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/Passes.h"
17 #include "llvm/ADT/SmallPtrSet.h"
18 #include "llvm/ADT/SmallVector.h"
19 #include "llvm/ADT/Statistic.h"
20 #include "llvm/Analysis/AliasAnalysis.h"
21 #include "llvm/Analysis/AssumptionCache.h"
22 #include "llvm/Analysis/CFG.h"
23 #include "llvm/Analysis/CaptureTracking.h"
24 #include "llvm/Analysis/InstructionSimplify.h"
25 #include "llvm/Analysis/LoopInfo.h"
26 #include "llvm/Analysis/MemoryBuiltins.h"
27 #include "llvm/Analysis/TargetLibraryInfo.h"
28 #include "llvm/Analysis/ValueTracking.h"
29 #include "llvm/IR/Constants.h"
30 #include "llvm/IR/DataLayout.h"
31 #include "llvm/IR/DerivedTypes.h"
32 #include "llvm/IR/Dominators.h"
33 #include "llvm/IR/Function.h"
34 #include "llvm/IR/GetElementPtrTypeIterator.h"
35 #include "llvm/IR/GlobalAlias.h"
36 #include "llvm/IR/GlobalVariable.h"
37 #include "llvm/IR/Instructions.h"
38 #include "llvm/IR/IntrinsicInst.h"
39 #include "llvm/IR/LLVMContext.h"
40 #include "llvm/IR/Operator.h"
41 #include "llvm/Pass.h"
42 #include "llvm/Support/ErrorHandling.h"
46 /// Enable analysis of recursive PHI nodes.
47 static cl::opt<bool> EnableRecPhiAnalysis("basicaa-recphi",
48 cl::Hidden, cl::init(false));
50 /// SearchLimitReached / SearchTimes shows how often the limit of
51 /// to decompose GEPs is reached. It will affect the precision
52 /// of basic alias analysis.
53 #define DEBUG_TYPE "basicaa"
54 STATISTIC(SearchLimitReached, "Number of times the limit to "
55 "decompose GEPs is reached");
56 STATISTIC(SearchTimes, "Number of times a GEP is decomposed");
58 /// Cutoff after which to stop analysing a set of phi nodes potentially involved
59 /// in a cycle. Because we are analysing 'through' phi nodes we need to be
60 /// careful with value equivalence. We use reachability to make sure a value
61 /// cannot be involved in a cycle.
62 const unsigned MaxNumPhiBBsValueReachabilityCheck = 20;
64 // The max limit of the search depth in DecomposeGEPExpression() and
65 // GetUnderlyingObject(), both functions need to use the same search
66 // depth otherwise the algorithm in aliasGEP will assert.
67 static const unsigned MaxLookupSearchDepth = 6;
69 //===----------------------------------------------------------------------===//
71 //===----------------------------------------------------------------------===//
73 /// isNonEscapingLocalObject - Return true if the pointer is to a function-local
74 /// object that never escapes from the function.
75 static bool isNonEscapingLocalObject(const Value *V) {
76 // If this is a local allocation, check to see if it escapes.
77 if (isa<AllocaInst>(V) || isNoAliasCall(V))
78 // Set StoreCaptures to True so that we can assume in our callers that the
79 // pointer is not the result of a load instruction. Currently
80 // PointerMayBeCaptured doesn't have any special analysis for the
81 // StoreCaptures=false case; if it did, our callers could be refined to be
83 return !PointerMayBeCaptured(V, false, /*StoreCaptures=*/true);
85 // If this is an argument that corresponds to a byval or noalias argument,
86 // then it has not escaped before entering the function. Check if it escapes
87 // inside the function.
88 if (const Argument *A = dyn_cast<Argument>(V))
89 if (A->hasByValAttr() || A->hasNoAliasAttr())
90 // Note even if the argument is marked nocapture we still need to check
91 // for copies made inside the function. The nocapture attribute only
92 // specifies that there are no copies made that outlive the function.
93 return !PointerMayBeCaptured(V, false, /*StoreCaptures=*/true);
98 /// isEscapeSource - Return true if the pointer is one which would have
99 /// been considered an escape by isNonEscapingLocalObject.
100 static bool isEscapeSource(const Value *V) {
101 if (isa<CallInst>(V) || isa<InvokeInst>(V) || isa<Argument>(V))
104 // The load case works because isNonEscapingLocalObject considers all
105 // stores to be escapes (it passes true for the StoreCaptures argument
106 // to PointerMayBeCaptured).
107 if (isa<LoadInst>(V))
113 /// getObjectSize - Return the size of the object specified by V, or
114 /// UnknownSize if unknown.
