1 //===- AliasAnalysis.cpp - Generic Alias Analysis Interface Implementation -==//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This file implements the generic AliasAnalysis interface which is used as the
11 // common interface used by all clients and implementations of alias analysis.
13 // This file also implements the default version of the AliasAnalysis interface
14 // that is to be used when no other implementation is specified. This does some
15 // simple tests that detect obvious cases: two different global pointers cannot
16 // alias, a global cannot alias a malloc, two different mallocs cannot alias,
19 // This alias analysis implementation really isn't very good for anything, but
20 // it is very fast, and makes a nice clean default implementation. Because it
21 // handles lots of little corner cases, other, more complex, alias analysis
22 // implementations may choose to rely on this pass to resolve these simple and
25 //===----------------------------------------------------------------------===//
27 #include "llvm/Analysis/AliasAnalysis.h"
28 #include "llvm/Pass.h"
29 #include "llvm/BasicBlock.h"
30 #include "llvm/Function.h"
31 #include "llvm/IntrinsicInst.h"
32 #include "llvm/Instructions.h"
33 #include "llvm/Type.h"
34 #include "llvm/Target/TargetData.h"
37 // Register the AliasAnalysis interface, providing a nice name to refer to.
38 static RegisterAnalysisGroup<AliasAnalysis> Z("Alias Analysis");
39 char AliasAnalysis::ID = 0;
41 //===----------------------------------------------------------------------===//
42 // Default chaining methods
43 //===----------------------------------------------------------------------===//
45 AliasAnalysis::AliasResult
46 AliasAnalysis::alias(const Value *V1, unsigned V1Size,
47 const Value *V2, unsigned V2Size) {
48 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
49 return AA->alias(V1, V1Size, V2, V2Size);
52 bool AliasAnalysis::pointsToConstantMemory(const Value *P) {
53 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
54 return AA->pointsToConstantMemory(P);
57 void AliasAnalysis::deleteValue(Value *V) {
58 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
62 void AliasAnalysis::copyValue(Value *From, Value *To) {
63 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
64 AA->copyValue(From, To);
67 AliasAnalysis::ModRefResult
68 AliasAnalysis::getModRefInfo(ImmutableCallSite CS,
69 const Value *P, unsigned Size) {
70 // Don't assert AA because BasicAA calls us in order to make use of the
73 ModRefBehavior MRB = getModRefBehavior(CS);
74 if (MRB == DoesNotAccessMemory)
77 ModRefResult Mask = ModRef;
78 if (MRB == OnlyReadsMemory)
80 else if (MRB == AliasAnalysis::AccessesArguments) {
81 bool doesAlias = false;
82 for (ImmutableCallSite::arg_iterator AI = CS.arg_begin(), AE = CS.arg_end();
84 if (!isNoAlias(*AI, ~0U, P, Size)) {
93 // If P points to a constant memory location, the call definitely could not
94 // modify the memory location.
95 if ((Mask & Mod) && pointsToConstantMemory(P))
96 Mask = ModRefResult(Mask & ~Mod);
98 // If this is BasicAA, don't forward.
101 // Otherwise, fall back to the next AA in the chain. But we can merge
102 // in any mask we've managed to compute.
103 return ModRefResult(AA->getModRefInfo(CS, P, Size) & Mask);
106 AliasAnalysis::ModRefResult
107 AliasAnalysis::getModRefInfo(ImmutableCallSite CS1, ImmutableCallSite CS2) {
108 // Don't assert AA because BasicAA calls us in order to make use of the
111 // If CS1 or CS2 are readnone, they don't interact.
112 ModRefBehavior CS1B = getModRefBehavior(CS1);
113 if (CS1B == DoesNotAccessMemory) return NoModRef;
115 ModRefBehavior CS2B = getModRefBehavior(CS2);
116 if (CS2B == DoesNotAccessMemory) return NoModRef;
118 // If they both only read from memory, there is no dependence.
