysis</tt></a>
393 virtual methods default to providing <a href="#chaining">chaining</a> to another
394 alias analysis implementation, which ends up returning conservatively correct
395 information (returning "May" Alias and "Mod/Ref" for alias and mod/ref queries
396 respectively). Depending on the capabilities of the analysis you are
397 implementing, you just override the interfaces you can improve.</p>
403 <!-- ======================================================================= -->
404 <div class="doc_subsection">
405 <a name="chaining"><tt>AliasAnalysis</tt> chaining behavior</a>
408 <div class="doc_text">
410 <p>With only two special exceptions (the <tt><a
411 href="#basic-aa">basicaa</a></tt> and <a href="#no-aa"><tt>no-aa</tt></a>
412 passes) every alias analysis pass chains to another alias analysis
413 implementation (for example, the user can specify "<tt>-basicaa -ds-aa
414 -licm</tt>" to get the maximum benefit from both alias
415 analyses). The alias analysis class automatically takes care of most of this
416 for methods that you don't override. For methods that you do override, in code
417 paths that return a conservative MayAlias or Mod/Ref result, simply return
418 whatever the superclass computes. For example:</p>
420 <div class="doc_code">
422 AliasAnalysis::AliasResult alias(const Value *V1, unsigned V1Size,
423 const Value *V2, unsigned V2Size) {
428 <i>// Couldn't determine a must or no-alias result.</i>
429 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.
476 <!-- _______________________________________________________________________ -->
477 <div class="doc_subsubsection">The <tt>copyValue</tt> method</div>
479 <div class="doc_text">
480 The <tt>copyValue</tt> method is used when a new value is introduced into the
481 program. There is no way to introduce a value into the program that did not
482 exist before (this doesn't make sense for a safe compiler transformation), so
483 this is the only way to introduce a new value. This method indicates that the
484 new value has exactly the same properties as the value being copied.
487 <!-- _______________________________________________________________________ -->
488 <div class="doc_subsubsection">The <tt>replaceWithNewValue</tt> method</div>
490 <div class="doc_text">
491 This method is a simple helper method that is provided to make clients easier to
492 use. It is implemented by copying the old analysis information to the new
493 value, then deleting the old value. This method cannot be overridden by alias
494 analysis implementations.
497 <!-- ======================================================================= -->
498 <div class="doc_subsection">
499 <a name="implefficiency">Efficiency Issues</a>
502 <div class="doc_text">
504 <p>From the LLVM perspective, the only thing you need to do to provide an
505 efficient alias analysis is to make sure that alias analysis <b>queries</b> are
506 serviced quickly. The actual calculation of the alias analysis results (the
507 "run" method) is only performed once, but many (perhaps duplicate) queries may
508 be performed. Because of this, try to move as much computation to the run
509 method as possible (within reason).</p>
513 <!-- ======================================================================= -->
514 <div class="doc_subsection">
515 <a name="passmanager">Pass Manager Issues</a>
518 <div class="doc_text">
520 <p>PassManager support for alternative AliasAnalysis implementation
523 <p>There is no way to override the default alias analysis. It would
524 be very useful to be able to do something like "opt -my-aa -O2" and
525 have it use -my-aa for all passes which need AliasAnalysis, but there
526 is currently no support for that, short of changing the source code
527 and recompiling. Similarly, there is also no way of setting a chain
528 of analyses as the default.</p>
530 <p>There is no way for transform passes to declare that they preserve
531 <tt>AliasAnalysis</tt> implementations. The <tt>AliasAnalysis</tt>
532 interface includes <tt>deleteValue</tt> and <tt>copyValue</tt> methods
533 which are intended to allow a pass to keep an AliasAnalysis consistent,
534 however there's no way for a pass to declare in its
535 <tt>getAnalysisUsage</tt> that it does so. Some passes attempt to use
536 <tt>AU.addPreserved<AliasAnalysis></tt>, however this doesn't
537 actually have any effect.</tt>
539 <p><tt>AliasAnalysisCounter</tt> (<tt>-count-aa</tt>) and <tt>AliasDebugger</tt>