115 static uint64_t getObjectSize(const Value *V, const DataLayout &DL,
116 const TargetLibraryInfo &TLI,
117 bool RoundToAlign = false) {
119 if (getObjectSize(V, Size, DL, &TLI, RoundToAlign))
121 return MemoryLocation::UnknownSize;
124 /// isObjectSmallerThan - Return true if we can prove that the object specified
125 /// by V is smaller than Size.
126 static bool isObjectSmallerThan(const Value *V, uint64_t Size,
127 const DataLayout &DL,
128 const TargetLibraryInfo &TLI) {
129 // Note that the meanings of the "object" are slightly different in the
130 // following contexts:
131 // c1: llvm::getObjectSize()
132 // c2: llvm.objectsize() intrinsic
133 // c3: isObjectSmallerThan()
134 // c1 and c2 share the same meaning; however, the meaning of "object" in c3
135 // refers to the "entire object".
137 // Consider this example:
138 // char *p = (char*)malloc(100)
141 // In the context of c1 and c2, the "object" pointed by q refers to the
142 // stretch of memory of q[0:19]. So, getObjectSize(q) should return 20.
144 // However, in the context of c3, the "object" refers to the chunk of memory
145 // being allocated. So, the "object" has 100 bytes, and q points to the middle
146 // the "object". In case q is passed to isObjectSmallerThan() as the 1st
147 // parameter, before the llvm::getObjectSize() is called to get the size of
148 // entire object, we should:
149 // - either rewind the pointer q to the base-address of the object in
150 // question (in this case rewind to p), or
151 // - just give up. It is up to caller to make sure the pointer is pointing
152 // to the base address the object.
154 // We go for 2nd option for simplicity.
155 if (!isIdentifiedObject(V))
158 // This function needs to use the aligned object size because we allow
159 // reads a bit past the end given sufficient alignment.
160 uint64_t ObjectSize = getObjectSize(V, DL, TLI, /*RoundToAlign*/true);
162 return ObjectSize != MemoryLocation::UnknownSize && ObjectSize < Size;
165 /// isObjectSize - Return true if we can prove that the object specified
166 /// by V has size Size.
167 static bool isObjectSize(const Value *V, uint64_t Size,
168 const DataLayout &DL, const TargetLibraryInfo &TLI) {
169 uint64_t ObjectSize = getObjectSize(V, DL, TLI);
170 return ObjectSize != MemoryLocation::UnknownSize && ObjectSize == Size;
173 //===----------------------------------------------------------------------===//
174 // GetElementPtr Instruction Decomposition and Analysis
175 //===----------------------------------------------------------------------===//
184 struct VariableGEPIndex {
186 ExtensionKind Extension;
189 bool operator==(const VariableGEPIndex &Other) const {
190 return V == Other.V && Extension == Other.Extension &&
191 Scale == Other.Scale;
194 bool operator!=(const VariableGEPIndex &Other) const {
195 return !operator==(Other);
201 /// GetLinearExpression - Analyze the specified value as a linear expression:
202 /// "A*V + B", where A and B are constant integers. Return the scale and offset
203 /// values as APInts and return V as a Value*, and return whether we looked
204 /// through any sign or zero extends. The incoming Value is known to have
205 /// IntegerType and it may already be sign or zero extended.
207 /// Note that this looks through extends, so the high bits may not be
208 /// represented in the result.
209 static Value *GetLinearExpression(Value *V, APInt &Scale, APInt &Offset,
210 ExtensionKind &Extension,
211 const DataLayout &DL, unsigned Depth,
212 AssumptionCache *AC, DominatorTree *DT) {
213 assert(V->getType()->isIntegerTy() && "Not an integer value");
215 // Limit our recursion depth.
222 if (ConstantInt *Const = dyn_cast<ConstantInt>(V)) {
223 // if it's a constant, just convert it to an offset
224 // and remove the variable.
225 Offset += Const->getValue();
226 assert(Scale == 0 && "Constant values don't have a scale");
230 if (BinaryOperator *BOp = dyn_cast<BinaryOperator>(V)) {
231 if (ConstantInt *RHSC = dyn_cast<ConstantInt>(BOp->getOperand(1))) {
232 switch (BOp->getOpcode()) {
234 case Instruction::Or:
235 // X|C == X+C if all the bits in C are unset in X. Otherwise we can't
237 if (!MaskedValueIsZero(BOp->getOperand(0), RHSC->getValue(), DL, 0, AC,
241 case Instruction::Add:
242 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension,
243 DL, Depth + 1, AC, DT);
244 Offset += RHSC->getValue();
246 case Instruction::Mul:
247 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension,
248 DL, Depth + 1, AC, DT);
249 Offset *= RHSC->getValue();
250 Scale *= RHSC->getValue();
252 case Instruction::Shl:
253 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension,
254 DL, Depth + 1, AC, DT);
255 Offset <<= RHSC->getValue().getLimitedValue();
256 Scale <<= RHSC->getValue().getLimitedValue();
262 // Since GEP indices are sign extended anyway, we don't care about the high
263 // bits of a sign or zero extended value - just scales and offsets. The
264 // extensions have to be consistent though.
