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| |
| <h1> |
| LLVM Alias Analysis Infrastructure |
| </h1> |
| |
| <ol> |
| <li><a href="#introduction">Introduction</a></li> |
| |
| <li><a href="#overview"><tt>AliasAnalysis</tt> Class Overview</a> |
| <ul> |
| <li><a href="#pointers">Representation of Pointers</a></li> |
| <li><a href="#alias">The <tt>alias</tt> method</a></li> |
| <li><a href="#ModRefInfo">The <tt>getModRefInfo</tt> methods</a></li> |
| <li><a href="#OtherItfs">Other useful <tt>AliasAnalysis</tt> methods</a></li> |
| </ul> |
| </li> |
| |
| <li><a href="#writingnew">Writing a new <tt>AliasAnalysis</tt> Implementation</a> |
| <ul> |
| <li><a href="#passsubclasses">Different Pass styles</a></li> |
| <li><a href="#requiredcalls">Required initialization calls</a></li> |
| <li><a href="#interfaces">Interfaces which may be specified</a></li> |
| <li><a href="#chaining"><tt>AliasAnalysis</tt> chaining behavior</a></li> |
| <li><a href="#updating">Updating analysis results for transformations</a></li> |
| <li><a href="#implefficiency">Efficiency Issues</a></li> |
| <li><a href="#limitations">Limitations</a></li> |
| </ul> |
| </li> |
| |
| <li><a href="#using">Using alias analysis results</a> |
| <ul> |
| <li><a href="#memdep">Using the <tt>MemoryDependenceAnalysis</tt> Pass</a></li> |
| <li><a href="#ast">Using the <tt>AliasSetTracker</tt> class</a></li> |
| <li><a href="#direct">Using the <tt>AliasAnalysis</tt> interface directly</a></li> |
| </ul> |
| </li> |
| |
| <li><a href="#exist">Existing alias analysis implementations and clients</a> |
| <ul> |
| <li><a href="#impls">Available <tt>AliasAnalysis</tt> implementations</a></li> |
| <li><a href="#aliasanalysis-xforms">Alias analysis driven transformations</a></li> |
| <li><a href="#aliasanalysis-debug">Clients for debugging and evaluation of |
| implementations</a></li> |
| </ul> |
| </li> |
| <li><a href="#memdep">Memory Dependence Analysis</a></li> |
| </ol> |
| |
| <div class="doc_author"> |
| <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p> |
| </div> |
| |
| <!-- *********************************************************************** --> |
| <h2> |
| <a name="introduction">Introduction</a> |
| </h2> |
| <!-- *********************************************************************** --> |
| |
| <div> |
| |
| <p>Alias Analysis (aka Pointer Analysis) is a class of techniques which attempt |
| to determine whether or not two pointers ever can point to the same object in |
| memory. There are many different algorithms for alias analysis and many |
| different ways of classifying them: flow-sensitive vs flow-insensitive, |
| context-sensitive vs context-insensitive, field-sensitive vs field-insensitive, |
| unification-based vs subset-based, etc. Traditionally, alias analyses respond |
| to a query with a <a href="#MustMayNo">Must, May, or No</a> alias response, |
| indicating that two pointers always point to the same object, might point to the |
| same object, or are known to never point to the same object.</p> |
| |
| <p>The LLVM <a |
| href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html"><tt>AliasAnalysis</tt></a> |
| class is the primary interface used by clients and implementations of alias |
| analyses in the LLVM system. This class is the common interface between clients |
| of alias analysis information and the implementations providing it, and is |
| designed to support a wide range of implementations and clients (but currently |
| all clients are assumed to be flow-insensitive). In addition to simple alias |
| analysis information, this class exposes Mod/Ref information from those |
| implementations which can provide it, allowing for powerful analyses and |
| transformations to work well together.</p> |
| |
| <p>This document contains information necessary to successfully implement this |
| interface, use it, and to test both sides. It also explains some of the finer |
| points about what exactly results mean. If you feel that something is unclear |
| or should be added, please <a href="mailto:sabre@nondot.org">let me |
| know</a>.</p> |
| |
| </div> |
| |
| <!-- *********************************************************************** --> |
| <h2> |
| <a name="overview"><tt>AliasAnalysis</tt> Class Overview</a> |
| </h2> |
| <!-- *********************************************************************** --> |
| |
| <div> |
| |
| <p>The <a |
| href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html"><tt>AliasAnalysis</tt></a> |
| class defines the interface that the various alias analysis implementations |
| should support. This class exports two important enums: <tt>AliasResult</tt> |
| and <tt>ModRefResult</tt> which represent the result of an alias query or a |
| mod/ref query, respectively.</p> |
| |
| <p>The <tt>AliasAnalysis</tt> interface exposes information about memory, |
| represented in several different ways. In particular, memory objects are |
| represented as a starting address and size, and function calls are represented |
| as the actual <tt>call</tt> or <tt>invoke</tt> instructions that performs the |
| call. The <tt>AliasAnalysis</tt> interface also exposes some helper methods |
| which allow you to get mod/ref information for arbitrary instructions.</p> |
| |
| <p>All <tt>AliasAnalysis</tt> interfaces require that in queries involving |
| multiple values, values which are not |