119 if (CS1B == OnlyReadsMemory && CS2B == OnlyReadsMemory)
122 AliasAnalysis::ModRefResult Mask = ModRef;
124 // If CS1 only reads memory, the only dependence on CS2 can be
125 // from CS1 reading memory written by CS2.
126 if (CS1B == OnlyReadsMemory)
127 Mask = ModRefResult(Mask & Ref);
129 // If CS2 only access memory through arguments, accumulate the mod/ref
130 // information from CS1's references to the memory referenced by
132 if (CS2B == AccessesArguments) {
133 AliasAnalysis::ModRefResult R = NoModRef;
134 for (ImmutableCallSite::arg_iterator
135 I = CS2.arg_begin(), E = CS2.arg_end(); I != E; ++I) {
136 R = ModRefResult((R | getModRefInfo(CS1, *I, UnknownSize)) & Mask);
143 // If CS1 only accesses memory through arguments, check if CS2 references
144 // any of the memory referenced by CS1's arguments. If not, return NoModRef.
145 if (CS1B == AccessesArguments) {
146 AliasAnalysis::ModRefResult R = NoModRef;
147 for (ImmutableCallSite::arg_iterator
148 I = CS1.arg_begin(), E = CS1.arg_end(); I != E; ++I)
149 if (getModRefInfo(CS2, *I, UnknownSize) != NoModRef) {
157 // If this is BasicAA, don't forward.
158 if (!AA) return Mask;
160 // Otherwise, fall back to the next AA in the chain. But we can merge
161 // in any mask we've managed to compute.
162 return ModRefResult(AA->getModRefInfo(CS1, CS2) & Mask);
165 AliasAnalysis::ModRefBehavior
166 AliasAnalysis::getModRefBehavior(ImmutableCallSite CS) {
167 // Don't assert AA because BasicAA calls us in order to make use of the
170 ModRefBehavior Min = UnknownModRefBehavior;
172 // Call back into the alias analysis with the other form of getModRefBehavior
173 // to see if it can give a better response.
174 if (const Function *F = CS.getCalledFunction())
175 Min = getModRefBehavior(F);
177 // If this is BasicAA, don't forward.
180 // Otherwise, fall back to the next AA in the chain. But we can merge
181 // in any result we've managed to compute.
182 return std::min(AA->getModRefBehavior(CS), Min);
185 AliasAnalysis::ModRefBehavior
186 AliasAnalysis::getModRefBehavior(const Function *F) {
187 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
188 return AA->getModRefBehavior(F);
191 AliasAnalysis::DependenceResult
192 AliasAnalysis::getDependence(const Instruction *First,
193 const Value *FirstPHITranslatedAddr,
194 DependenceQueryFlags FirstFlags,
195 const Instruction *Second,
196 const Value *SecondPHITranslatedAddr,
197 DependenceQueryFlags SecondFlags) {
198 assert(AA && "AA didn't call InitializeAliasAnalyais in its run method!");
199 return AA->getDependence(First, FirstPHITranslatedAddr, FirstFlags,
200 Second, SecondPHITranslatedAddr, SecondFlags);
203 //===----------------------------------------------------------------------===//
204 // AliasAnalysis non-virtual helper method implementation
205 //===----------------------------------------------------------------------===//
207 AliasAnalysis::ModRefResult
208 AliasAnalysis::getModRefInfo(const LoadInst *L, const Value *P, unsigned Size) {
209 // Be conservative in the face of volatile.
213 // If the load address doesn't alias the given address, it doesn't read
214 // or write the specified memory.
215 if (!alias(L->getOperand(0), getTypeStoreSize(L->getType()), P, Size))
218 // Otherwise, a load just reads.
222 AliasAnalysis::ModRefResult
223 AliasAnalysis::getModRefInfo(const StoreInst *S, const Value *P, unsigned Size) {
224 // Be conservative in the face of volatile.
228 // If the store address cannot alias the pointer in question, then the
229 // specified memory cannot be modified by the store.
230 if (!alias(S->getOperand(1),
231 getTypeStoreSize(S->getOperand(0)->getType()), P, Size))
234 // If the pointer is a pointer to constant memory, then it could not have been
235 // modified by this store.