540 (<tt>-debug-aa</tt>) are implemented as <tt>ModulePass</tt> classes, so if your
541 alias analysis uses <tt>FunctionPass</tt>, it won't be able to use
542 these utilities. If you try to use them, the pass manager will
543 silently route alias analysis queries directly to
544 <tt>BasicAliasAnalysis</tt> instead.</p>
546 <p>Similarly, the <tt>opt -p</tt> option introduces <tt>ModulePass</tt>
547 passes between each pass, which prevents the use of <tt>FunctionPass</tt>
548 alias analysis passes.</p>
552 <!-- *********************************************************************** -->
553 <div class="doc_section">
554 <a name="using">Using alias analysis results</a>
556 <!-- *********************************************************************** -->
558 <div class="doc_text">
560 <p>There are several different ways to use alias analysis results. In order of
561 preference, these are...</p>
565 <!-- ======================================================================= -->
566 <div class="doc_subsection">
567 <a name="memdep">Using the <tt>MemoryDependenceAnalysis</tt> Pass</a>
570 <div class="doc_text">
572 <p>The <tt>memdep</tt> pass uses alias analysis to provide high-level dependence
573 information about memory-using instructions. This will tell you which store
574 feeds into a load, for example. It uses caching and other techniques to be
575 efficient, and is used by Dead Store Elimination, GVN, and memcpy optimizations.
580 <!-- ======================================================================= -->
581 <div class="doc_subsection">
582 <a name="ast">Using the <tt>AliasSetTracker</tt> class</a>
585 <div class="doc_text">
587 <p>Many transformations need information about alias <b>sets</b> that are active
588 in some scope, rather than information about pairwise aliasing. The <tt><a
589 href="/doxygen/classllvm_1_1AliasSetTracker.html">AliasSetTracker</a></tt> class
590 is used to efficiently build these Alias Sets from the pairwise alias analysis
591 information provided by the <tt>AliasAnalysis</tt> interface.</p>
593 <p>First you initialize the AliasSetTracker by using the "<tt>add</tt>" methods
594 to add information about various potentially aliasing instructions in the scope
595 you are interested in. Once all of the alias sets are completed, your pass
596 should simply iterate through the constructed alias sets, using the
597 <tt>AliasSetTracker</tt> <tt>begin()</tt>/<tt>end()</tt> methods.</p>
599 <p>The <tt>AliasSet</tt>s formed by the <tt>AliasSetTracker</tt> are guaranteed
600 to be disjoint, calculate mod/ref information and volatility for the set, and
601 keep track of whether or not all of the pointers in the set are Must aliases.
602 The AliasSetTracker also makes sure that sets are properly folded due to call
603 instructions, and can provide a list of pointers in each set.</p>
605 <p>As an example user of this, the <a href="/doxygen/structLICM.html">Loop
606 Invariant Code Motion</a> pass uses <tt>AliasSetTracker</tt>s to calculate alias
607 sets for each loop nest. If an <tt>AliasSet</tt> in a loop is not modified,
608 then all load instructions from that set may be hoisted out of the loop. If any
609 alias sets are stored to <b>and</b> are must alias sets, then the stores may be
610 sunk to outside of the loop, promoting the memory location to a register for the
611 duration of the loop nest. Both of these transformations only apply if the
612 pointer argument is loop-invariant.</p>
616 <!-- _______________________________________________________________________ -->
617 <div class="doc_subsubsection">
618 The AliasSetTracker implementation
621 <div class="doc_text">
623 <p>The AliasSetTracker class is implemented to be as efficient as possible. It
624 uses the union-find algorithm to efficiently merge AliasSets when a pointer is
625 inserted into the AliasSetTracker that aliases multiple sets. The primary data
626 structure is a hash table mapping pointers to the AliasSet they are in.</p>
628 <p>The AliasSetTracker class must maintain a list of all of the LLVM Value*'s
629 that are in each AliasSet. Since the hash table already has entries for each
630 LLVM Value* of interest, the AliasesSets thread the linked list through these
631 hash-table nodes to avoid having to allocate memory unnecessarily, and to make
632 merging alias sets extremely efficient (the linked list merge is constant time).