| <a href="LangRef.html#constants">constants</a> are all defined within the |
| same function.</p> |
| |
| <!-- ======================================================================= --> |
| <h3> |
| <a name="pointers">Representation of Pointers</a> |
| </h3> |
| |
| <div> |
| |
| <p>Most importantly, the <tt>AliasAnalysis</tt> class provides several methods |
| which are used to query whether or not two memory objects alias, whether |
| function calls can modify or read a memory object, etc. For all of these |
| queries, memory objects are represented as a pair of their starting address (a |
| symbolic LLVM <tt>Value*</tt>) and a static size.</p> |
| |
| <p>Representing memory objects as a starting address and a size is critically |
| important for correct Alias Analyses. For example, consider this (silly, but |
| possible) C code:</p> |
| |
| <div class="doc_code"> |
| <pre> |
| int i; |
| char C[2]; |
| char A[10]; |
| /* ... */ |
| for (i = 0; i != 10; ++i) { |
| C[0] = A[i]; /* One byte store */ |
| C[1] = A[9-i]; /* One byte store */ |
| } |
| </pre> |
| </div> |
| |
| <p>In this case, the <tt>basicaa</tt> pass will disambiguate the stores to |
| <tt>C[0]</tt> and <tt>C[1]</tt> because they are accesses to two distinct |
| locations one byte apart, and the accesses are each one byte. In this case, the |
| LICM pass can use store motion to remove the stores from the loop. In |
| constrast, the following code:</p> |
| |
| <div class="doc_code"> |
| <pre> |
| int i; |
| char C[2]; |
| char A[10]; |
| /* ... */ |
| for (i = 0; i != 10; ++i) { |
| ((short*)C)[0] = A[i]; /* Two byte store! */ |
| C[1] = A[9-i]; /* One byte store */ |
| } |
| </pre> |
| </div> |
| |
| <p>In this case, the two stores to C do alias each other, because the access to |
| the <tt>&C[0]</tt> element is a two byte access. If size information wasn't |
| available in the query, even the first case would have to conservatively assume |
| that the accesses alias.</p> |
| |
| </div> |
| |
| <!-- ======================================================================= --> |
| <h3> |
| <a name="alias">The <tt>alias</tt> method</a> |
| </h3> |
| |
| <div> |
| <p>The <tt>alias</tt> method is the primary interface used to determine whether |
| or not two memory objects alias each other. It takes two memory objects as |
| input and returns MustAlias, PartialAlias, MayAlias, or NoAlias as |
| appropriate.</p> |
| |
| <p>Like all <tt>AliasAnalysis</tt> interfaces, the <tt>alias</tt> method requires |
| that either the two pointer values be defined within the same function, or at |
| least one of the values is a <a href="LangRef.html#constants">constant</a>.</p> |
| |
| <!-- _______________________________________________________________________ --> |
| <h4> |
| <a name="MustMayNo">Must, May, and No Alias Responses</a> |
| </h4> |
| |
| <div> |
| <p>The NoAlias response may be used when there is never an immediate dependence |
| between any memory reference <i>based</i> on one pointer and any memory |
| reference <i>based</i> the other. The most obvious example is when the two |
| pointers point to non-overlapping memory ranges. Another is when the two |
| pointers are only ever used for reading memory. Another is when the memory is |
| freed and reallocated between accesses through one pointer and accesses through |
| the other -- in this case, there is a dependence, but it's mediated by the free |
| and reallocation.</p> |
| |
| <p>As an exception to this is with the |
| <a href="LangRef.html#noalias"><tt>noalias</tt></a> keyword; the "irrelevant" |
| dependencies are ignored.</p> |
| |
| <p>The MayAlias response is used whenever the two pointers might refer to the |
| same object.</p> |
| |
| <p>The PartialAlias response is used when the two memory objects are known |
| to be overlapping in some way, but do not start at the same address.</p> |
| |
| <p>The MustAlias response may only be returned if the two memory objects are |
| guaranteed to always start at exactly the same location. A MustAlias response |
| implies that the pointers compare equal.</p> |
| |
| </div> |
| |
| </div> |
| |
| <!-- ======================================================================= --> |
| <h3> |
| <a name="ModRefInfo">The <tt>getModRefInfo</tt> methods</a> |
| </h3> |
| |
| <div> |
| |
| <p>The <tt>getModRefInfo</tt> methods return information about whether the |
| execution of an instruction can read or modify a memory location. Mod/Ref |
| information is always conservative: if an instruction <b>might</b> read or write |
| a location, ModRef is returned.</p> |
| |
| <p>The <tt>AliasAnalysis</tt> class also provides a <tt>getModRefInfo</tt> |
| method for testing dependencies between function calls. This method takes two |
| call sites (CS1 & CS2), returns NoModRef if neither call writes to memory |
| read or written by the other, Ref if CS1 reads memory written by CS2, Mod if CS1 |
| writes to memory read or written by CS2, or ModRef if CS1 might read or write |
| memory written to by CS2. Note that this relation is not commutative.</p> |
| |
| </div> |
| |
| |
| <!-- ======================================================================= --> |
| <h3> |
| <a name="OtherItfs">Other useful <tt>AliasAnalysis</tt> methods</a> |
| </h3> |
| |
| <div> |
| |
| <p> |
| Several other tidbits of information are often collected by various alias |
