1 //===- BasicAliasAnalysis.cpp - Stateless Alias Analysis Impl -------------===//
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 defines the primary stateless implementation of the
11 // Alias Analysis interface that implements identities (two different
12 // globals cannot alias, etc), but does no stateful analysis.
14 //===----------------------------------------------------------------------===//
16 #include "llvm/Analysis/AliasAnalysis.h"
17 #include "llvm/Analysis/Passes.h"
18 #include "llvm/Constants.h"
19 #include "llvm/DerivedTypes.h"
20 #include "llvm/Function.h"
21 #include "llvm/GlobalAlias.h"
22 #include "llvm/GlobalVariable.h"
23 #include "llvm/Instructions.h"
24 #include "llvm/IntrinsicInst.h"
25 #include "llvm/LLVMContext.h"
26 #include "llvm/Operator.h"
27 #include "llvm/Pass.h"
28 #include "llvm/Analysis/CaptureTracking.h"
29 #include "llvm/Analysis/MemoryBuiltins.h"
30 #include "llvm/Analysis/InstructionSimplify.h"
31 #include "llvm/Analysis/ValueTracking.h"
32 #include "llvm/Target/TargetData.h"
33 #include "llvm/Target/TargetLibraryInfo.h"
34 #include "llvm/ADT/SmallPtrSet.h"
35 #include "llvm/ADT/SmallVector.h"
36 #include "llvm/Support/ErrorHandling.h"
37 #include "llvm/Support/GetElementPtrTypeIterator.h"
41 //===----------------------------------------------------------------------===//
43 //===----------------------------------------------------------------------===//
45 /// isNonEscapingLocalObject - Return true if the pointer is to a function-local
46 /// object that never escapes from the function.
47 static bool isNonEscapingLocalObject(const Value *V) {
48 // If this is a local allocation, check to see if it escapes.
49 if (isa<AllocaInst>(V) || isNoAliasCall(V))
50 // Set StoreCaptures to True so that we can assume in our callers that the
51 // pointer is not the result of a load instruction. Currently
52 // PointerMayBeCaptured doesn't have any special analysis for the
53 // StoreCaptures=false case; if it did, our callers could be refined to be
55 return !PointerMayBeCaptured(V, false, /*StoreCaptures=*/true);
57 // If this is an argument that corresponds to a byval or noalias argument,
58 // then it has not escaped before entering the function. Check if it escapes
59 // inside the function.
60 if (const Argument *A = dyn_cast<Argument>(V))
61 if (A->hasByValAttr() || A->hasNoAliasAttr()) {
62 // Don't bother analyzing arguments already known not to escape.
63 if (A->hasNoCaptureAttr())
65 return !PointerMayBeCaptured(V, false, /*StoreCaptures=*/true);
70 /// isEscapeSource - Return true if the pointer is one which would have
71 /// been considered an escape by isNonEscapingLocalObject.
72 static bool isEscapeSource(const Value *V) {
73 if (isa<CallInst>(V) || isa<InvokeInst>(V) || isa<Argument>(V))
76 // The load case works because isNonEscapingLocalObject considers all
77 // stores to be escapes (it passes true for the StoreCaptures argument
78 // to PointerMayBeCaptured).
85 /// getObjectSize - Return the size of the object specified by V, or
86 /// UnknownSize if unknown.
87 static uint64_t getObjectSize(const Value *V, const TargetData &TD,
88 bool RoundToAlign = false) {
90 if (getObjectSize(V, Size, &TD, RoundToAlign))
92 return AliasAnalysis::UnknownSize;
95 /// isObjectSmallerThan - Return true if we can prove that the object specified
96 /// by V is smaller than Size.
97 static bool isObjectSmallerThan(const Value *V, uint64_t Size,
98 const TargetData &TD) {
99 // This function needs to use the aligned object size because we allow
100 // reads a bit past the end given sufficient alignment.
101 uint64_t ObjectSize = getObjectSize(V, TD, /*RoundToAlign*/true);
103 return ObjectSize != AliasAnalysis::UnknownSize && ObjectSize < Size;
106 /// isObjectSize - Return true if we can prove that the object specified
107 /// by V has size Size.
108 static bool isObjectSize(const Value *V, uint64_t Size,
109 const TargetData &TD) {
110 uint64_t ObjectSize = getObjectSize(V, TD);
111 return ObjectSize != AliasAnalysis::UnknownSize && ObjectSize == Size;
114 //===----------------------------------------------------------------------===//
115 // GetElementPtr Instruction Decomposition and Analysis
116 //===----------------------------------------------------------------------===//
125 struct VariableGEPIndex {
127 ExtensionKind Extension;
133 /// GetLinearExpression - Analyze the specified value as a linear expression:
134 /// "A*V + B", where A and B are constant integers. Return the scale and offset
135 /// values as APInts and return V as a Value*, and return whether we looked
136 /// through any sign or zero extends. The incoming Value is known to have
137 /// IntegerType and it may already be sign or zero extended.
139 /// Note that this looks through extends, so the high bits may not be
140 /// represented in the result.
141 static Value *GetLinearExpression(Value *V, APInt &Scale, APInt &Offset,
142 ExtensionKind &Extension,
143 const TargetData &TD, unsigned Depth) {
144 assert(V->getType()->isIntegerTy() && "Not an integer value");
146 // Limit our recursion depth.