635 <p>You shouldn't need to understand these details if you are just a client of
636 the AliasSetTracker, but if you look at the code, hopefully this brief
637 description will help make sense of why things are designed the way they
642 <!-- ======================================================================= -->
643 <div class="doc_subsection">
644 <a name="direct">Using the <tt>AliasAnalysis</tt> interface directly</a>
647 <div class="doc_text">
649 <p>If neither of these utility class are what your pass needs, you should use
650 the interfaces exposed by the <tt>AliasAnalysis</tt> class directly. Try to use
651 the higher-level methods when possible (e.g., use mod/ref information instead of
652 the <a href="#alias"><tt>alias</tt></a> method directly if possible) to get the
653 best precision and efficiency.</p>
657 <!-- *********************************************************************** -->
658 <div class="doc_section">
659 <a name="exist">Existing alias analysis implementations and clients</a>
661 <!-- *********************************************************************** -->
663 <div class="doc_text">
665 <p>If you're going to be working with the LLVM alias analysis infrastructure,
666 you should know what clients and implementations of alias analysis are
667 available. In particular, if you are implementing an alias analysis, you should
668 be aware of the <a href="#aliasanalysis-debug">the clients</a> that are useful
669 for monitoring and evaluating different implementations.</p>
673 <!-- ======================================================================= -->
674 <div class="doc_subsection">
675 <a name="impls">Available <tt>AliasAnalysis</tt> implementations</a>
678 <div class="doc_text">
680 <p>This section lists the various implementations of the <tt>AliasAnalysis</tt>
681 interface. With the exception of the <a href="#no-aa"><tt>-no-aa</tt></a> and
682 <a href="#basic-aa"><tt>-basicaa</tt></a> implementations, all of these <a
683 href="#chaining">chain</a> to other alias analysis implementations.</p>
687 <!-- _______________________________________________________________________ -->
688 <div class="doc_subsubsection">
689 <a name="no-aa">The <tt>-no-aa</tt> pass</a>
692 <div class="doc_text">
694 <p>The <tt>-no-aa</tt> pass is just like what it sounds: an alias analysis that
695 never returns any useful information. This pass can be useful if you think that
696 alias analysis is doing something wrong and are trying to narrow down a
701 <!-- _______________________________________________________________________ -->
702 <div class="doc_subsubsection">
703 <a name="basic-aa">The <tt>-basicaa</tt> pass</a>
706 <div class="doc_text">
708 <p>The <tt>-basicaa</tt> pass is the default LLVM alias analysis. It is an
709 aggressive local analysis that "knows" many important facts:</p>
712 <li>Distinct globals, stack allocations, and heap allocations can never
714 <li>Globals, stack allocations, and heap allocations never alias the null
716 <li>Different fields of a structure do not alias.</li>
717 <li>Indexes into arrays with statically differing subscripts cannot alias.</li>
718 <li>Many common standard C library functions <a
719 href="#simplemodref">never access memory or only read memory</a>.</li>
720 <li>Pointers that obviously point to constant globals
721 "<tt>pointToConstantMemory</tt>".</li>
722 <li>Function calls can not modify or references stack allocations if they never
723 escape from the function that allocates them (a common case for automatic
729 <!-- _______________________________________________________________________ -->
730 <div class="doc_subsubsection">
731 <a name="globalsmodref">The <tt>-globalsmodref-aa</tt> pass</a>
734 <div class="doc_text">
736 <p>This pass implements a simple context-sensitive mod/ref and alias analysis
737 for internal global variables that don't "have their address taken". If a
738 global does not have its address taken, the pass knows that no pointers alias
739 the global. This pass also keeps track of functions that it knows never access
740 memory or never read memory. This allows certain optimizations (e.g. GVN) to
741 eliminate call instructions entirely.