| analysis implementations and can be put to good use by various clients. |
| </p> |
| |
| <!-- _______________________________________________________________________ --> |
| <h4> |
| The <tt>pointsToConstantMemory</tt> method |
| </h4> |
| |
| <div> |
| |
| <p>The <tt>pointsToConstantMemory</tt> method returns true if and only if the |
| analysis can prove that the pointer only points to unchanging memory locations |
| (functions, constant global variables, and the null pointer). This information |
| can be used to refine mod/ref information: it is impossible for an unchanging |
| memory location to be modified.</p> |
| |
| </div> |
| |
| <!-- _______________________________________________________________________ --> |
| <h4> |
| <a name="simplemodref">The <tt>doesNotAccessMemory</tt> and |
| <tt>onlyReadsMemory</tt> methods</a> |
| </h4> |
| |
| <div> |
| |
| <p>These methods are used to provide very simple mod/ref information for |
| function calls. The <tt>doesNotAccessMemory</tt> method returns true for a |
| function if the analysis can prove that the function never reads or writes to |
| memory, or if the function only reads from constant memory. Functions with this |
| property are side-effect free and only depend on their input arguments, allowing |
| them to be eliminated if they form common subexpressions or be hoisted out of |
| loops. Many common functions behave this way (e.g., <tt>sin</tt> and |
| <tt>cos</tt>) but many others do not (e.g., <tt>acos</tt>, which modifies the |
| <tt>errno</tt> variable).</p> |
| |
| <p>The <tt>onlyReadsMemory</tt> method returns true for a function if analysis |
| can prove that (at most) the function only reads from non-volatile memory. |
| Functions with this property are side-effect free, only depending on their input |
| arguments and the state of memory when they are called. This property allows |
| calls to these functions to be eliminated and moved around, as long as there is |
| no store instruction that changes the contents of memory. Note that all |
| functions that satisfy the <tt>doesNotAccessMemory</tt> method also satisfies |
| <tt>onlyReadsMemory</tt>.</p> |
| |
| </div> |
| |
| </div> |
| |
| </div> |
| |
| <!-- *********************************************************************** --> |
| <h2> |
| <a name="writingnew">Writing a new <tt>AliasAnalysis</tt> Implementation</a> |
| </h2> |
| <!-- *********************************************************************** --> |
| |
| <div> |
| |
| <p>Writing a new alias analysis implementation for LLVM is quite |
| straight-forward. There are already several implementations that you can use |
| for examples, and the following information should help fill in any details. |
| For a examples, take a look at the <a href="#impls">various alias analysis |
| implementations</a> included with LLVM.</p> |
| |
| <!-- ======================================================================= --> |
| <h3> |
| <a name="passsubclasses">Different Pass styles</a> |
| </h3> |
| |
| <div> |
| |
| <p>The first step to determining what type of <a |
| href="WritingAnLLVMPass.html">LLVM pass</a> you need to use for your Alias |
| Analysis. As is the case with most other analyses and transformations, the |
| answer should be fairly obvious from what type of problem you are trying to |
| solve:</p> |
| |
| <ol> |
| <li>If you require interprocedural analysis, it should be a |
| <tt>Pass</tt>.</li> |
| <li>If you are a function-local analysis, subclass <tt>FunctionPass</tt>.</li> |
| <li>If you don't need to look at the program at all, subclass |
| <tt>ImmutablePass</tt>.</li> |
| </ol> |
| |
| <p>In addition to the pass that you subclass, you should also inherit from the |
| <tt>AliasAnalysis</tt> interface, of course, and use the |
| <tt>RegisterAnalysisGroup</tt> template to register as an implementation of |
| <tt>AliasAnalysis</tt>.</p> |
| |
| </div> |
| |
| <!-- ======================================================================= --> |
| <h3> |
| <a name="requiredcalls">Required initialization calls</a> |
| </h3> |
| |
| <div> |
| |
| <p>Your subclass of <tt>AliasAnalysis</tt> is required to invoke two methods on |
| the <tt>AliasAnalysis</tt> base class: <tt>getAnalysisUsage</tt> and |
| <tt>InitializeAliasAnalysis</tt>. In particular, your implementation of |
| <tt>getAnalysisUsage</tt> should explicitly call into the |
| <tt>AliasAnalysis::getAnalysisUsage</tt> method in addition to doing any |
| declaring any pass dependencies your pass has. Thus you should have something |
| like this:</p> |
| |
| <div class="doc_code"> |
| <pre> |
| void getAnalysisUsage(AnalysisUsage &AU) const { |
| AliasAnalysis::getAnalysisUsage(AU); |
| <i>// declare your dependencies here.</i> |
| } |
| </pre> |
| </div> |
| |
| <p>Additionally, your must invoke the <tt>InitializeAliasAnalysis</tt> method |
| from your analysis run method (<tt>run</tt> for a <tt>Pass</tt>, |
| <tt>runOnFunction</tt> for a <tt>FunctionPass</tt>, or <tt>InitializePass</tt> |
| for an <tt>ImmutablePass</tt>). For example (as part of a <tt>Pass</tt>):</p> |
| |
| <div class="doc_code"> |
| <pre> |
| bool run(Module &M) { |
| InitializeAliasAnalysis(this); |
| <i>// Perform analysis here...</i> |
| return false; |
| } |
| </pre> |
| </div> |
| |
| </div> |
| |
| <!-- ======================================================================= --> |
