1 //===- llvm/Analysis/AliasAnalysis.h - Alias Analysis Interface -*- C++ -*-===//
3 // The LLVM Compiler Infrastructure
5 // This file was developed by the LLVM research group and is distributed under
6 // the University of Illinois Open Source License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This file defines the generic AliasAnalysis interface, which is used as the
11 // common interface used by all clients of alias analysis information, and
12 // implemented by all alias analysis implementations. Mod/Ref information is
13 // also captured by this interface.
15 // Implementations of this interface must implement the various virtual methods,
16 // which automatically provides functionality for the entire suite of client
19 // This API represents memory as a (Pointer, Size) pair. The Pointer component
20 // specifies the base memory address of the region, the Size specifies how large
21 // of an area is being queried. If Size is 0, two pointers only alias if they
22 // are exactly equal. If size is greater than zero, but small, the two pointers
23 // alias if the areas pointed to overlap. If the size is very large (ie, ~0U),
24 // then the two pointers alias if they may be pointing to components of the same
25 // memory object. Pointers that point to two completely different objects in
26 // memory never alias, regardless of the value of the Size component.
28 //===----------------------------------------------------------------------===//
30 #ifndef LLVM_ANALYSIS_ALIAS_ANALYSIS_H
31 #define LLVM_ANALYSIS_ALIAS_ANALYSIS_H
33 #include "llvm/Support/CallSite.h"
34 #include "llvm/Pass.h" // Need this for IncludeFile
46 AliasAnalysis *AA; // Previous Alias Analysis to chain to.
48 /// InitializeAliasAnalysis - Subclasses must call this method to initialize
49 /// the AliasAnalysis interface before any other methods are called. This is
50 /// typically called by the run* methods of these subclasses. This may be
51 /// called multiple times.
53 void InitializeAliasAnalysis(Pass *P);
55 // getAnalysisUsage - All alias analysis implementations should invoke this
56 // directly (using AliasAnalysis::getAnalysisUsage(AU)) to make sure that
57 // TargetData is required by the pass.
58 virtual void getAnalysisUsage(AnalysisUsage &AU) const;
61 AliasAnalysis() : TD(0), AA(0) {}
62 virtual ~AliasAnalysis(); // We want to be subclassed
64 /// getTargetData - Every alias analysis implementation depends on the size of
65 /// data items in the current Target. This provides a uniform way to handle
68 const TargetData &getTargetData() const { return *TD; }
70 //===--------------------------------------------------------------------===//
74 /// Alias analysis result - Either we know for sure that it does not alias, we
75 /// know for sure it must alias, or we don't know anything: The two pointers
76 /// _might_ alias. This enum is designed so you can do things like:
77 /// if (AA.alias(P1, P2)) { ... }
78 /// to check to see if two pointers might alias.
80 enum AliasResult { NoAlias = 0, MayAlias = 1, MustAlias = 2 };
82 /// alias - The main low level interface to the alias analysis implementation.
83 /// Returns a Result indicating whether the two pointers are aliased to each
84 /// other. This is the interface that must be implemented by specific alias
85 /// analysis implementations.
87 virtual AliasResult alias(const Value *V1, unsigned V1Size,
88 const Value *V2, unsigned V2Size);
90 /// getMustAliases - If there are any pointers known that must alias this
91 /// pointer, return them now. This allows alias-set based alias analyses to
92 /// perform a form a value numbering (which is exposed by load-vn). If an
93 /// alias analysis supports this, it should ADD any must aliased pointers to
94 /// the specified vector.
96 virtual void getMustAliases(Value *P, std::vector<Value*> &RetVals);
98 /// pointsToConstantMemory - If the specified pointer is known to point into
99 /// constant global memory, return true. This allows disambiguation of store
100 /// instructions from constant pointers.
102 virtual bool pointsToConstantMemory(const Value *P);
104 //===--------------------------------------------------------------------===//
105 /// Simple mod/ref information...
108 /// ModRefResult - Represent the result of a mod/ref query. Mod and Ref are
109 /// bits which may be or'd together.
111 enum ModRefResult { NoModRef = 0, Ref = 1, Mod = 2, ModRef = 3 };
114 /// ModRefBehavior - Summary of how a function affects memory in the program.