153 if (BinaryOperator *BOp = dyn_cast<BinaryOperator>(V)) {
154 if (ConstantInt *RHSC = dyn_cast<ConstantInt>(BOp->getOperand(1))) {
155 switch (BOp->getOpcode()) {
157 case Instruction::Or:
158 // X|C == X+C if all the bits in C are unset in X. Otherwise we can't
160 if (!MaskedValueIsZero(BOp->getOperand(0), RHSC->getValue(), &TD))
163 case Instruction::Add:
164 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension,
166 Offset += RHSC->getValue();
168 case Instruction::Mul:
169 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension,
171 Offset *= RHSC->getValue();
172 Scale *= RHSC->getValue();
174 case Instruction::Shl:
175 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension,
177 Offset <<= RHSC->getValue().getLimitedValue();
178 Scale <<= RHSC->getValue().getLimitedValue();
184 // Since GEP indices are sign extended anyway, we don't care about the high
185 // bits of a sign or zero extended value - just scales and offsets. The
186 // extensions have to be consistent though.
187 if ((isa<SExtInst>(V) && Extension != EK_ZeroExt) ||
188 (isa<ZExtInst>(V) && Extension != EK_SignExt)) {
189 Value *CastOp = cast<CastInst>(V)->getOperand(0);
190 unsigned OldWidth = Scale.getBitWidth();
191 unsigned SmallWidth = CastOp->getType()->getPrimitiveSizeInBits();
192 Scale = Scale.trunc(SmallWidth);
193 Offset = Offset.trunc(SmallWidth);
194 Extension = isa<SExtInst>(V) ? EK_SignExt : EK_ZeroExt;
196 Value *Result = GetLinearExpression(CastOp, Scale, Offset, Extension,
198 Scale = Scale.zext(OldWidth);
199 Offset = Offset.zext(OldWidth);
209 /// DecomposeGEPExpression - If V is a symbolic pointer expression, decompose it
210 /// into a base pointer with a constant offset and a number of scaled symbolic
213 /// The scaled symbolic offsets (represented by pairs of a Value* and a scale in
214 /// the VarIndices vector) are Value*'s that are known to be scaled by the
215 /// specified amount, but which may have other unrepresented high bits. As such,
216 /// the gep cannot necessarily be reconstructed from its decomposed form.
218 /// When TargetData is around, this function is capable of analyzing everything
219 /// that GetUnderlyingObject can look through. When not, it just looks
220 /// through pointer casts.
223 DecomposeGEPExpression(const Value *V, int64_t &BaseOffs,
224 SmallVectorImpl<VariableGEPIndex> &VarIndices,
225 const TargetData *TD) {
226 // Limit recursion depth to limit compile time in crazy cases.
227 unsigned MaxLookup = 6;
231 // See if this is a bitcast or GEP.
232 const Operator *Op = dyn_cast<Operator>(V);
234 // The only non-operator case we can handle are GlobalAliases.
235 if (const GlobalAlias *GA = dyn_cast<GlobalAlias>(V)) {
236 if (!GA->mayBeOverridden()) {
237 V = GA->getAliasee();
244 if (Op->getOpcode() == Instruction::BitCast) {
245 V = Op->getOperand(0);
249 const GEPOperator *GEPOp = dyn_cast<GEPOperator>(Op);
251 // If it's not a GEP, hand it off to SimplifyInstruction to see if it
252 // can come up with something. This matches what GetUnderlyingObject does.
253 if (const Instruction *I = dyn_cast<Instruction>(V))
254 // TODO: Get a DominatorTree and use it here.
255 if (const Value *Simplified =
256 SimplifyInstruction(const_cast<Instruction *>(I), TD)) {
264 // Don't attempt to analyze GEPs over unsized objects.
265 if (!cast<PointerType>(GEPOp->getOperand(0)->getType())
266 ->getElementType()->isSized())
269 // If we are lacking TargetData information, we can't compute the offets of
270 // elements computed by GEPs. However, we can handle bitcast equivalent
273 if (!GEPOp->hasAllZeroIndices())
275 V = GEPOp->getOperand(0);
279 // Walk the indices of the GEP, accumulating them into BaseOff/VarIndices.
280 gep_type_iterator GTI = gep_type_begin(GEPOp);
281 for (User::const_op_iterator I = GEPOp->op_begin()+1,
282 E = GEPOp->op_end(); I != E; ++I) {
284 // Compute the (potentially symbolic) offset in bytes for this index.
285 if (StructType *STy = dyn_cast<StructType>(*GTI++)) {
286 // For a struct, add the member offset.
287 unsigned FieldNo = cast<ConstantInt>(Index)->getZExtValue();
288 if (FieldNo == 0) continue;
290 BaseOffs += TD->getStructLayout(STy)->getElementOffset(FieldNo);
294 // For an array/pointer, add the element offset, explicitly scaled.
295 if (ConstantInt *CIdx = dyn_cast<ConstantInt>(Index)) {
296 if (CIdx->isZero()) continue;
297 BaseOffs += TD->getTypeAllocSize(*GTI)*CIdx->getSExtValue();
301 uint64_t Scale = TD->getTypeAllocSize(*GTI);
302 ExtensionKind Extension = EK_NotExtended;
304 // If the integer type is smaller than the pointer size, it is implicitly
305 // sign extended to pointer size.
306 unsigned Width = cast<IntegerType>(Index->getType())->getBitWidth();
307 if (TD->getPointerSizeInBits() > Width)
308 Extension = EK_SignExt;
310 // Use GetLinearExpression to decompose the index into a C1*V+C2 form.
311 APInt IndexScale(Width, 0), IndexOffset(Width, 0);
312 Index = GetLinearExpression(Index, IndexScale, IndexOffset, Extension,
315 // The GEP index scale ("Scale") scales C1*V+C2, yielding (C1*V+C2)*Scale.
316 // This gives us an aggregate computation of (C1*Scale)*V + C2*Scale.
317 BaseOffs += IndexOffset.getSExtValue()*Scale;
318 Scale *= IndexScale.getSExtValue();
321 // If we already had an occurrence of this index variable, merge this
322 // scale into it. For example, we want to handle:
323 // A[x][x] -> x*16 + x*4 -> x*20
324 // This also ensures that 'x' only appears in the index list once.