265 if ((isa<SExtInst>(V) && Extension != EK_ZeroExt) ||
266 (isa<ZExtInst>(V) && Extension != EK_SignExt)) {
267 Value *CastOp = cast<CastInst>(V)->getOperand(0);
268 unsigned OldWidth = Scale.getBitWidth();
269 unsigned SmallWidth = CastOp->getType()->getPrimitiveSizeInBits();
270 Scale = Scale.trunc(SmallWidth);
271 Offset = Offset.trunc(SmallWidth);
272 Extension = isa<SExtInst>(V) ? EK_SignExt : EK_ZeroExt;
274 Value *Result = GetLinearExpression(CastOp, Scale, Offset, Extension, DL,
276 Scale = Scale.zext(OldWidth);
278 // We have to sign-extend even if Extension == EK_ZeroExt as we can't
279 // decompose a sign extension (i.e. zext(x - 1) != zext(x) - zext(-1)).
280 Offset = Offset.sext(OldWidth);
290 /// DecomposeGEPExpression - If V is a symbolic pointer expression, decompose it
291 /// into a base pointer with a constant offset and a number of scaled symbolic
294 /// The scaled symbolic offsets (represented by pairs of a Value* and a scale in
295 /// the VarIndices vector) are Value*'s that are known to be scaled by the
296 /// specified amount, but which may have other unrepresented high bits. As such,
297 /// the gep cannot necessarily be reconstructed from its decomposed form.
299 /// When DataLayout is around, this function is capable of analyzing everything
300 /// that GetUnderlyingObject can look through. To be able to do that
301 /// GetUnderlyingObject and DecomposeGEPExpression must use the same search
302 /// depth (MaxLookupSearchDepth).
303 /// When DataLayout not is around, it just looks through pointer casts.
306 DecomposeGEPExpression(const Value *V, int64_t &BaseOffs,
307 SmallVectorImpl<VariableGEPIndex> &VarIndices,
308 bool &MaxLookupReached, const DataLayout &DL,
309 AssumptionCache *AC, DominatorTree *DT) {
310 // Limit recursion depth to limit compile time in crazy cases.
311 unsigned MaxLookup = MaxLookupSearchDepth;
312 MaxLookupReached = false;
317 // See if this is a bitcast or GEP.
318 const Operator *Op = dyn_cast<Operator>(V);
320 // The only non-operator case we can handle are GlobalAliases.
321 if (const GlobalAlias *GA = dyn_cast<GlobalAlias>(V)) {
322 if (!GA->mayBeOverridden()) {
323 V = GA->getAliasee();
330 if (Op->getOpcode() == Instruction::BitCast ||
331 Op->getOpcode() == Instruction::AddrSpaceCast) {
332 V = Op->getOperand(0);
336 const GEPOperator *GEPOp = dyn_cast<GEPOperator>(Op);
338 // If it's not a GEP, hand it off to SimplifyInstruction to see if it
339 // can come up with something. This matches what GetUnderlyingObject does.
340 if (const Instruction *I = dyn_cast<Instruction>(V))
341 // TODO: Get a DominatorTree and AssumptionCache and use them here
342 // (these are both now available in this function, but this should be
343 // updated when GetUnderlyingObject is updated). TLI should be
345 if (const Value *Simplified =
346 SimplifyInstruction(const_cast<Instruction *>(I), DL)) {
354 // Don't attempt to analyze GEPs over unsized objects.
355 if (!GEPOp->getOperand(0)->getType()->getPointerElementType()->isSized())
358 unsigned AS = GEPOp->getPointerAddressSpace();
359 // Walk the indices of the GEP, accumulating them into BaseOff/VarIndices.
360 gep_type_iterator GTI = gep_type_begin(GEPOp);
361 for (User::const_op_iterator I = GEPOp->op_begin()+1,
362 E = GEPOp->op_end(); I != E; ++I) {
364 // Compute the (potentially symbolic) offset in bytes for this index.