236 if (pointsToConstantMemory(P))
239 // Otherwise, a store just writes.
243 AliasAnalysis::ModRefResult
244 AliasAnalysis::getModRefInfo(const VAArgInst *V, const Value *P, unsigned Size) {
245 // If the va_arg address cannot alias the pointer in question, then the
246 // specified memory cannot be accessed by the va_arg.
247 if (!alias(V->getOperand(0), UnknownSize, P, Size))
250 // If the pointer is a pointer to constant memory, then it could not have been
251 // modified by this va_arg.
252 if (pointsToConstantMemory(P))
255 // Otherwise, a va_arg reads and writes.
259 AliasAnalysis::DependenceResult
260 AliasAnalysis::getDependenceViaModRefInfo(const Instruction *First,
261 const Value *FirstPHITranslatedAddr,
262 DependenceQueryFlags FirstFlags,
263 const Instruction *Second,
264 const Value *SecondPHITranslatedAddr,
265 DependenceQueryFlags SecondFlags) {
266 if (const LoadInst *L = dyn_cast<LoadInst>(First)) {
267 // Be over-conservative with volatile for now.
271 // If we don't have a phi-translated address, use the actual one.
272 if (!FirstPHITranslatedAddr)
273 FirstPHITranslatedAddr = L->getPointerOperand();
275 // Forward this query to getModRefInfo.
276 switch (getModRefInfo(Second,
277 FirstPHITranslatedAddr,
278 getTypeStoreSize(L->getType()))) {
280 // Second doesn't reference First's memory, so they're independent.
284 // Second only reads from the memory read from by First. If it
285 // also writes to any other memory, be conservative.
286 if (Second->mayWriteToMemory())
289 // If it's loading the same size from the same address, we can
290 // give a more precise result.
291 if (const LoadInst *SecondL = dyn_cast<LoadInst>(Second)) {
292 // If we don't have a phi-translated address, use the actual one.
293 if (!SecondPHITranslatedAddr)
294 SecondPHITranslatedAddr = SecondL->getPointerOperand();
296 unsigned LSize = getTypeStoreSize(L->getType());
297 unsigned SecondLSize = getTypeStoreSize(SecondL->getType());
298 if (alias(FirstPHITranslatedAddr, LSize,
299 SecondPHITranslatedAddr, SecondLSize) ==
301 // If the loads are the same size, it's ReadThenRead.
302 if (LSize == SecondLSize)
305 // If the second load is smaller, it's only ReadThenReadSome.
306 if (LSize > SecondLSize)
307 return ReadThenReadSome;
311 // Otherwise it's just two loads.
315 // Second only writes to the memory read from by First. If it
316 // also reads from any other memory, be conservative.
317 if (Second->mayReadFromMemory())
320 // If it's storing the same size to the same address, we can
321 // give a more precise result.
322 if (const StoreInst *SecondS = dyn_cast<StoreInst>(Second)) {
323 // If we don't have a phi-translated address, use the actual one.
324 if (!SecondPHITranslatedAddr)
325 SecondPHITranslatedAddr = SecondS->getPointerOperand();
327 unsigned LSize = getTypeStoreSize(L->getType());
328 unsigned SecondSSize = getTypeStoreSize(SecondS->getType());
329 if (alias(FirstPHITranslatedAddr, LSize,
330 SecondPHITranslatedAddr, SecondSSize) ==
332 // If the load and the store are the same size, it's ReadThenWrite.
333 if (LSize == SecondSSize)
334 return ReadThenWrite;
338 // Otherwise we don't know if it could be writing to other memory.
342 // Second reads and writes to the memory read from by First.
343 // We don't have a way to express that.
347 } else if (const StoreInst *S = dyn_cast<StoreInst>(First)) {
348 // Be over-conservative with volatile for now.
352 // If we don't have a phi-translated address, use the actual one.
353 if (!FirstPHITranslatedAddr)
354 FirstPHITranslatedAddr = S->getPointerOperand();
356 // Forward this query to getModRefInfo.