744 <p>The real power of this pass is that it provides context-sensitive mod/ref
745 information for call instructions. This allows the optimizer to know that
746 calls to a function do not clobber or read the value of the global, allowing
747 loads and stores to be eliminated.</p>
749 <p>Note that this pass is somewhat limited in its scope (only support
750 non-address taken globals), but is very quick analysis.</p>
753 <!-- _______________________________________________________________________ -->
754 <div class="doc_subsubsection">
755 <a name="steens-aa">The <tt>-steens-aa</tt> pass</a>
758 <div class="doc_text">
760 <p>The <tt>-steens-aa</tt> pass implements a variation on the well-known
761 "Steensgaard's algorithm" for interprocedural alias analysis. Steensgaard's
762 algorithm is a unification-based, flow-insensitive, context-insensitive, and
763 field-insensitive alias analysis that is also very scalable (effectively linear
766 <p>The LLVM <tt>-steens-aa</tt> pass implements a "speculatively
767 field-<b>sensitive</b>" version of Steensgaard's algorithm using the Data
768 Structure Analysis framework. This gives it substantially more precision than
769 the standard algorithm while maintaining excellent analysis scalability.</p>
771 <p>Note that <tt>-steens-aa</tt> is available in the optional "poolalloc"
772 module, it is not part of the LLVM core.</p>
776 <!-- _______________________________________________________________________ -->
777 <div class="doc_subsubsection">
778 <a name="ds-aa">The <tt>-ds-aa</tt> pass</a>
781 <div class="doc_text">
783 <p>The <tt>-ds-aa</tt> pass implements the full Data Structure Analysis
784 algorithm. Data Structure Analysis is a modular unification-based,
785 flow-insensitive, context-<b>sensitive</b>, and speculatively
786 field-<b>sensitive</b> alias analysis that is also quite scalable, usually at
789 <p>This algorithm is capable of responding to a full variety of alias analysis
790 queries, and can provide context-sensitive mod/ref information as well. The
791 only major facility not implemented so far is support for must-alias
794 <p>Note that <tt>-ds-aa</tt> is available in the optional "poolalloc"
795 module, it is not part of the LLVM core.</p>
799 <!-- _______________________________________________________________________ -->
800 <div class="doc_subsubsection">
801 <a name="scev-aa">The <tt>-scev-aa</tt> pass</a>
804 <div class="doc_text">
806 <p>The <tt>-scev-aa</tt> pass implements AliasAnalysis queries by
807 translating them into ScalarEvolution queries. This gives it a
808 more complete understanding of <tt>getelementptr</tt> instructions
809 and loop induction variables than other alias analyses have.</p>
813 <!-- ======================================================================= -->
814 <div class="doc_subsection">
815 <a name="aliasanalysis-xforms">Alias analysis driven transformations</a>
818 <div class="doc_text">
819 LLVM includes several alias-analysis driven transformations which can be used
820 with any of the implementations above.
823 <!-- _______________________________________________________________________ -->
824 <div class="doc_subsubsection">
825 <a name="adce">The <tt>-adce</tt> pass</a>
828 <div class="doc_text">
830 <p>The <tt>-adce</tt> pass, which implements Aggressive Dead Code Elimination
831 uses the <tt>AliasAnalysis</tt> interface to delete calls to functions that do
832 not have side-effects and are not used.</p>
837 <!-- _______________________________________________________________________ -->
838 <div class="doc_subsubsection">
839 <a name="licm">The <tt>-licm</tt> pass</a>
842 <div class="doc_text">
844 <p>The <tt>-licm</tt> pass implements various Loop Invariant Code Motion related
845 transformations. It uses the <tt>AliasAnalysis</tt> interface for several
846 different transformations:</p>
849 <li>It uses mod/ref information to hoist or sink load instructions out of loops
850 if there are no instructions in the loop that modifies the memory loaded.</li>
852 <li>It uses mod/ref information to hoist function calls out of loops that do not
853 write to memory and are loop-invariant.</li>
855 <li>If uses alias information to promote memory objects that are loaded and
856 stored to in loops to live in a register instead. It can do this if there are
857 no may aliases to the loaded/stored memory location.</li>
862 <!-- _______________________________________________________________________ -->
863 <div class="doc_subsubsection">
864 <a name="argpromotion">The <tt>-argpromotion</tt> pass</a>
867 <div class="doc_text">
869 The <tt>-argpromotion</tt> pass promotes by-reference arguments to be passed in
870 by-value instead. In particular, if pointer arguments are only loaded from it
871 passes in the value loaded instead of the address to the function. This pass
872 uses alias information to make sure that the value loaded from the argument
873 pointer is not modified between the entry of the function and any load of the
877 <!-- _______________________________________________________________________ -->
878 <div class="doc_subsubsection">
879 <a name="gvn">The <tt>-gvn</tt>, <tt>-memcpyopt</tt>, and <tt>-dse</tt>
883 <div class="doc_text">
885 <p>These passes use AliasAnalysis information to reason about loads and stores.