365 if (StructType *STy = dyn_cast<StructType>(*GTI++)) {
366 // For a struct, add the member offset.
367 unsigned FieldNo = cast<ConstantInt>(Index)->getZExtValue();
368 if (FieldNo == 0) continue;
370 BaseOffs += DL.getStructLayout(STy)->getElementOffset(FieldNo);
374 // For an array/pointer, add the element offset, explicitly scaled.
375 if (ConstantInt *CIdx = dyn_cast<ConstantInt>(Index)) {
376 if (CIdx->isZero()) continue;
377 BaseOffs += DL.getTypeAllocSize(*GTI) * CIdx->getSExtValue();
381 uint64_t Scale = DL.getTypeAllocSize(*GTI);
382 ExtensionKind Extension = EK_NotExtended;
384 // If the integer type is smaller than the pointer size, it is implicitly
385 // sign extended to pointer size.
386 unsigned Width = Index->getType()->getIntegerBitWidth();
387 if (DL.getPointerSizeInBits(AS) > Width)
388 Extension = EK_SignExt;
390 // Use GetLinearExpression to decompose the index into a C1*V+C2 form.
391 APInt IndexScale(Width, 0), IndexOffset(Width, 0);
392 Index = GetLinearExpression(Index, IndexScale, IndexOffset, Extension, DL,
395 // The GEP index scale ("Scale") scales C1*V+C2, yielding (C1*V+C2)*Scale.
396 // This gives us an aggregate computation of (C1*Scale)*V + C2*Scale.
397 BaseOffs += IndexOffset.getSExtValue()*Scale;
398 Scale *= IndexScale.getSExtValue();
400 // If we already had an occurrence of this index variable, merge this
401 // scale into it. For example, we want to handle:
402 // A[x][x] -> x*16 + x*4 -> x*20
403 // This also ensures that 'x' only appears in the index list once.
404 for (unsigned i = 0, e = VarIndices.size(); i != e; ++i) {
405 if (VarIndices[i].V == Index &&
406 VarIndices[i].Extension == Extension) {
407 Scale += VarIndices[i].Scale;
408 VarIndices.erase(VarIndices.begin()+i);
413 // Make sure that we have a scale that makes sense for this target's
415 if (unsigned ShiftBits = 64 - DL.getPointerSizeInBits(AS)) {
417 Scale = (int64_t)Scale >> ShiftBits;
421 VariableGEPIndex Entry = {Index, Extension,
422 static_cast<int64_t>(Scale)};
423 VarIndices.push_back(Entry);
427 // Analyze the base pointer next.
428 V = GEPOp->getOperand(0);
429 } while (--MaxLookup);
431 // If the chain of expressions is too deep, just return early.
432 MaxLookupReached = true;
433 SearchLimitReached++;
437 //===----------------------------------------------------------------------===//
438 // BasicAliasAnalysis Pass
439 //===----------------------------------------------------------------------===//
442 static const Function *getParent(const Value *V) {
443 if (const Instruction *inst = dyn_cast<Instruction>(V))
444 return inst->getParent()->getParent();
446 if (const Argument *arg = dyn_cast<Argument>(V))
447 return arg->getParent();
452 static bool notDifferentParent(const Value *O1, const Value *O2) {
454 const Function *F1 = getParent(O1);
455 const Function *F2 = getParent(O2);
457 return !F1 || !F2 || F1 == F2;
462 /// BasicAliasAnalysis - This is the primary alias analysis implementation.
463 struct BasicAliasAnalysis : public ImmutablePass, public AliasAnalysis {
464 static char ID; // Class identification, replacement for typeinfo
465 BasicAliasAnalysis() : ImmutablePass(ID) {
466 initializeBasicAliasAnalysisPass(*PassRegistry::getPassRegistry());
469 bool doInitialization(Module &M) override;
471 void getAnalysisUsage(AnalysisUsage &AU) const override {
472 AU.addRequired<AliasAnalysis>();
473 AU.addRequired<AssumptionCacheTracker>();
474 AU.addRequired<TargetLibraryInfoWrapperPass>();
477 AliasResult alias(const MemoryLocation &LocA,
478 const MemoryLocation &LocB) override {
479 assert(AliasCache.empty() && "AliasCache must be cleared after use!");
480 assert(notDifferentParent(LocA.Ptr, LocB.Ptr) &&
481 "BasicAliasAnalysis doesn't support interprocedural queries.");
482 AliasResult Alias = aliasCheck(LocA.Ptr, LocA.Size, LocA.AATags,
483 LocB.Ptr, LocB.Size, LocB.AATags);
484 // AliasCache rarely has more than 1 or 2 elements, always use
485 // shrink_and_clear so it quickly returns to the inline capacity of the
486 // SmallDenseMap if it ever grows larger.