325 for (unsigned i = 0, e = VarIndices.size(); i != e; ++i) {
326 if (VarIndices[i].V == Index &&
327 VarIndices[i].Extension == Extension) {
328 Scale += VarIndices[i].Scale;
329 VarIndices.erase(VarIndices.begin()+i);
334 // Make sure that we have a scale that makes sense for this target's
336 if (unsigned ShiftBits = 64-TD->getPointerSizeInBits()) {
338 Scale = (int64_t)Scale >> ShiftBits;
342 VariableGEPIndex Entry = {Index, Extension,
343 static_cast<int64_t>(Scale)};
344 VarIndices.push_back(Entry);
348 // Analyze the base pointer next.
349 V = GEPOp->getOperand(0);
350 } while (--MaxLookup);
352 // If the chain of expressions is too deep, just return early.
356 /// GetIndexDifference - Dest and Src are the variable indices from two
357 /// decomposed GetElementPtr instructions GEP1 and GEP2 which have common base
358 /// pointers. Subtract the GEP2 indices from GEP1 to find the symbolic
359 /// difference between the two pointers.
360 static void GetIndexDifference(SmallVectorImpl<VariableGEPIndex> &Dest,
361 const SmallVectorImpl<VariableGEPIndex> &Src) {
362 if (Src.empty()) return;
364 for (unsigned i = 0, e = Src.size(); i != e; ++i) {
365 const Value *V = Src[i].V;
366 ExtensionKind Extension = Src[i].Extension;
367 int64_t Scale = Src[i].Scale;
369 // Find V in Dest. This is N^2, but pointer indices almost never have more
370 // than a few variable indexes.
371 for (unsigned j = 0, e = Dest.size(); j != e; ++j) {
372 if (Dest[j].V != V || Dest[j].Extension != Extension) continue;
374 // If we found it, subtract off Scale V's from the entry in Dest. If it
375 // goes to zero, remove the entry.
376 if (Dest[j].Scale != Scale)
377 Dest[j].Scale -= Scale;
379 Dest.erase(Dest.begin()+j);
384 // If we didn't consume this entry, add it to the end of the Dest list.
386 VariableGEPIndex Entry = { V, Extension, -Scale };
387 Dest.push_back(Entry);
392 //===----------------------------------------------------------------------===//
393 // BasicAliasAnalysis Pass
394 //===----------------------------------------------------------------------===//
397 static const Function *getParent(const Value *V) {
398 if (const Instruction *inst = dyn_cast<Instruction>(V))
399 return inst->getParent()->getParent();
401 if (const Argument *arg = dyn_cast<Argument>(V))
402 return arg->getParent();
407 static bool notDifferentParent(const Value *O1, const Value *O2) {
409 const Function *F1 = getParent(O1);
410 const Function *F2 = getParent(O2);
412 return !F1 || !F2 || F1 == F2;
417 /// BasicAliasAnalysis - This is the primary alias analysis implementation.
418 struct BasicAliasAnalysis : public ImmutablePass, public AliasAnalysis {
419 static char ID; // Class identification, replacement for typeinfo
420 BasicAliasAnalysis() : ImmutablePass(ID),
421 // AliasCache rarely has more than 1 or 2 elements,
422 // so start it off fairly small so that clear()
423 // doesn't have to tromp through 64 (the default)
424 // elements on each alias query. This really wants
425 // something like a SmallDenseMap.
427 initializeBasicAliasAnalysisPass(*PassRegistry::getPassRegistry());
430 virtual void initializePass() {
431 InitializeAliasAnalysis(this);
434 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
435 AU.addRequired<AliasAnalysis>();
436 AU.addRequired<TargetLibraryInfo>();
439 virtual AliasResult alias(const Location &LocA,
440 const Location &LocB) {
441 assert(AliasCache.empty() && "AliasCache must be cleared after use!");
442 assert(notDifferentParent(LocA.Ptr, LocB.Ptr) &&
443 "BasicAliasAnalysis doesn't support interprocedural queries.");
444 AliasResult Alias = aliasCheck(LocA.Ptr, LocA.Size, LocA.TBAATag,
445 LocB.Ptr, LocB.Size, LocB.TBAATag);
450 virtual ModRefResult getModRefInfo(ImmutableCallSite CS,
451 const Location &Loc);
453 virtual ModRefResult getModRefInfo(ImmutableCallSite CS1,
454 ImmutableCallSite CS2) {
455 // The AliasAnalysis base class has some smarts, lets use them.
456 return AliasAnalysis::getModRefInfo(CS1, CS2);
459 /// pointsToConstantMemory - Chase pointers until we find a (constant
461 virtual bool pointsToConstantMemory(const Location &Loc, bool OrLocal);
463 /// getModRefBehavior - Return the behavior when calling the given
465 virtual ModRefBehavior getModRefBehavior(ImmutableCallSite CS);
467 /// getModRefBehavior - Return the behavior when calling the given function.
468 /// For use when the call site is not known.
469 virtual ModRefBehavior getModRefBehavior(const Function *F);
471 /// getAdjustedAnalysisPointer - This method is used when a pass implements
472 /// an analysis interface through multiple inheritance. If needed, it
473 /// should override this to adjust the this pointer as needed for the
474 /// specified pass info.
475 virtual void *getAdjustedAnalysisPointer(const void *ID) {
476 if (ID == &AliasAnalysis::ID)
477 return (AliasAnalysis*)this;
482 // AliasCache - Track alias queries to guard against recursion.
483 typedef std::pair<Location, Location> LocPair;
484 typedef DenseMap<LocPair, AliasResult> AliasCacheTy;
485 AliasCacheTy AliasCache;
487 // Visited - Track instructions visited by pointsToConstantMemory.