890 <!-- ======================================================================= -->
891 <div class="doc_subsection">
892 <a name="aliasanalysis-debug">Clients for debugging and evaluation of
896 <div class="doc_text">
898 <p>These passes are useful for evaluating the various alias analysis
899 implementations. You can use them with commands like '<tt>opt -ds-aa
900 -aa-eval foo.bc -disable-output -stats</tt>'.</p>
904 <!-- _______________________________________________________________________ -->
905 <div class="doc_subsubsection">
906 <a name="print-alias-sets">The <tt>-print-alias-sets</tt> pass</a>
909 <div class="doc_text">
911 <p>The <tt>-print-alias-sets</tt> pass is exposed as part of the
912 <tt>opt</tt> tool to print out the Alias Sets formed by the <a
913 href="#ast"><tt>AliasSetTracker</tt></a> class. This is useful if you're using
914 the <tt>AliasSetTracker</tt> class. To use it, use something like:</p>
916 <div class="doc_code">
918 % opt -ds-aa -print-alias-sets -disable-output
925 <!-- _______________________________________________________________________ -->
926 <div class="doc_subsubsection">
927 <a name="count-aa">The <tt>-count-aa</tt> pass</a>
930 <div class="doc_text">
932 <p>The <tt>-count-aa</tt> pass is useful to see how many queries a particular
933 pass is making and what responses are returned by the alias analysis. As an
936 <div class="doc_code">
938 % opt -basicaa -count-aa -ds-aa -count-aa -licm
942 <p>will print out how many queries (and what responses are returned) by the
943 <tt>-licm</tt> pass (of the <tt>-ds-aa</tt> pass) and how many queries are made
944 of the <tt>-basicaa</tt> pass by the <tt>-ds-aa</tt> pass. This can be useful
945 when debugging a transformation or an alias analysis implementation.</p>
949 <!-- _______________________________________________________________________ -->
950 <div class="doc_subsubsection">
951 <a name="aa-eval">The <tt>-aa-eval</tt> pass</a>
954 <div class="doc_text">
956 <p>The <tt>-aa-eval</tt> pass simply iterates through all pairs of pointers in a
957 function and asks an alias analysis whether or not the pointers alias. This
958 gives an indication of the precision of the alias analysis. Statistics are
959 printed indicating the percent of no/may/must aliases found (a more precise
960 algorithm will have a lower number of may aliases).</p>
964 <!-- *********************************************************************** -->
965 <div class="doc_section">
966 <a name="memdep">Memory Dependence Analysis</a>
968 <!-- *********************************************************************** -->
970 <div class="doc_text">
972 <p>If you're just looking to be a client of alias analysis information, consider
973 using the Memory Dependence Analysis interface instead. MemDep is a lazy,
974 caching layer on top of alias analysis that is able to answer the question of
975 what preceding memory operations a given instruction depends on, either at an
976 intra- or inter-block level. Because of its laziness and caching
977 policy, using MemDep can be a significant performance win over accessing alias
978 analysis directly.</p>
982 <!-- *********************************************************************** -->
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991 <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
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