487 // FIXME: This should really be shrink_to_inline_capacity_and_clear().
488 AliasCache.shrink_and_clear();
489 VisitedPhiBBs.clear();
493 ModRefInfo getModRefInfo(ImmutableCallSite CS,
494 const MemoryLocation &Loc) override;
496 ModRefInfo getModRefInfo(ImmutableCallSite CS1,
497 ImmutableCallSite CS2) override;
499 /// pointsToConstantMemory - Chase pointers until we find a (constant
501 bool pointsToConstantMemory(const MemoryLocation &Loc,
502 bool OrLocal) override;
504 /// Get the location associated with a pointer argument of a callsite.
505 ModRefInfo getArgModRefInfo(ImmutableCallSite CS, unsigned ArgIdx) override;
507 /// getModRefBehavior - Return the behavior when calling the given
509 FunctionModRefBehavior getModRefBehavior(ImmutableCallSite CS) override;
511 /// getModRefBehavior - Return the behavior when calling the given function.
512 /// For use when the call site is not known.
513 FunctionModRefBehavior getModRefBehavior(const Function *F) override;
515 /// getAdjustedAnalysisPointer - This method is used when a pass implements
516 /// an analysis interface through multiple inheritance. If needed, it
517 /// should override this to adjust the this pointer as needed for the
518 /// specified pass info.
519 void *getAdjustedAnalysisPointer(const void *ID) override {
520 if (ID == &AliasAnalysis::ID)
521 return (AliasAnalysis*)this;
526 // AliasCache - Track alias queries to guard against recursion.
527 typedef std::pair<MemoryLocation, MemoryLocation> LocPair;
528 typedef SmallDenseMap<LocPair, AliasResult, 8> AliasCacheTy;
529 AliasCacheTy AliasCache;
531 /// \brief Track phi nodes we have visited. When interpret "Value" pointer
532 /// equality as value equality we need to make sure that the "Value" is not
533 /// part of a cycle. Otherwise, two uses could come from different
534 /// "iterations" of a cycle and see different values for the same "Value"
536 /// The following example shows the problem:
537 /// %p = phi(%alloca1, %addr2)
539 /// %addr1 = gep, %alloca2, 0, %l
540 /// %addr2 = gep %alloca2, 0, (%l + 1)
541 /// alias(%p, %addr1) -> MayAlias !
543 SmallPtrSet<const BasicBlock*, 8> VisitedPhiBBs;
545 // Visited - Track instructions visited by pointsToConstantMemory.
546 SmallPtrSet<const Value*, 16> Visited;
548 /// \brief Check whether two Values can be considered equivalent.
550 /// In addition to pointer equivalence of \p V1 and \p V2 this checks
551 /// whether they can not be part of a cycle in the value graph by looking at
552 /// all visited phi nodes an making sure that the phis cannot reach the
553 /// value. We have to do this because we are looking through phi nodes (That
554 /// is we say noalias(V, phi(VA, VB)) if noalias(V, VA) and noalias(V, VB).
555 bool isValueEqualInPotentialCycles(const Value *V1, const Value *V2);
557 /// \brief Dest and Src are the variable indices from two decomposed
558 /// GetElementPtr instructions GEP1 and GEP2 which have common base
559 /// pointers. Subtract the GEP2 indices from GEP1 to find the symbolic
560 /// difference between the two pointers.
561 void GetIndexDifference(SmallVectorImpl<VariableGEPIndex> &Dest,
562 const SmallVectorImpl<VariableGEPIndex> &Src);
564 // aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP
565 // instruction against another.
566 AliasResult aliasGEP(const GEPOperator *V1, uint64_t V1Size,
567 const AAMDNodes &V1AAInfo,
568 const Value *V2, uint64_t V2Size,
569 const AAMDNodes &V2AAInfo,
570 const Value *UnderlyingV1, const Value *UnderlyingV2);
572 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI
573 // instruction against another.
574 AliasResult aliasPHI(const PHINode *PN, uint64_t PNSize,
575 const AAMDNodes &PNAAInfo,
576 const Value *V2, uint64_t V2Size,
577 const AAMDNodes &V2AAInfo);
579 /// aliasSelect - Disambiguate a Select instruction against another value.