| <h3> |
| <a name="interfaces">Interfaces which may be specified</a> |
| </h3> |
| |
| <div> |
| |
| <p>All of the <a |
| href="/doxygen/classllvm_1_1AliasAnalysis.html"><tt>AliasAnalysis</tt></a> |
| virtual methods default to providing <a href="#chaining">chaining</a> to another |
| alias analysis implementation, which ends up returning conservatively correct |
| information (returning "May" Alias and "Mod/Ref" for alias and mod/ref queries |
| respectively). Depending on the capabilities of the analysis you are |
| implementing, you just override the interfaces you can improve.</p> |
| |
| </div> |
| |
| |
| |
| <!-- ======================================================================= --> |
| <h3> |
| <a name="chaining"><tt>AliasAnalysis</tt> chaining behavior</a> |
| </h3> |
| |
| <div> |
| |
| <p>With only one special exception (the <a href="#no-aa"><tt>no-aa</tt></a> |
| pass) every alias analysis pass chains to another alias analysis |
| implementation (for example, the user can specify "<tt>-basicaa -ds-aa |
| -licm</tt>" to get the maximum benefit from both alias |
| analyses). The alias analysis class automatically takes care of most of this |
| for methods that you don't override. For methods that you do override, in code |
| paths that return a conservative MayAlias or Mod/Ref result, simply return |
| whatever the superclass computes. For example:</p> |
| |
| <div class="doc_code"> |
| <pre> |
| AliasAnalysis::AliasResult alias(const Value *V1, unsigned V1Size, |
| const Value *V2, unsigned V2Size) { |
| if (...) |
| return NoAlias; |
| ... |
| |
| <i>// Couldn't determine a must or no-alias result.</i> |
| return AliasAnalysis::alias(V1, V1Size, V2, V2Size); |
| } |
| </pre> |
| </div> |
| |
| <p>In addition to analysis queries, you must make sure to unconditionally pass |
| LLVM <a href="#updating">update notification</a> methods to the superclass as |
| well if you override them, which allows all alias analyses in a change to be |
| updated.</p> |
| |
| </div> |
| |
| |
| <!-- ======================================================================= --> |
| <h3> |
| <a name="updating">Updating analysis results for transformations</a> |
| </h3> |
| |
| <div> |
| <p> |
| Alias analysis information is initially computed for a static snapshot of the |
| program, but clients will use this information to make transformations to the |
| code. All but the most trivial forms of alias analysis will need to have their |
| analysis results updated to reflect the changes made by these transformations. |
| </p> |
| |
| <p> |
| The <tt>AliasAnalysis</tt> interface exposes four methods which are used to |
| communicate program changes from the clients to the analysis implementations. |
| Various alias analysis implementations should use these methods to ensure that |
| their internal data structures are kept up-to-date as the program changes (for |
| example, when an instruction is deleted), and clients of alias analysis must be |
| sure to call these interfaces appropriately. |
| </p> |
| |
| <!-- _______________________________________________________________________ --> |
| <h4>The <tt>deleteValue</tt> method</h4> |
| |
| <div> |
| The <tt>deleteValue</tt> method is called by transformations when they remove an |
| instruction or any other value from the program (including values that do not |
| use pointers). Typically alias analyses keep data structures that have entries |
| for each value in the program. When this method is called, they should remove |
| any entries for the specified value, if they exist. |
| </div> |
| |
| <!-- _______________________________________________________________________ --> |
| <h4>The <tt>copyValue</tt> method</h4> |
| |
| <div> |
| The <tt>copyValue</tt> method is used when a new value is introduced into the |
| program. There is no way to introduce a value into the program that did not |
| exist before (this doesn't make sense for a safe compiler transformation), so |
| this is the only way to introduce a new value. This method indicates that the |
| new value has exactly the same properties as the value being copied. |
| </div> |
| |
| <!-- _______________________________________________________________________ --> |
| <h4>The <tt>replaceWithNewValue</tt> method</h4> |
| |
| <div> |
| This method is a simple helper method that is provided to make clients easier to |
| use. It is implemented by copying the old analysis information to the new |
| value, then deleting the old value. This method cannot be overridden by alias |
| analysis implementations. |
| </div> |
| |
| <!-- _______________________________________________________________________ --> |
| <h4>The <tt>addEscapingUse</tt> method</h4> |
| |
| <div> |
| <p>The <tt>addEscapingUse</tt> method is used when the uses of a pointer |
| value have changed in ways that may invalidate precomputed analysis information. |
| Implementations may either use this callback to provide conservative responses |
| for points whose uses have change since analysis time, or may recompute some |
| or all of their internal state to continue providing accurate responses.</p> |
| |
| <p>In general, any new use of a pointer value is considered an escaping use, |
| and must be reported through this callback, <em>except</em> for the |
| uses below:</p> |
| |
| <ul> |
| <li>A <tt>bitcast</tt> or <tt>getelementptr</tt> of the pointer</li> |