115 /// Loads from constant globals are not considered memory accesses for this
116 /// interface. Also, functions may freely modify stack space local to their
117 /// invocation without having to report it through these interfaces.
118 enum ModRefBehavior {
119 // DoesNotAccessMemory - This function does not perform any non-local loads
120 // or stores to memory.
122 // This property corresponds to the GCC 'const' attribute.
125 // AccessesArguments - This function accesses function arguments in
126 // non-volatile and well known ways, but does not access any other memory.
128 // Clients may call getArgumentAccesses to get specific information about
129 // how pointer arguments are used.
132 // AccessesArgumentsAndGlobals - This function has accesses function
133 // arguments and global variables in non-volatile and well-known ways, but
134 // does not access any other memory.
136 // Clients may call getArgumentAccesses to get specific information about
137 // how pointer arguments and globals are used.
138 AccessesArgumentsAndGlobals,
140 // OnlyReadsMemory - This function does not perform any non-local stores or
141 // volatile loads, but may read from any memory location.
143 // This property corresponds to the GCC 'pure' attribute.
146 // UnknownModRefBehavior - This indicates that the function could not be
147 // classified into one of the behaviors above.
148 UnknownModRefBehavior
151 /// PointerAccessInfo - This struct is used to return results for pointers,
152 /// globals, and the return value of a function.
153 struct PointerAccessInfo {
154 /// V - The value this record corresponds to. This may be an Argument for
155 /// the function, a GlobalVariable, or null, corresponding to the return
156 /// value for the function.
159 /// ModRefInfo - Whether the pointer is loaded or stored to/from.
161 ModRefResult ModRefInfo;
163 /// AccessType - Specific fine-grained access information for the argument.
164 /// If none of these classifications is general enough, the
165 /// getModRefBehavior method should not return AccessesArguments*. If a
166 /// record is not returned for a particular argument, the argument is never
167 /// dead and never dereferenced.
169 /// ScalarAccess - The pointer is dereferenced.
173 /// ArrayAccess - The pointer is indexed through as an array of elements.
177 /// ElementAccess ?? P->F only?
179 /// CallsThrough - Indirect calls are made through the specified function
185 /// getModRefBehavior - Return the behavior of the specified function if
186 /// called from the specified call site. The call site may be null in which
187 /// case the most generic behavior of this function should be returned.
188 virtual ModRefBehavior getModRefBehavior(Function *F, CallSite CS,
189 std::vector<PointerAccessInfo> *Info = 0);
191 /// doesNotAccessMemory - If the specified function is known to never read or
192 /// write memory, return true. If the function only reads from known-constant
193 /// memory, it is also legal to return true. Functions that unwind the stack
194 /// are not legal for this predicate.
196 /// Many optimizations (such as CSE and LICM) can be performed on calls to it,
197 /// without worrying about aliasing properties, and many functions have this
198 /// property (e.g. 'sin' and 'cos').
200 /// This property corresponds to the GCC 'const' attribute.
202 bool doesNotAccessMemory(Function *F) {
203 return getModRefBehavior(F, CallSite()) == DoesNotAccessMemory;
206 /// onlyReadsMemory - If the specified function is known to only read from
207 /// non-volatile memory (or not access memory at all), return true. Functions
208 /// that unwind the stack are not legal for this predicate.
210 /// This property allows many common optimizations to be performed in the
211 /// absence of interfering store instructions, such as CSE of strlen calls.
213 /// This property corresponds to the GCC 'pure' attribute.
215 bool onlyReadsMemory(Function *F) {
216 /// FIXME: If the analysis returns more precise info, we can reduce it to
218 ModRefBehavior MRB = getModRefBehavior(F, CallSite());
219 return MRB == DoesNotAccessMemory || MRB == OnlyReadsMemory;
223 /// getModRefInfo - Return information about whether or not an instruction may
224 /// read or write memory specified by the pointer operand. An instruction
225 /// that doesn't read or write memory may be trivially LICM'd for example.