580 AliasResult aliasSelect(const SelectInst *SI, uint64_t SISize,
581 const AAMDNodes &SIAAInfo,
582 const Value *V2, uint64_t V2Size,
583 const AAMDNodes &V2AAInfo);
585 AliasResult aliasCheck(const Value *V1, uint64_t V1Size,
587 const Value *V2, uint64_t V2Size,
590 } // End of anonymous namespace
592 // Register this pass...
593 char BasicAliasAnalysis::ID = 0;
594 INITIALIZE_AG_PASS_BEGIN(BasicAliasAnalysis, AliasAnalysis, "basicaa",
595 "Basic Alias Analysis (stateless AA impl)",
597 INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
598 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
599 INITIALIZE_AG_PASS_END(BasicAliasAnalysis, AliasAnalysis, "basicaa",
600 "Basic Alias Analysis (stateless AA impl)",
604 ImmutablePass *llvm::createBasicAliasAnalysisPass() {
605 return new BasicAliasAnalysis();
608 /// pointsToConstantMemory - Returns whether the given pointer value
609 /// points to memory that is local to the function, with global constants being
610 /// considered local to all functions.
611 bool BasicAliasAnalysis::pointsToConstantMemory(const MemoryLocation &Loc,
613 assert(Visited.empty() && "Visited must be cleared after use!");
615 unsigned MaxLookup = 8;
616 SmallVector<const Value *, 16> Worklist;
617 Worklist.push_back(Loc.Ptr);
619 const Value *V = GetUnderlyingObject(Worklist.pop_back_val(), *DL);
620 if (!Visited.insert(V).second) {
622 return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);
625 // An alloca instruction defines local memory.
626 if (OrLocal && isa<AllocaInst>(V))
629 // A global constant counts as local memory for our purposes.
630 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) {
631 // Note: this doesn't require GV to be "ODR" because it isn't legal for a
632 // global to be marked constant in some modules and non-constant in
633 // others. GV may even be a declaration, not a definition.
634 if (!GV->isConstant()) {
636 return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);
641 // If both select values point to local memory, then so does the select.
642 if (const SelectInst *SI = dyn_cast<SelectInst>(V)) {
643 Worklist.push_back(SI->getTrueValue());
644 Worklist.push_back(SI->getFalseValue());
648 // If all values incoming to a phi node point to local memory, then so does
650 if (const PHINode *PN = dyn_cast<PHINode>(V)) {
651 // Don't bother inspecting phi nodes with many operands.
652 if (PN->getNumIncomingValues() > MaxLookup) {
654 return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);
656 for (Value *IncValue : PN->incoming_values())
657 Worklist.push_back(IncValue);
661 // Otherwise be conservative.
663 return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);
665 } while (!Worklist.empty() && --MaxLookup);
668 return Worklist.empty();
671 // FIXME: This code is duplicated with MemoryLocation and should be hoisted to
672 // some common utility location.
673 static bool isMemsetPattern16(const Function *MS,
674 const TargetLibraryInfo &TLI) {
675 if (TLI.has(LibFunc::memset_pattern16) &&
676 MS->getName() == "memset_pattern16") {
677 FunctionType *MemsetType = MS->getFunctionType();
678 if (!MemsetType->isVarArg() && MemsetType->getNumParams() == 3 &&
679 isa<PointerType>(MemsetType->getParamType(0)) &&
680 isa<PointerType>(MemsetType->getParamType(1)) &&
681 isa<IntegerType>(MemsetType->getParamType(2)))
688 /// getModRefBehavior - Return the behavior when calling the given call site.
689 FunctionModRefBehavior
690 BasicAliasAnalysis::getModRefBehavior(ImmutableCallSite CS) {
691 if (CS.doesNotAccessMemory())
692 // Can't do better than this.
693 return FMRB_DoesNotAccessMemory;
695 FunctionModRefBehavior Min = FMRB_UnknownModRefBehavior;
697 // If the callsite knows it only reads memory, don't return worse
699 if (CS.onlyReadsMemory())
700 Min = FMRB_OnlyReadsMemory;
702 if (CS.onlyAccessesArgMemory())
703 Min = FunctionModRefBehavior(Min & FMRB_OnlyAccessesArgumentPointees);
705 // The AliasAnalysis base class has some smarts, lets use them.
706 return FunctionModRefBehavior(AliasAnalysis::getModRefBehavior(CS) & Min);
709 /// getModRefBehavior - Return the behavior when calling the given function.
710 /// For use when the call site is not known.
711 FunctionModRefBehavior
712 BasicAliasAnalysis::getModRefBehavior(const Function *F) {
713 // If the function declares it doesn't access memory, we can't do better.