488 SmallPtrSet<const Value*, 16> Visited;
490 // aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP
491 // instruction against another.
492 AliasResult aliasGEP(const GEPOperator *V1, uint64_t V1Size,
493 const Value *V2, uint64_t V2Size,
494 const MDNode *V2TBAAInfo,
495 const Value *UnderlyingV1, const Value *UnderlyingV2);
497 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI
498 // instruction against another.
499 AliasResult aliasPHI(const PHINode *PN, uint64_t PNSize,
500 const MDNode *PNTBAAInfo,
501 const Value *V2, uint64_t V2Size,
502 const MDNode *V2TBAAInfo);
504 /// aliasSelect - Disambiguate a Select instruction against another value.
505 AliasResult aliasSelect(const SelectInst *SI, uint64_t SISize,
506 const MDNode *SITBAAInfo,
507 const Value *V2, uint64_t V2Size,
508 const MDNode *V2TBAAInfo);
510 AliasResult aliasCheck(const Value *V1, uint64_t V1Size,
511 const MDNode *V1TBAATag,
512 const Value *V2, uint64_t V2Size,
513 const MDNode *V2TBAATag);
515 } // End of anonymous namespace
517 // Register this pass...
518 char BasicAliasAnalysis::ID = 0;
519 INITIALIZE_AG_PASS_BEGIN(BasicAliasAnalysis, AliasAnalysis, "basicaa",
520 "Basic Alias Analysis (stateless AA impl)",
522 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfo)
523 INITIALIZE_AG_PASS_END(BasicAliasAnalysis, AliasAnalysis, "basicaa",
524 "Basic Alias Analysis (stateless AA impl)",
528 ImmutablePass *llvm::createBasicAliasAnalysisPass() {
529 return new BasicAliasAnalysis();
532 /// pointsToConstantMemory - Returns whether the given pointer value
533 /// points to memory that is local to the function, with global constants being
534 /// considered local to all functions.
536 BasicAliasAnalysis::pointsToConstantMemory(const Location &Loc, bool OrLocal) {
537 assert(Visited.empty() && "Visited must be cleared after use!");
539 unsigned MaxLookup = 8;
540 SmallVector<const Value *, 16> Worklist;
541 Worklist.push_back(Loc.Ptr);
543 const Value *V = GetUnderlyingObject(Worklist.pop_back_val(), TD);
544 if (!Visited.insert(V)) {
546 return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);
549 // An alloca instruction defines local memory.
550 if (OrLocal && isa<AllocaInst>(V))
553 // A global constant counts as local memory for our purposes.
554 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) {
555 // Note: this doesn't require GV to be "ODR" because it isn't legal for a
556 // global to be marked constant in some modules and non-constant in
557 // others. GV may even be a declaration, not a definition.
558 if (!GV->isConstant()) {
560 return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);
565 // If both select values point to local memory, then so does the select.
566 if (const SelectInst *SI = dyn_cast<SelectInst>(V)) {
567 Worklist.push_back(SI->getTrueValue());
568 Worklist.push_back(SI->getFalseValue());
572 // If all values incoming to a phi node point to local memory, then so does
574 if (const PHINode *PN = dyn_cast<PHINode>(V)) {
575 // Don't bother inspecting phi nodes with many operands.
576 if (PN->getNumIncomingValues() > MaxLookup) {
578 return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);
580 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
581 Worklist.push_back(PN->getIncomingValue(i));
585 // Otherwise be conservative.
587 return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);
589 } while (!Worklist.empty() && --MaxLookup);
592 return Worklist.empty();
595 /// getModRefBehavior - Return the behavior when calling the given call site.
596 AliasAnalysis::ModRefBehavior
597 BasicAliasAnalysis::getModRefBehavior(ImmutableCallSite CS) {
598 if (CS.doesNotAccessMemory())
599 // Can't do better than this.
600 return DoesNotAccessMemory;
602 ModRefBehavior Min = UnknownModRefBehavior;
604 // If the callsite knows it only reads memory, don't return worse
606 if (CS.onlyReadsMemory())
607 Min = OnlyReadsMemory;
609 // The AliasAnalysis base class has some smarts, lets use them.
610 return ModRefBehavior(AliasAnalysis::getModRefBehavior(CS) & Min);
613 /// getModRefBehavior - Return the behavior when calling the given function.
614 /// For use when the call site is not known.
615 AliasAnalysis::ModRefBehavior
616 BasicAliasAnalysis::getModRefBehavior(const Function *F) {
617 // If the function declares it doesn't access memory, we can't do better.
618 if (F->doesNotAccessMemory())
619 return DoesNotAccessMemory;
621 // For intrinsics, we can check the table.
622 if (unsigned iid = F->getIntrinsicID()) {
623 #define GET_INTRINSIC_MODREF_BEHAVIOR
624 #include "llvm/Intrinsics.gen"
625 #undef GET_INTRINSIC_MODREF_BEHAVIOR
628 ModRefBehavior Min = UnknownModRefBehavior;
630 // If the function declares it only reads memory, go with that.
631 if (F->onlyReadsMemory())
632 Min = OnlyReadsMemory;
634 // Otherwise be conservative.
635 return ModRefBehavior(AliasAnalysis::getModRefBehavior(F) & Min);
638 /// getModRefInfo - Check to see if the specified callsite can clobber the
639 /// specified memory object. Since we only look at local properties of this
640 /// function, we really can't say much about this query. We do, however, use
641 /// simple "address taken" analysis on local objects.