357 switch (getModRefInfo(Second,
358 FirstPHITranslatedAddr,
359 getTypeStoreSize(S->getValueOperand()->getType()))) {
361 // Second doesn't reference First's memory, so they're independent.
365 // Second only reads from the memory written to by First. If it
366 // also writes to any other memory, be conservative.
367 if (Second->mayWriteToMemory())
370 // If it's loading the same size from the same address, we can
371 // give a more precise result.
372 if (const LoadInst *SecondL = dyn_cast<LoadInst>(Second)) {
373 // If we don't have a phi-translated address, use the actual one.
374 if (!SecondPHITranslatedAddr)
375 SecondPHITranslatedAddr = SecondL->getPointerOperand();
377 unsigned SSize = getTypeStoreSize(S->getValueOperand()->getType());
378 unsigned SecondLSize = getTypeStoreSize(SecondL->getType());
379 if (alias(FirstPHITranslatedAddr, SSize,
380 SecondPHITranslatedAddr, SecondLSize) ==
382 // If the store and the load are the same size, it's WriteThenRead.
383 if (SSize == SecondLSize)
384 return WriteThenRead;
386 // If the load is smaller, it's only WriteThenReadSome.
387 if (SSize > SecondLSize)
388 return WriteThenReadSome;
392 // Otherwise we don't know if it could be reading from other memory.
396 // Second only writes to the memory written to by First. If it
397 // also reads from any other memory, be conservative.
398 if (Second->mayReadFromMemory())
401 // If it's storing the same size to the same address, we can
402 // give a more precise result.
403 if (const StoreInst *SecondS = dyn_cast<StoreInst>(Second)) {
404 // If we don't have a phi-translated address, use the actual one.
405 if (!SecondPHITranslatedAddr)
406 SecondPHITranslatedAddr = SecondS->getPointerOperand();
408 unsigned SSize = getTypeStoreSize(S->getValueOperand()->getType());
409 unsigned SecondSSize = getTypeStoreSize(SecondS->getType());
410 if (alias(FirstPHITranslatedAddr, SSize,
411 SecondPHITranslatedAddr, SecondSSize) ==
413 // If the stores are the same size, it's WriteThenWrite.
414 if (SSize == SecondSSize)
415 return WriteThenWrite;
417 // If the second store is larger, it's only WriteSomeThenWrite.
418 if (SSize < SecondSSize)
419 return WriteSomeThenWrite;
423 // Otherwise we don't know if it could be writing to other memory.
427 // Second reads and writes to the memory written to by First.
428 // We don't have a way to express that.
432 } else if (const VAArgInst *V = dyn_cast<VAArgInst>(First)) {
433 // If we don't have a phi-translated address, use the actual one.
434 if (!FirstPHITranslatedAddr)
435 FirstPHITranslatedAddr = V->getPointerOperand();
437 // Forward this query to getModRefInfo.
438 if (getModRefInfo(Second, FirstPHITranslatedAddr, UnknownSize) == NoModRef)
439 // Second doesn't reference First's memory, so they're independent.
442 } else if (ImmutableCallSite FirstCS = cast<Value>(First)) {
443 assert(!FirstPHITranslatedAddr &&
444 !SecondPHITranslatedAddr &&
445 "PHI translation with calls not supported yet!");
447 // If both instructions are calls/invokes we can use the two-callsite
448 // form of getModRefInfo.
449 if (ImmutableCallSite SecondCS = cast<Value>(Second))
450 // getModRefInfo's arguments are backwards from intuition.
451 switch (getModRefInfo(SecondCS, FirstCS)) {
453 // Second doesn't reference First's memory, so they're independent.
457 // If they're both read-only, there's no dependence.
458 if (FirstCS.onlyReadsMemory() && SecondCS.onlyReadsMemory())
461 // Otherwise it's not obvious what we can do here.
465 // It's not obvious what we can do here.
474 // For anything else, be conservative.