| <li>A <tt>store</tt> through the pointer (but not a <tt>store</tt> |
| <em>of</em> the pointer)</li> |
| <li>A <tt>load</tt> through the pointer</li> |
| </ul> |
| </div> |
| |
| </div> |
| |
| <!-- ======================================================================= --> |
| <h3> |
| <a name="implefficiency">Efficiency Issues</a> |
| </h3> |
| |
| <div> |
| |
| <p>From the LLVM perspective, the only thing you need to do to provide an |
| efficient alias analysis is to make sure that alias analysis <b>queries</b> are |
| serviced quickly. The actual calculation of the alias analysis results (the |
| "run" method) is only performed once, but many (perhaps duplicate) queries may |
| be performed. Because of this, try to move as much computation to the run |
| method as possible (within reason).</p> |
| |
| </div> |
| |
| <!-- ======================================================================= --> |
| <h3> |
| <a name="limitations">Limitations</a> |
| </h3> |
| |
| <div> |
| |
| <p>The AliasAnalysis infrastructure has several limitations which make |
| writing a new <tt>AliasAnalysis</tt> implementation difficult.</p> |
| |
| <p>There is no way to override the default alias analysis. It would |
| be very useful to be able to do something like "opt -my-aa -O2" and |
| have it use -my-aa for all passes which need AliasAnalysis, but there |
| is currently no support for that, short of changing the source code |
| and recompiling. Similarly, there is also no way of setting a chain |
| of analyses as the default.</p> |
| |
| <p>There is no way for transform passes to declare that they preserve |
| <tt>AliasAnalysis</tt> implementations. The <tt>AliasAnalysis</tt> |
| interface includes <tt>deleteValue</tt> and <tt>copyValue</tt> methods |
| which are intended to allow a pass to keep an AliasAnalysis consistent, |
| however there's no way for a pass to declare in its |
| <tt>getAnalysisUsage</tt> that it does so. Some passes attempt to use |
| <tt>AU.addPreserved<AliasAnalysis></tt>, however this doesn't |
| actually have any effect.</p> |
| |
| <p><tt>AliasAnalysisCounter</tt> (<tt>-count-aa</tt>) and <tt>AliasDebugger</tt> |
| (<tt>-debug-aa</tt>) are implemented as <tt>ModulePass</tt> classes, so if your |
| alias analysis uses <tt>FunctionPass</tt>, it won't be able to use |
| these utilities. If you try to use them, the pass manager will |
| silently route alias analysis queries directly to |
| <tt>BasicAliasAnalysis</tt> instead.</p> |
| |
| <p>Similarly, the <tt>opt -p</tt> option introduces <tt>ModulePass</tt> |
| passes between each pass, which prevents the use of <tt>FunctionPass</tt> |
| alias analysis passes.</p> |
| |
| <p>The <tt>AliasAnalysis</tt> API does have functions for notifying |
| implementations when values are deleted or copied, however these |
| aren't sufficient. There are many other ways that LLVM IR can be |
| modified which could be relevant to <tt>AliasAnalysis</tt> |
| implementations which can not be expressed.</p> |
| |
| <p>The <tt>AliasAnalysisDebugger</tt> utility seems to suggest that |
| <tt>AliasAnalysis</tt> implementations can expect that they will be |
| informed of any relevant <tt>Value</tt> before it appears in an |
| alias query. However, popular clients such as <tt>GVN</tt> don't |
| support this, and are known to trigger errors when run with the |
| <tt>AliasAnalysisDebugger</tt>.</p> |
| |
| <p>Due to several of the above limitations, the most obvious use for |
| the <tt>AliasAnalysisCounter</tt> utility, collecting stats on all |
| alias queries in a compilation, doesn't work, even if the |
| <tt>AliasAnalysis</tt> implementations don't use <tt>FunctionPass</tt>. |
| There's no way to set a default, much less a default sequence, |
| and there's no way to preserve it.</p> |
| |
| <p>The <tt>AliasSetTracker</tt> class (which is used by <tt>LICM</tt> |
| makes a non-deterministic number of alias queries. This can cause stats |
| collected by <tt>AliasAnalysisCounter</tt> to have fluctuations among |
| identical runs, for example. Another consequence is that debugging |
| techniques involving pausing execution after a predetermined number |
| of queries can be unreliable.</p> |
| |
| <p>Many alias queries can be reformulated in terms of other alias |
| queries. When multiple <tt>AliasAnalysis</tt> queries are chained together, |
| it would make sense to start those queries from the beginning of the chain, |
| with care taken to avoid infinite looping, however currently an |
| implementation which wants to do this can only start such queries |
| from itself.</p> |
| |
| </div> |
| |
| </div> |
| |
| <!-- *********************************************************************** --> |
| <h2> |
| <a name="using">Using alias analysis results</a> |
| </h2> |
| <!-- *********************************************************************** --> |
| |
| <div> |
| |
| <p>There are several different ways to use alias analysis results. In order of |
| preference, these are...</p> |
| |
| <!-- ======================================================================= --> |
| <h3> |
| <a name="memdep">Using the <tt>MemoryDependenceAnalysis</tt> Pass</a> |
| </h3> |
| |
| <div> |
| |
| <p>The <tt>memdep</tt> pass uses alias analysis to provide high-level dependence |
| information about memory-using instructions. This will tell you which store |