642 AliasAnalysis::ModRefResult
643 BasicAliasAnalysis::getModRefInfo(ImmutableCallSite CS,
644 const Location &Loc) {
645 assert(notDifferentParent(CS.getInstruction(), Loc.Ptr) &&
646 "AliasAnalysis query involving multiple functions!");
648 const Value *Object = GetUnderlyingObject(Loc.Ptr, TD);
650 // If this is a tail call and Loc.Ptr points to a stack location, we know that
651 // the tail call cannot access or modify the local stack.
652 // We cannot exclude byval arguments here; these belong to the caller of
653 // the current function not to the current function, and a tail callee
654 // may reference them.
655 if (isa<AllocaInst>(Object))
656 if (const CallInst *CI = dyn_cast<CallInst>(CS.getInstruction()))
657 if (CI->isTailCall())
660 // If the pointer is to a locally allocated object that does not escape,
661 // then the call can not mod/ref the pointer unless the call takes the pointer
662 // as an argument, and itself doesn't capture it.
663 if (!isa<Constant>(Object) && CS.getInstruction() != Object &&
664 isNonEscapingLocalObject(Object)) {
665 bool PassedAsArg = false;
667 for (ImmutableCallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end();
668 CI != CE; ++CI, ++ArgNo) {
669 // Only look at the no-capture or byval pointer arguments. If this
670 // pointer were passed to arguments that were neither of these, then it
671 // couldn't be no-capture.
672 if (!(*CI)->getType()->isPointerTy() ||
673 (!CS.doesNotCapture(ArgNo) && !CS.isByValArgument(ArgNo)))
676 // If this is a no-capture pointer argument, see if we can tell that it
677 // is impossible to alias the pointer we're checking. If not, we have to
678 // assume that the call could touch the pointer, even though it doesn't
680 if (!isNoAlias(Location(*CI), Location(Object))) {
690 const TargetLibraryInfo &TLI = getAnalysis<TargetLibraryInfo>();
691 ModRefResult Min = ModRef;
693 // Finally, handle specific knowledge of intrinsics.
694 const IntrinsicInst *II = dyn_cast<IntrinsicInst>(CS.getInstruction());
696 switch (II->getIntrinsicID()) {
698 case Intrinsic::memcpy:
699 case Intrinsic::memmove: {
700 uint64_t Len = UnknownSize;
701 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2)))
702 Len = LenCI->getZExtValue();
703 Value *Dest = II->getArgOperand(0);
704 Value *Src = II->getArgOperand(1);
705 // If it can't overlap the source dest, then it doesn't modref the loc.
706 if (isNoAlias(Location(Dest, Len), Loc)) {
707 if (isNoAlias(Location(Src, Len), Loc))
709 // If it can't overlap the dest, then worst case it reads the loc.
711 } else if (isNoAlias(Location(Src, Len), Loc)) {
712 // If it can't overlap the source, then worst case it mutates the loc.
717 case Intrinsic::memset:
718 // Since memset is 'accesses arguments' only, the AliasAnalysis base class
719 // will handle it for the variable length case.
720 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2))) {
721 uint64_t Len = LenCI->getZExtValue();
722 Value *Dest = II->getArgOperand(0);
723 if (isNoAlias(Location(Dest, Len), Loc))
726 // We know that memset doesn't load anything.
729 case Intrinsic::lifetime_start:
730 case Intrinsic::lifetime_end:
731 case Intrinsic::invariant_start: {
733 cast<ConstantInt>(II->getArgOperand(0))->getZExtValue();
734 if (isNoAlias(Location(II->getArgOperand(1),
736 II->getMetadata(LLVMContext::MD_tbaa)),
741 case Intrinsic::invariant_end: {
743 cast<ConstantInt>(II->getArgOperand(1))->getZExtValue();
744 if (isNoAlias(Location(II->getArgOperand(2),
746 II->getMetadata(LLVMContext::MD_tbaa)),
751 case Intrinsic::arm_neon_vld1: {
752 // LLVM's vld1 and vst1 intrinsics currently only support a single
755 TD ? TD->getTypeStoreSize(II->getType()) : UnknownSize;
756 if (isNoAlias(Location(II->getArgOperand(0), Size,
757 II->getMetadata(LLVMContext::MD_tbaa)),
762 case Intrinsic::arm_neon_vst1: {
764 TD ? TD->getTypeStoreSize(II->getArgOperand(1)->getType()) : UnknownSize;
765 if (isNoAlias(Location(II->getArgOperand(0), Size,
766 II->getMetadata(LLVMContext::MD_tbaa)),
773 // We can bound the aliasing properties of memset_pattern16 just as we can
774 // for memcpy/memset. This is particularly important because the
775 // LoopIdiomRecognizer likes to turn loops into calls to memset_pattern16
776 // whenever possible.
777 else if (TLI.has(LibFunc::memset_pattern16) &&
778 CS.getCalledFunction() &&
779 CS.getCalledFunction()->getName() == "memset_pattern16") {
780 const Function *MS = CS.getCalledFunction();
781 FunctionType *MemsetType = MS->getFunctionType();
782 if (!MemsetType->isVarArg() && MemsetType->getNumParams() == 3 &&
783 isa<PointerType>(MemsetType->getParamType(0)) &&
784 isa<PointerType>(MemsetType->getParamType(1)) &&
785 isa<IntegerType>(MemsetType->getParamType(2))) {
786 uint64_t Len = UnknownSize;
787 if (const ConstantInt *LenCI = dyn_cast<ConstantInt>(CS.getArgument(2)))
788 Len = LenCI->getZExtValue();
789 const Value *Dest = CS.getArgument(0);
790 const Value *Src = CS.getArgument(1);
791 // If it can't overlap the source dest, then it doesn't modref the loc.
792 if (isNoAlias(Location(Dest, Len), Loc)) {
793 // Always reads 16 bytes of the source.