714 if (F->doesNotAccessMemory())
715 return FMRB_DoesNotAccessMemory;
717 // For intrinsics, we can check the table.
718 if (Intrinsic::ID iid = F->getIntrinsicID()) {
719 #define GET_INTRINSIC_MODREF_BEHAVIOR
720 #include "llvm/IR/Intrinsics.gen"
721 #undef GET_INTRINSIC_MODREF_BEHAVIOR
724 FunctionModRefBehavior Min = FMRB_UnknownModRefBehavior;
726 // If the function declares it only reads memory, go with that.
727 if (F->onlyReadsMemory())
728 Min = FMRB_OnlyReadsMemory;
730 if (F->onlyAccessesArgMemory())
731 Min = FunctionModRefBehavior(Min & FMRB_OnlyAccessesArgumentPointees);
733 const TargetLibraryInfo &TLI =
734 getAnalysis<TargetLibraryInfoWrapperPass>().getTLI();
735 if (isMemsetPattern16(F, TLI))
736 Min = FMRB_OnlyAccessesArgumentPointees;
738 // Otherwise be conservative.
739 return FunctionModRefBehavior(AliasAnalysis::getModRefBehavior(F) & Min);
742 ModRefInfo BasicAliasAnalysis::getArgModRefInfo(ImmutableCallSite CS,
744 if (const IntrinsicInst *II = dyn_cast<IntrinsicInst>(CS.getInstruction()))
745 switch (II->getIntrinsicID()) {
748 case Intrinsic::memset:
749 case Intrinsic::memcpy:
750 case Intrinsic::memmove:
751 assert((ArgIdx == 0 || ArgIdx == 1) &&
752 "Invalid argument index for memory intrinsic");
753 return ArgIdx ? MRI_Ref : MRI_Mod;
756 // We can bound the aliasing properties of memset_pattern16 just as we can
757 // for memcpy/memset. This is particularly important because the
758 // LoopIdiomRecognizer likes to turn loops into calls to memset_pattern16
759 // whenever possible.
760 if (CS.getCalledFunction() &&
761 isMemsetPattern16(CS.getCalledFunction(), *TLI)) {
762 assert((ArgIdx == 0 || ArgIdx == 1) &&
763 "Invalid argument index for memset_pattern16");
764 return ArgIdx ? MRI_Ref : MRI_Mod;
766 // FIXME: Handle memset_pattern4 and memset_pattern8 also.
768 return AliasAnalysis::getArgModRefInfo(CS, ArgIdx);
771 static bool isAssumeIntrinsic(ImmutableCallSite CS) {
772 const IntrinsicInst *II = dyn_cast<IntrinsicInst>(CS.getInstruction());
773 if (II && II->getIntrinsicID() == Intrinsic::assume)
779 bool BasicAliasAnalysis::doInitialization(Module &M) {
780 InitializeAliasAnalysis(this, &M.getDataLayout());
784 /// getModRefInfo - Check to see if the specified callsite can clobber the
785 /// specified memory object. Since we only look at local properties of this
786 /// function, we really can't say much about this query. We do, however, use
787 /// simple "address taken" analysis on local objects.
788 ModRefInfo BasicAliasAnalysis::getModRefInfo(ImmutableCallSite CS,
789 const MemoryLocation &Loc) {
790 assert(notDifferentParent(CS.getInstruction(), Loc.Ptr) &&
791 "AliasAnalysis query involving multiple functions!");
793 const Value *Object = GetUnderlyingObject(Loc.Ptr, *DL);
795 // If this is a tail call and Loc.Ptr points to a stack location, we know that
796 // the tail call cannot access or modify the local stack.
797 // We cannot exclude byval arguments here; these belong to the caller of
798 // the current function not to the current function, and a tail callee
799 // may reference them.
800 if (isa<AllocaInst>(Object))
801 if (const CallInst *CI = dyn_cast<CallInst>(CS.getInstruction()))
802 if (CI->isTailCall())
805 // If the pointer is to a locally allocated object that does not escape,
806 // then the call can not mod/ref the pointer unless the call takes the pointer
807 // as an argument, and itself doesn't capture it.
808 if (!isa<Constant>(Object) && CS.getInstruction() != Object &&
809 isNonEscapingLocalObject(Object)) {
810 bool PassedAsArg = false;
812 for (ImmutableCallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end();
813 CI != CE; ++CI, ++ArgNo) {
814 // Only look at the no-capture or byval pointer arguments. If this
815 // pointer were passed to arguments that were neither of these, then it
816 // couldn't be no-capture.