478 AliasAnalysis::ModRefBehavior
479 AliasAnalysis::getIntrinsicModRefBehavior(unsigned iid) {
480 #define GET_INTRINSIC_MODREF_BEHAVIOR
481 #include "llvm/Intrinsics.gen"
482 #undef GET_INTRINSIC_MODREF_BEHAVIOR
485 // AliasAnalysis destructor: DO NOT move this to the header file for
486 // AliasAnalysis or else clients of the AliasAnalysis class may not depend on
487 // the AliasAnalysis.o file in the current .a file, causing alias analysis
488 // support to not be included in the tool correctly!
490 AliasAnalysis::~AliasAnalysis() {}
492 /// InitializeAliasAnalysis - Subclasses must call this method to initialize the
493 /// AliasAnalysis interface before any other methods are called.
495 void AliasAnalysis::InitializeAliasAnalysis(Pass *P) {
496 TD = P->getAnalysisIfAvailable<TargetData>();
497 AA = &P->getAnalysis<AliasAnalysis>();
500 // getAnalysisUsage - All alias analysis implementations should invoke this
501 // directly (using AliasAnalysis::getAnalysisUsage(AU)).
502 void AliasAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
503 AU.addRequired<AliasAnalysis>(); // All AA's chain
506 /// getTypeStoreSize - Return the TargetData store size for the given type,
507 /// if known, or a conservative value otherwise.
509 unsigned AliasAnalysis::getTypeStoreSize(const Type *Ty) {
510 return TD ? TD->getTypeStoreSize(Ty) : ~0u;
513 /// canBasicBlockModify - Return true if it is possible for execution of the
514 /// specified basic block to modify the value pointed to by Ptr.
516 bool AliasAnalysis::canBasicBlockModify(const BasicBlock &BB,
517 const Value *Ptr, unsigned Size) {
518 return canInstructionRangeModify(BB.front(), BB.back(), Ptr, Size);
521 /// canInstructionRangeModify - Return true if it is possible for the execution
522 /// of the specified instructions to modify the value pointed to by Ptr. The
523 /// instructions to consider are all of the instructions in the range of [I1,I2]
524 /// INCLUSIVE. I1 and I2 must be in the same basic block.
526 bool AliasAnalysis::canInstructionRangeModify(const Instruction &I1,
527 const Instruction &I2,
528 const Value *Ptr, unsigned Size) {
529 assert(I1.getParent() == I2.getParent() &&
530 "Instructions not in same basic block!");
531 BasicBlock::const_iterator I = &I1;
532 BasicBlock::const_iterator E = &I2;
533 ++E; // Convert from inclusive to exclusive range.
535 for (; I != E; ++I) // Check every instruction in range
536 if (getModRefInfo(I, Ptr, Size) & Mod)
541 /// isNoAliasCall - Return true if this pointer is returned by a noalias
543 bool llvm::isNoAliasCall(const Value *V) {
544 if (isa<CallInst>(V) || isa<InvokeInst>(V))
545 return ImmutableCallSite(cast<Instruction>(V))
546 .paramHasAttr(0, Attribute::NoAlias);
550 /// isIdentifiedObject - Return true if this pointer refers to a distinct and
551 /// identifiable object. This returns true for:
552 /// Global Variables and Functions (but not Global Aliases)
553 /// Allocas and Mallocs
554 /// ByVal and NoAlias Arguments
557 bool llvm::isIdentifiedObject(const Value *V) {
558 if (isa<AllocaInst>(V))
560 if (isa<GlobalValue>(V) && !isa<GlobalAlias>(V))
562 if (isNoAliasCall(V))
564 if (const Argument *A = dyn_cast<Argument>(V))
565 return A->hasNoAliasAttr() || A->hasByValAttr();
569 // Because of the way .a files work, we must force the BasicAA implementation to
570 // be pulled in if the AliasAnalysis classes are pulled in. Otherwise we run
571 // the risk of AliasAnalysis being used, but the default implementation not
572 // being linked into the tool that uses it.
573 DEFINING_FILE_FOR(AliasAnalysis)