794 if (isNoAlias(Location(Src, 16), Loc))
796 // If it can't overlap the dest, then worst case it reads the loc.
798 // Always reads 16 bytes of the source.
799 } else if (isNoAlias(Location(Src, 16), Loc)) {
800 // If it can't overlap the source, then worst case it mutates the loc.
806 // The AliasAnalysis base class has some smarts, lets use them.
807 return ModRefResult(AliasAnalysis::getModRefInfo(CS, Loc) & Min);
810 /// aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP instruction
811 /// against another pointer. We know that V1 is a GEP, but we don't know
812 /// anything about V2. UnderlyingV1 is GetUnderlyingObject(GEP1, TD),
813 /// UnderlyingV2 is the same for V2.
815 AliasAnalysis::AliasResult
816 BasicAliasAnalysis::aliasGEP(const GEPOperator *GEP1, uint64_t V1Size,
817 const Value *V2, uint64_t V2Size,
818 const MDNode *V2TBAAInfo,
819 const Value *UnderlyingV1,
820 const Value *UnderlyingV2) {
821 int64_t GEP1BaseOffset;
822 SmallVector<VariableGEPIndex, 4> GEP1VariableIndices;
824 // If we have two gep instructions with must-alias'ing base pointers, figure
825 // out if the indexes to the GEP tell us anything about the derived pointer.
826 if (const GEPOperator *GEP2 = dyn_cast<GEPOperator>(V2)) {
827 // Do the base pointers alias?
828 AliasResult BaseAlias = aliasCheck(UnderlyingV1, UnknownSize, 0,
829 UnderlyingV2, UnknownSize, 0);
831 // If we get a No or May, then return it immediately, no amount of analysis
832 // will improve this situation.
833 if (BaseAlias != MustAlias) return BaseAlias;
835 // Otherwise, we have a MustAlias. Since the base pointers alias each other
836 // exactly, see if the computed offset from the common pointer tells us
837 // about the relation of the resulting pointer.
838 const Value *GEP1BasePtr =
839 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD);
841 int64_t GEP2BaseOffset;
842 SmallVector<VariableGEPIndex, 4> GEP2VariableIndices;
843 const Value *GEP2BasePtr =
844 DecomposeGEPExpression(GEP2, GEP2BaseOffset, GEP2VariableIndices, TD);
846 // If DecomposeGEPExpression isn't able to look all the way through the
847 // addressing operation, we must not have TD and this is too complex for us
848 // to handle without it.
849 if (GEP1BasePtr != UnderlyingV1 || GEP2BasePtr != UnderlyingV2) {
851 "DecomposeGEPExpression and GetUnderlyingObject disagree!");
855 // Subtract the GEP2 pointer from the GEP1 pointer to find out their
856 // symbolic difference.
857 GEP1BaseOffset -= GEP2BaseOffset;
858 GetIndexDifference(GEP1VariableIndices, GEP2VariableIndices);
861 // Check to see if these two pointers are related by the getelementptr
862 // instruction. If one pointer is a GEP with a non-zero index of the other
863 // pointer, we know they cannot alias.
865 // If both accesses are unknown size, we can't do anything useful here.
866 if (V1Size == UnknownSize && V2Size == UnknownSize)
869 AliasResult R = aliasCheck(UnderlyingV1, UnknownSize, 0,
870 V2, V2Size, V2TBAAInfo);
872 // If V2 may alias GEP base pointer, conservatively returns MayAlias.
873 // If V2 is known not to alias GEP base pointer, then the two values
874 // cannot alias per GEP semantics: "A pointer value formed from a
875 // getelementptr instruction is associated with the addresses associated
876 // with the first operand of the getelementptr".
879 const Value *GEP1BasePtr =
880 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD);
882 // If DecomposeGEPExpression isn't able to look all the way through the
883 // addressing operation, we must not have TD and this is too complex for us
884 // to handle without it.
885 if (GEP1BasePtr != UnderlyingV1) {
887 "DecomposeGEPExpression and GetUnderlyingObject disagree!");
892 // In the two GEP Case, if there is no difference in the offsets of the
893 // computed pointers, the resultant pointers are a must alias. This
894 // hapens when we have two lexically identical GEP's (for example).
896 // In the other case, if we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2
897 // must aliases the GEP, the end result is a must alias also.
898 if (GEP1BaseOffset == 0 && GEP1VariableIndices.empty())
901 // If there is a constant difference between the pointers, but the difference
902 // is less than the size of the associated memory object, then we know
903 // that the objects are partially overlapping. If the difference is
904 // greater, we know they do not overlap.
905 if (GEP1BaseOffset != 0 && GEP1VariableIndices.empty()) {
906 if (GEP1BaseOffset >= 0) {
907 if (V2Size != UnknownSize) {
908 if ((uint64_t)GEP1BaseOffset < V2Size)
913 if (V1Size != UnknownSize) {
914 if (-(uint64_t)GEP1BaseOffset < V1Size)
921 // Try to distinguish something like &A[i][1] against &A[42][0].
922 // Grab the least significant bit set in any of the scales.
923 if (!GEP1VariableIndices.empty()) {
925 for (unsigned i = 0, e = GEP1VariableIndices.size(); i != e; ++i)
926 Modulo |= (uint64_t)GEP1VariableIndices[i].Scale;
927 Modulo = Modulo ^ (Modulo & (Modulo - 1));
929 // We can compute the difference between the two addresses
930 // mod Modulo. Check whether that difference guarantees that the
931 // two locations do not alias.
932 uint64_t ModOffset = (uint64_t)GEP1BaseOffset & (Modulo - 1);
933 if (V1Size != UnknownSize && V2Size != UnknownSize &&
934 ModOffset >= V2Size && V1Size <= Modulo - ModOffset)
938 // Statically, we can see that the base objects are the same, but the
939 // pointers have dynamic offsets which we can't resolve. And none of our
940 // little tricks above worked.