227 /// getModRefInfo (for call sites) - Return whether information about whether
228 /// a particular call site modifies or reads the memory specified by the
231 virtual ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size);
233 /// getModRefInfo - Return information about whether two call sites may refer
234 /// to the same set of memory locations. This function returns NoModRef if
235 /// the two calls refer to disjoint memory locations, Ref if CS1 reads memory
236 /// written by CS2, Mod if CS1 writes to memory read or written by CS2, or
237 /// ModRef if CS1 might read or write memory accessed by CS2.
239 virtual ModRefResult getModRefInfo(CallSite CS1, CallSite CS2);
241 /// hasNoModRefInfoForCalls - Return true if the analysis has no mod/ref
242 /// information for pairs of function calls (other than "pure" and "const"
243 /// functions). This can be used by clients to avoid many pointless queries.
244 /// Remember that if you override this and chain to another analysis, you must
245 /// make sure that it doesn't have mod/ref info either.
247 virtual bool hasNoModRefInfoForCalls() const;
249 /// Convenience functions...
250 ModRefResult getModRefInfo(LoadInst *L, Value *P, unsigned Size);
251 ModRefResult getModRefInfo(StoreInst *S, Value *P, unsigned Size);
252 ModRefResult getModRefInfo(CallInst *C, Value *P, unsigned Size) {
253 return getModRefInfo(CallSite(C), P, Size);
255 ModRefResult getModRefInfo(InvokeInst *I, Value *P, unsigned Size) {
256 return getModRefInfo(CallSite(I), P, Size);
258 ModRefResult getModRefInfo(VAArgInst* I, Value* P, unsigned Size) {
259 return AliasAnalysis::Mod;
261 ModRefResult getModRefInfo(Instruction *I, Value *P, unsigned Size) {
262 switch (I->getOpcode()) {
263 case Instruction::VAArg: return getModRefInfo((VAArgInst*)I, P, Size);
264 case Instruction::Load: return getModRefInfo((LoadInst*)I, P, Size);
265 case Instruction::Store: return getModRefInfo((StoreInst*)I, P, Size);
266 case Instruction::Call: return getModRefInfo((CallInst*)I, P, Size);
267 case Instruction::Invoke: return getModRefInfo((InvokeInst*)I, P, Size);
268 default: return NoModRef;
272 //===--------------------------------------------------------------------===//
273 /// Higher level methods for querying mod/ref information.
276 /// canBasicBlockModify - Return true if it is possible for execution of the
277 /// specified basic block to modify the value pointed to by Ptr.
279 bool canBasicBlockModify(const BasicBlock &BB, const Value *P, unsigned Size);
281 /// canInstructionRangeModify - Return true if it is possible for the
282 /// execution of the specified instructions to modify the value pointed to by
283 /// Ptr. The instructions to consider are all of the instructions in the
284 /// range of [I1,I2] INCLUSIVE. I1 and I2 must be in the same basic block.
286 bool canInstructionRangeModify(const Instruction &I1, const Instruction &I2,
287 const Value *Ptr, unsigned Size);
289 //===--------------------------------------------------------------------===//
290 /// Methods that clients should call when they transform the program to allow
291 /// alias analyses to update their internal data structures. Note that these
292 /// methods may be called on any instruction, regardless of whether or not
293 /// they have pointer-analysis implications.
296 /// deleteValue - This method should be called whenever an LLVM Value is
297 /// deleted from the program, for example when an instruction is found to be
298 /// redundant and is eliminated.
300 virtual void deleteValue(Value *V);
302 /// copyValue - This method should be used whenever a preexisting value in the
303 /// program is copied or cloned, introducing a new value. Note that analysis
304 /// implementations should tolerate clients that use this method to introduce
305 /// the same value multiple times: if the analysis already knows about a
306 /// value, it should ignore the request.
308 virtual void copyValue(Value *From, Value *To);
310 /// replaceWithNewValue - This method is the obvious combination of the two
311 /// above, and it provided as a helper to simplify client code.
313 void replaceWithNewValue(Value *Old, Value *New) {
319 // Because of the way .a files work, we must force the BasicAA implementation to
320 // be pulled in if the AliasAnalysis header is included. Otherwise we run
321 // the risk of AliasAnalysis being used, but the default implementation not
322 // being linked into the tool that uses it.
324 extern void BasicAAStub();
325 static IncludeFile HDR_INCLUDE_BASICAA_CPP((void*)&BasicAAStub);
327 } // End llvm namespace