| feeds into a load, for example. It uses caching and other techniques to be |
| efficient, and is used by Dead Store Elimination, GVN, and memcpy optimizations. |
| </p> |
| |
| </div> |
| |
| <!-- ======================================================================= --> |
| <h3> |
| <a name="ast">Using the <tt>AliasSetTracker</tt> class</a> |
| </h3> |
| |
| <div> |
| |
| <p>Many transformations need information about alias <b>sets</b> that are active |
| in some scope, rather than information about pairwise aliasing. The <tt><a |
| href="/doxygen/classllvm_1_1AliasSetTracker.html">AliasSetTracker</a></tt> class |
| is used to efficiently build these Alias Sets from the pairwise alias analysis |
| information provided by the <tt>AliasAnalysis</tt> interface.</p> |
| |
| <p>First you initialize the AliasSetTracker by using the "<tt>add</tt>" methods |
| to add information about various potentially aliasing instructions in the scope |
| you are interested in. Once all of the alias sets are completed, your pass |
| should simply iterate through the constructed alias sets, using the |
| <tt>AliasSetTracker</tt> <tt>begin()</tt>/<tt>end()</tt> methods.</p> |
| |
| <p>The <tt>AliasSet</tt>s formed by the <tt>AliasSetTracker</tt> are guaranteed |
| to be disjoint, calculate mod/ref information and volatility for the set, and |
| keep track of whether or not all of the pointers in the set are Must aliases. |
| The AliasSetTracker also makes sure that sets are properly folded due to call |
| instructions, and can provide a list of pointers in each set.</p> |
| |
| <p>As an example user of this, the <a href="/doxygen/structLICM.html">Loop |
| Invariant Code Motion</a> pass uses <tt>AliasSetTracker</tt>s to calculate alias |
| sets for each loop nest. If an <tt>AliasSet</tt> in a loop is not modified, |
| then all load instructions from that set may be hoisted out of the loop. If any |
| alias sets are stored to <b>and</b> are must alias sets, then the stores may be |
| sunk to outside of the loop, promoting the memory location to a register for the |
| duration of the loop nest. Both of these transformations only apply if the |
| pointer argument is loop-invariant.</p> |
| |
| <!-- _______________________________________________________________________ --> |
| <h4> |
| The AliasSetTracker implementation |
| </h4> |
| |
| <div> |
| |
| <p>The AliasSetTracker class is implemented to be as efficient as possible. It |
| uses the union-find algorithm to efficiently merge AliasSets when a pointer is |
| inserted into the AliasSetTracker that aliases multiple sets. The primary data |
| structure is a hash table mapping pointers to the AliasSet they are in.</p> |
| |
| <p>The AliasSetTracker class must maintain a list of all of the LLVM Value*'s |
| that are in each AliasSet. Since the hash table already has entries for each |
| LLVM Value* of interest, the AliasesSets thread the linked list through these |
| hash-table nodes to avoid having to allocate memory unnecessarily, and to make |
| merging alias sets extremely efficient (the linked list merge is constant time). |
| </p> |
| |
| <p>You shouldn't need to understand these details if you are just a client of |
| the AliasSetTracker, but if you look at the code, hopefully this brief |
| description will help make sense of why things are designed the way they |
| are.</p> |
| |
| </div> |
| |
| </div> |
| |
| <!-- ======================================================================= --> |
| <h3> |
| <a name="direct">Using the <tt>AliasAnalysis</tt> interface directly</a> |
| </h3> |
| |
| <div> |
| |
| <p>If neither of these utility class are what your pass needs, you should use |
| the interfaces exposed by the <tt>AliasAnalysis</tt> class directly. Try to use |
| the higher-level methods when possible (e.g., use mod/ref information instead of |
| the <a href="#alias"><tt>alias</tt></a> method directly if possible) to get the |
| best precision and efficiency.</p> |
| |
| </div> |
| |
| </div> |
| |
| <!-- *********************************************************************** --> |
| <h2> |
| <a name="exist">Existing alias analysis implementations and clients</a> |
| </h2> |
| <!-- *********************************************************************** --> |
| |
| <div> |
| |
| <p>If you're going to be working with the LLVM alias analysis infrastructure, |
| you should know what clients and implementations of alias analysis are |
| available. In particular, if you are implementing an alias analysis, you should |
| be aware of the <a href="#aliasanalysis-debug">the clients</a> that are useful |
| for monitoring and evaluating different implementations.</p> |
| |
| <!-- ======================================================================= --> |
| <h3> |
| <a name="impls">Available <tt>AliasAnalysis</tt> implementations</a> |
| </h3> |
| |
| <div> |
| |
| <p>This section lists the various implementations of the <tt>AliasAnalysis</tt> |
| interface. With the exception of the <a href="#no-aa"><tt>-no-aa</tt></a> |
| implementation, all of these <a href="#chaining">chain</a> to other alias |
| analysis implementations.</p> |
| |
| <!-- _______________________________________________________________________ --> |
| <h4> |
| <a name="no-aa">The <tt>-no-aa</tt> pass</a> |
| </h4> |
| |
| <div> |
| |
| <p>The <tt>-no-aa</tt> pass is just like what it sounds: an alias analysis that |