942 // TODO: Returning PartialAlias instead of MayAlias is a mild hack; the
943 // practical effect of this is protecting TBAA in the case of dynamic
944 // indices into arrays of unions or malloc'd memory.
948 static AliasAnalysis::AliasResult
949 MergeAliasResults(AliasAnalysis::AliasResult A, AliasAnalysis::AliasResult B) {
950 // If the results agree, take it.
953 // A mix of PartialAlias and MustAlias is PartialAlias.
954 if ((A == AliasAnalysis::PartialAlias && B == AliasAnalysis::MustAlias) ||
955 (B == AliasAnalysis::PartialAlias && A == AliasAnalysis::MustAlias))
956 return AliasAnalysis::PartialAlias;
957 // Otherwise, we don't know anything.
958 return AliasAnalysis::MayAlias;
961 /// aliasSelect - Provide a bunch of ad-hoc rules to disambiguate a Select
962 /// instruction against another.
963 AliasAnalysis::AliasResult
964 BasicAliasAnalysis::aliasSelect(const SelectInst *SI, uint64_t SISize,
965 const MDNode *SITBAAInfo,
966 const Value *V2, uint64_t V2Size,
967 const MDNode *V2TBAAInfo) {
968 // If the values are Selects with the same condition, we can do a more precise
969 // check: just check for aliases between the values on corresponding arms.
970 if (const SelectInst *SI2 = dyn_cast<SelectInst>(V2))
971 if (SI->getCondition() == SI2->getCondition()) {
973 aliasCheck(SI->getTrueValue(), SISize, SITBAAInfo,
974 SI2->getTrueValue(), V2Size, V2TBAAInfo);
975 if (Alias == MayAlias)
977 AliasResult ThisAlias =
978 aliasCheck(SI->getFalseValue(), SISize, SITBAAInfo,
979 SI2->getFalseValue(), V2Size, V2TBAAInfo);
980 return MergeAliasResults(ThisAlias, Alias);
983 // If both arms of the Select node NoAlias or MustAlias V2, then returns
984 // NoAlias / MustAlias. Otherwise, returns MayAlias.
986 aliasCheck(V2, V2Size, V2TBAAInfo, SI->getTrueValue(), SISize, SITBAAInfo);
987 if (Alias == MayAlias)
990 AliasResult ThisAlias =
991 aliasCheck(V2, V2Size, V2TBAAInfo, SI->getFalseValue(), SISize, SITBAAInfo);
992 return MergeAliasResults(ThisAlias, Alias);
995 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI instruction
997 AliasAnalysis::AliasResult
998 BasicAliasAnalysis::aliasPHI(const PHINode *PN, uint64_t PNSize,
999 const MDNode *PNTBAAInfo,
1000 const Value *V2, uint64_t V2Size,
1001 const MDNode *V2TBAAInfo) {
1002 // If the values are PHIs in the same block, we can do a more precise
1003 // as well as efficient check: just check for aliases between the values
1004 // on corresponding edges.
1005 if (const PHINode *PN2 = dyn_cast<PHINode>(V2))
1006 if (PN2->getParent() == PN->getParent()) {
1008 aliasCheck(PN->getIncomingValue(0), PNSize, PNTBAAInfo,
1009 PN2->getIncomingValueForBlock(PN->getIncomingBlock(0)),
1010 V2Size, V2TBAAInfo);
1011 if (Alias == MayAlias)
1013 for (unsigned i = 1, e = PN->getNumIncomingValues(); i != e; ++i) {
1014 AliasResult ThisAlias =
1015 aliasCheck(PN->getIncomingValue(i), PNSize, PNTBAAInfo,
1016 PN2->getIncomingValueForBlock(PN->getIncomingBlock(i)),
1017 V2Size, V2TBAAInfo);
1018 Alias = MergeAliasResults(ThisAlias, Alias);
1019 if (Alias == MayAlias)
1025 SmallPtrSet<Value*, 4> UniqueSrc;
1026 SmallVector<Value*, 4> V1Srcs;
1027 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
1028 Value *PV1 = PN->getIncomingValue(i);
1029 if (isa<PHINode>(PV1))
1030 // If any of the source itself is a PHI, return MayAlias conservatively
1031 // to avoid compile time explosion. The worst possible case is if both
1032 // sides are PHI nodes. In which case, this is O(m x n) time where 'm'
1033 // and 'n' are the number of PHI sources.
1035 if (UniqueSrc.insert(PV1))
1036 V1Srcs.push_back(PV1);
1039 AliasResult Alias = aliasCheck(V2, V2Size, V2TBAAInfo,
1040 V1Srcs[0], PNSize, PNTBAAInfo);
1041 // Early exit if the check of the first PHI source against V2 is MayAlias.
1042 // Other results are not possible.
1043 if (Alias == MayAlias)
1046 // If all sources of the PHI node NoAlias or MustAlias V2, then returns
1047 // NoAlias / MustAlias. Otherwise, returns MayAlias.
1048 for (unsigned i = 1, e = V1Srcs.size(); i != e; ++i) {
1049 Value *V = V1Srcs[i];
1051 AliasResult ThisAlias = aliasCheck(V2, V2Size, V2TBAAInfo,
1052 V, PNSize, PNTBAAInfo);
1053 Alias = MergeAliasResults(ThisAlias, Alias);
1054 if (Alias == MayAlias)
1061 // aliasCheck - Provide a bunch of ad-hoc rules to disambiguate in common cases,
1062 // such as array references.