| never returns any useful information. This pass can be useful if you think that |
| alias analysis is doing something wrong and are trying to narrow down a |
| problem.</p> |
| |
| </div> |
| |
| <!-- _______________________________________________________________________ --> |
| <h4> |
| <a name="basic-aa">The <tt>-basicaa</tt> pass</a> |
| </h4> |
| |
| <div> |
| |
| <p>The <tt>-basicaa</tt> pass is an aggressive local analysis that "knows" |
| many important facts:</p> |
| |
| <ul> |
| <li>Distinct globals, stack allocations, and heap allocations can never |
| alias.</li> |
| <li>Globals, stack allocations, and heap allocations never alias the null |
| pointer.</li> |
| <li>Different fields of a structure do not alias.</li> |
| <li>Indexes into arrays with statically differing subscripts cannot alias.</li> |
| <li>Many common standard C library functions <a |
| href="#simplemodref">never access memory or only read memory</a>.</li> |
| <li>Pointers that obviously point to constant globals |
| "<tt>pointToConstantMemory</tt>".</li> |
| <li>Function calls can not modify or references stack allocations if they never |
| escape from the function that allocates them (a common case for automatic |
| arrays).</li> |
| </ul> |
| |
| </div> |
| |
| <!-- _______________________________________________________________________ --> |
| <h4> |
| <a name="globalsmodref">The <tt>-globalsmodref-aa</tt> pass</a> |
| </h4> |
| |
| <div> |
| |
| <p>This pass implements a simple context-sensitive mod/ref and alias analysis |
| for internal global variables that don't "have their address taken". If a |
| global does not have its address taken, the pass knows that no pointers alias |
| the global. This pass also keeps track of functions that it knows never access |
| memory or never read memory. This allows certain optimizations (e.g. GVN) to |
| eliminate call instructions entirely. |
| </p> |
| |
| <p>The real power of this pass is that it provides context-sensitive mod/ref |
| information for call instructions. This allows the optimizer to know that |
| calls to a function do not clobber or read the value of the global, allowing |
| loads and stores to be eliminated.</p> |
| |
| <p>Note that this pass is somewhat limited in its scope (only support |
| non-address taken globals), but is very quick analysis.</p> |
| </div> |
| |
| <!-- _______________________________________________________________________ --> |
| <h4> |
| <a name="steens-aa">The <tt>-steens-aa</tt> pass</a> |
| </h4> |
| |
| <div> |
| |
| <p>The <tt>-steens-aa</tt> pass implements a variation on the well-known |
| "Steensgaard's algorithm" for interprocedural alias analysis. Steensgaard's |
| algorithm is a unification-based, flow-insensitive, context-insensitive, and |
| field-insensitive alias analysis that is also very scalable (effectively linear |
| time).</p> |
| |
| <p>The LLVM <tt>-steens-aa</tt> pass implements a "speculatively |
| field-<b>sensitive</b>" version of Steensgaard's algorithm using the Data |
| Structure Analysis framework. This gives it substantially more precision than |
| the standard algorithm while maintaining excellent analysis scalability.</p> |
| |
| <p>Note that <tt>-steens-aa</tt> is available in the optional "poolalloc" |
| module, it is not part of the LLVM core.</p> |
| |
| </div> |
| |
| <!-- _______________________________________________________________________ --> |
| <h4> |
| <a name="ds-aa">The <tt>-ds-aa</tt> pass</a> |
| </h4> |
| |
| <div> |
| |
| <p>The <tt>-ds-aa</tt> pass implements the full Data Structure Analysis |
| algorithm. Data Structure Analysis is a modular unification-based, |
| flow-insensitive, context-<b>sensitive</b>, and speculatively |
| field-<b>sensitive</b> alias analysis that is also quite scalable, usually at |
| O(n*log(n)).</p> |
| |
| <p>This algorithm is capable of responding to a full variety of alias analysis |
| queries, and can provide context-sensitive mod/ref information as well. The |
| only major facility not implemented so far is support for must-alias |
| information.</p> |
| |
| <p>Note that <tt>-ds-aa</tt> is available in the optional "poolalloc" |
| module, it is not part of the LLVM core.</p> |
| |
| </div> |
| |
| <!-- _______________________________________________________________________ --> |
| <h4> |
| <a name="scev-aa">The <tt>-scev-aa</tt> pass</a> |
| </h4> |
| |
| <div> |
| |
| <p>The <tt>-scev-aa</tt> pass implements AliasAnalysis queries by |
| translating them into ScalarEvolution queries. This gives it a |
| more complete understanding of <tt>getelementptr</tt> instructions |
| and loop induction variables than other alias analyses have.</p> |
| |
| </div> |
| |
| </div> |
| |
| <!-- ======================================================================= --> |
| <h3> |
| <a name="aliasanalysis-xforms">Alias analysis driven transformations</a> |
| </h3> |
| |
| <div> |
| LLVM includes several alias-analysis driven transformations which can be used |
| with any of the implementations above. |
| |
| <!-- _______________________________________________________________________ --> |
| <h4> |
| <a name="adce">The <tt>-adce</tt> pass</a> |
| </h4> |
| |
| <div> |
| |
| <p>The <tt>-adce</tt> pass, which implements Aggressive Dead Code Elimination |
| uses the <tt>AliasAnalysis</tt> interface to delete calls to functions that do |