1064 AliasAnalysis::AliasResult
1065 BasicAliasAnalysis::aliasCheck(const Value *V1, uint64_t V1Size,
1066 const MDNode *V1TBAAInfo,
1067 const Value *V2, uint64_t V2Size,
1068 const MDNode *V2TBAAInfo) {
1069 // If either of the memory references is empty, it doesn't matter what the
1070 // pointer values are.
1071 if (V1Size == 0 || V2Size == 0)
1074 // Strip off any casts if they exist.
1075 V1 = V1->stripPointerCasts();
1076 V2 = V2->stripPointerCasts();
1078 // Are we checking for alias of the same value?
1079 if (V1 == V2) return MustAlias;
1081 if (!V1->getType()->isPointerTy() || !V2->getType()->isPointerTy())
1082 return NoAlias; // Scalars cannot alias each other
1084 // Figure out what objects these things are pointing to if we can.
1085 const Value *O1 = GetUnderlyingObject(V1, TD);
1086 const Value *O2 = GetUnderlyingObject(V2, TD);
1088 // Null values in the default address space don't point to any object, so they
1089 // don't alias any other pointer.
1090 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O1))
1091 if (CPN->getType()->getAddressSpace() == 0)
1093 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O2))
1094 if (CPN->getType()->getAddressSpace() == 0)
1098 // If V1/V2 point to two different objects we know that we have no alias.
1099 if (isIdentifiedObject(O1) && isIdentifiedObject(O2))
1102 // Constant pointers can't alias with non-const isIdentifiedObject objects.
1103 if ((isa<Constant>(O1) && isIdentifiedObject(O2) && !isa<Constant>(O2)) ||
1104 (isa<Constant>(O2) && isIdentifiedObject(O1) && !isa<Constant>(O1)))
1107 // Arguments can't alias with local allocations or noalias calls
1108 // in the same function.
1109 if (((isa<Argument>(O1) && (isa<AllocaInst>(O2) || isNoAliasCall(O2))) ||
1110 (isa<Argument>(O2) && (isa<AllocaInst>(O1) || isNoAliasCall(O1)))))
1113 // Most objects can't alias null.
1114 if ((isa<ConstantPointerNull>(O2) && isKnownNonNull(O1)) ||
1115 (isa<ConstantPointerNull>(O1) && isKnownNonNull(O2)))
1118 // If one pointer is the result of a call/invoke or load and the other is a
1119 // non-escaping local object within the same function, then we know the
1120 // object couldn't escape to a point where the call could return it.
1122 // Note that if the pointers are in different functions, there are a
1123 // variety of complications. A call with a nocapture argument may still
1124 // temporary store the nocapture argument's value in a temporary memory
1125 // location if that memory location doesn't escape. Or it may pass a
1126 // nocapture value to other functions as long as they don't capture it.
1127 if (isEscapeSource(O1) && isNonEscapingLocalObject(O2))
1129 if (isEscapeSource(O2) && isNonEscapingLocalObject(O1))
1133 // If the size of one access is larger than the entire object on the other
1134 // side, then we know such behavior is undefined and can assume no alias.
1136 if ((V1Size != UnknownSize && isObjectSmallerThan(O2, V1Size, *TD)) ||
1137 (V2Size != UnknownSize && isObjectSmallerThan(O1, V2Size, *TD)))
1140 // Check the cache before climbing up use-def chains. This also terminates
1141 // otherwise infinitely recursive queries.
1142 LocPair Locs(Location(V1, V1Size, V1TBAAInfo),
1143 Location(V2, V2Size, V2TBAAInfo));
1145 std::swap(Locs.first, Locs.second);
1146 std::pair<AliasCacheTy::iterator, bool> Pair =
1147 AliasCache.insert(std::make_pair(Locs, MayAlias));
1149 return Pair.first->second;
1151 // FIXME: This isn't aggressively handling alias(GEP, PHI) for example: if the
1152 // GEP can't simplify, we don't even look at the PHI cases.
1153 if (!isa<GEPOperator>(V1) && isa<GEPOperator>(V2)) {
1155 std::swap(V1Size, V2Size);
1158 if (const GEPOperator *GV1 = dyn_cast<GEPOperator>(V1)) {
1159 AliasResult Result = aliasGEP(GV1, V1Size, V2, V2Size, V2TBAAInfo, O1, O2);
1160 if (Result != MayAlias) return AliasCache[Locs] = Result;
1163 if (isa<PHINode>(V2) && !isa<PHINode>(V1)) {
1165 std::swap(V1Size, V2Size);
1167 if (const PHINode *PN = dyn_cast<PHINode>(V1)) {
1168 AliasResult Result = aliasPHI(PN, V1Size, V1TBAAInfo,
1169 V2, V2Size, V2TBAAInfo);
1170 if (Result != MayAlias) return AliasCache[Locs] = Result;
1173 if (isa<SelectInst>(V2) && !isa<SelectInst>(V1)) {
1175 std::swap(V1Size, V2Size);
1177 if (const SelectInst *S1 = dyn_cast<SelectInst>(V1)) {
1178 AliasResult Result = aliasSelect(S1, V1Size, V1TBAAInfo,
1179 V2, V2Size, V2TBAAInfo);
1180 if (Result != MayAlias) return AliasCache[Locs] = Result;
1183 // If both pointers are pointing into the same object and one of them
1184 // accesses is accessing the entire object, then the accesses must
1185 // overlap in some way.
1187 if ((V1Size != UnknownSize && isObjectSize(O1, V1Size, *TD)) ||
1188 (V2Size != UnknownSize && isObjectSize(O2, V2Size, *TD)))
1189 return AliasCache[Locs] = PartialAlias;
1191 AliasResult Result =
1192 AliasAnalysis::alias(Location(V1, V1Size, V1TBAAInfo),
1193 Location(V2, V2Size, V2TBAAInfo));
1194 return AliasCache[Locs] = Result;