| not have side-effects and are not used.</p> |
| |
| </div> |
| |
| |
| <!-- _______________________________________________________________________ --> |
| <h4> |
| <a name="licm">The <tt>-licm</tt> pass</a> |
| </h4> |
| |
| <div> |
| |
| <p>The <tt>-licm</tt> pass implements various Loop Invariant Code Motion related |
| transformations. It uses the <tt>AliasAnalysis</tt> interface for several |
| different transformations:</p> |
| |
| <ul> |
| <li>It uses mod/ref information to hoist or sink load instructions out of loops |
| if there are no instructions in the loop that modifies the memory loaded.</li> |
| |
| <li>It uses mod/ref information to hoist function calls out of loops that do not |
| write to memory and are loop-invariant.</li> |
| |
| <li>If uses alias information to promote memory objects that are loaded and |
| stored to in loops to live in a register instead. It can do this if there are |
| no may aliases to the loaded/stored memory location.</li> |
| </ul> |
| |
| </div> |
| |
| <!-- _______________________________________________________________________ --> |
| <h4> |
| <a name="argpromotion">The <tt>-argpromotion</tt> pass</a> |
| </h4> |
| |
| <div> |
| <p> |
| The <tt>-argpromotion</tt> pass promotes by-reference arguments to be passed in |
| by-value instead. In particular, if pointer arguments are only loaded from it |
| passes in the value loaded instead of the address to the function. This pass |
| uses alias information to make sure that the value loaded from the argument |
| pointer is not modified between the entry of the function and any load of the |
| pointer.</p> |
| </div> |
| |
| <!-- _______________________________________________________________________ --> |
| <h4> |
| <a name="gvn">The <tt>-gvn</tt>, <tt>-memcpyopt</tt>, and <tt>-dse</tt> |
| passes</a> |
| </h4> |
| |
| <div> |
| |
| <p>These passes use AliasAnalysis information to reason about loads and stores. |
| </p> |
| |
| </div> |
| |
| </div> |
| |
| <!-- ======================================================================= --> |
| <h3> |
| <a name="aliasanalysis-debug">Clients for debugging and evaluation of |
| implementations</a> |
| </h3> |
| |
| <div> |
| |
| <p>These passes are useful for evaluating the various alias analysis |
| implementations. You can use them with commands like '<tt>opt -ds-aa |
| -aa-eval foo.bc -disable-output -stats</tt>'.</p> |
| |
| <!-- _______________________________________________________________________ --> |
| <h4> |
| <a name="print-alias-sets">The <tt>-print-alias-sets</tt> pass</a> |
| </h4> |
| |
| <div> |
| |
| <p>The <tt>-print-alias-sets</tt> pass is exposed as part of the |
| <tt>opt</tt> tool to print out the Alias Sets formed by the <a |
| href="#ast"><tt>AliasSetTracker</tt></a> class. This is useful if you're using |
| the <tt>AliasSetTracker</tt> class. To use it, use something like:</p> |
| |
| <div class="doc_code"> |
| <pre> |
| % opt -ds-aa -print-alias-sets -disable-output |
| </pre> |
| </div> |
| |
| </div> |
| |
| |
| <!-- _______________________________________________________________________ --> |
| <h4> |
| <a name="count-aa">The <tt>-count-aa</tt> pass</a> |
| </h4> |
| |
| <div> |
| |
| <p>The <tt>-count-aa</tt> pass is useful to see how many queries a particular |
| pass is making and what responses are returned by the alias analysis. As an |
| example,</p> |
| |
| <div class="doc_code"> |
| <pre> |
| % opt -basicaa -count-aa -ds-aa -count-aa -licm |
| </pre> |
| </div> |
| |
| <p>will print out how many queries (and what responses are returned) by the |
| <tt>-licm</tt> pass (of the <tt>-ds-aa</tt> pass) and how many queries are made |
| of the <tt>-basicaa</tt> pass by the <tt>-ds-aa</tt> pass. This can be useful |
| when debugging a transformation or an alias analysis implementation.</p> |
| |
| </div> |
| |
| <!-- _______________________________________________________________________ --> |
| <h4> |
| <a name="aa-eval">The <tt>-aa-eval</tt> pass</a> |
| </h4> |
| |
| <div> |
| |
| <p>The <tt>-aa-eval</tt> pass simply iterates through all pairs of pointers in a |
| function and asks an alias analysis whether or not the pointers alias. This |
| gives an indication of the precision of the alias analysis. Statistics are |
| printed indicating the percent of no/may/must aliases found (a more precise |
| algorithm will have a lower number of may aliases).</p> |
| |
| </div> |
| |
| </div> |
| |
| </div> |
| |
| <!-- *********************************************************************** --> |
| <h2> |
| <a name="memdep">Memory Dependence Analysis</a> |
| </h2> |
| <!-- *********************************************************************** --> |
| |
| <div> |
| |
| <p>If you're just looking to be a client of alias analysis information, consider |
| using the Memory Dependence Analysis interface instead. MemDep is a lazy, |
| caching layer on top of alias analysis that is able to answer the question of |
| what preceding memory operations a given instruction depends on, either at an |
| intra- or inter-block level. Because of its laziness and caching |
| policy, using MemDep can be a significant performance win over accessing alias |
| analysis directly.</p> |
| |
| </div> |
| |
| <!-- *********************************************************************** --> |
| |
| <hr> |
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