817 if (!(*CI)->getType()->isPointerTy() ||
818 (!CS.doesNotCapture(ArgNo) && !CS.isByValArgument(ArgNo)))
821 // If this is a no-capture pointer argument, see if we can tell that it
822 // is impossible to alias the pointer we're checking. If not, we have to
823 // assume that the call could touch the pointer, even though it doesn't
825 if (!isNoAlias(MemoryLocation(*CI), MemoryLocation(Object))) {
835 // While the assume intrinsic is marked as arbitrarily writing so that
836 // proper control dependencies will be maintained, it never aliases any
837 // particular memory location.
838 if (isAssumeIntrinsic(CS))
841 // The AliasAnalysis base class has some smarts, lets use them.
842 return AliasAnalysis::getModRefInfo(CS, Loc);
845 ModRefInfo BasicAliasAnalysis::getModRefInfo(ImmutableCallSite CS1,
846 ImmutableCallSite CS2) {
847 // While the assume intrinsic is marked as arbitrarily writing so that
848 // proper control dependencies will be maintained, it never aliases any
849 // particular memory location.
850 if (isAssumeIntrinsic(CS1) || isAssumeIntrinsic(CS2))
853 // The AliasAnalysis base class has some smarts, lets use them.
854 return AliasAnalysis::getModRefInfo(CS1, CS2);
857 /// \brief Provide ad-hoc rules to disambiguate accesses through two GEP
858 /// operators, both having the exact same pointer operand.
859 static AliasResult aliasSameBasePointerGEPs(const GEPOperator *GEP1,
861 const GEPOperator *GEP2,
863 const DataLayout &DL) {
865 assert(GEP1->getPointerOperand() == GEP2->getPointerOperand() &&
866 "Expected GEPs with the same pointer operand");
868 // Try to determine whether GEP1 and GEP2 index through arrays, into structs,
869 // such that the struct field accesses provably cannot alias.
870 // We also need at least two indices (the pointer, and the struct field).
871 if (GEP1->getNumIndices() != GEP2->getNumIndices() ||
872 GEP1->getNumIndices() < 2)
875 // If we don't know the size of the accesses through both GEPs, we can't
876 // determine whether the struct fields accessed can't alias.
877 if (V1Size == MemoryLocation::UnknownSize ||
878 V2Size == MemoryLocation::UnknownSize)
882 dyn_cast<ConstantInt>(GEP1->getOperand(GEP1->getNumOperands() - 1));
884 dyn_cast<ConstantInt>(GEP2->getOperand(GEP2->getNumOperands() - 1));
886 // If the last (struct) indices aren't constants, we can't say anything.
887 // If they're identical, the other indices might be also be dynamically
888 // equal, so the GEPs can alias.
889 if (!C1 || !C2 || C1 == C2)
892 // Find the last-indexed type of the GEP, i.e., the type you'd get if
893 // you stripped the last index.
894 // On the way, look at each indexed type. If there's something other
895 // than an array, different indices can lead to different final types.
896 SmallVector<Value *, 8> IntermediateIndices;
898 // Insert the first index; we don't need to check the type indexed
899 // through it as it only drops the pointer indirection.
900 assert(GEP1->getNumIndices() > 1 && "Not enough GEP indices to examine");
901 IntermediateIndices.push_back(GEP1->getOperand(1));
903 // Insert all the remaining indices but the last one.
904 // Also, check that they all index through arrays.
905 for (unsigned i = 1, e = GEP1->getNumIndices() - 1; i != e; ++i) {
906 if (!isa<ArrayType>(GetElementPtrInst::getIndexedType(
907 GEP1->getSourceElementType(), IntermediateIndices)))
909 IntermediateIndices.push_back(GEP1->getOperand(i + 1));
912 StructType *LastIndexedStruct =
913 dyn_cast<StructType>(GetElementPtrInst::getIndexedType(
914 GEP1->getSourceElementType(), IntermediateIndices));
916 if (!LastIndexedStruct)
920 // - both GEPs begin indexing from the exact same pointer;
921 // - the last indices in both GEPs are constants, indexing into a struct;
922 // - said indices are different, hence, the pointed-to fields are different;
923 // - both GEPs only index through arrays prior to that.
925 // This lets us determine that the struct that GEP1 indexes into and the
926 // struct that GEP2 indexes into must either precisely overlap or be
927 // completely disjoint. Because they cannot partially overlap, indexing into
928 // different non-overlapping fields of the struct will never alias.
930 // Therefore, the only remaining thing needed to show that both GEPs can't
931 // alias is that the fields are not overlapping.
932 const StructLayout *SL = DL.getStructLayout(LastIndexedStruct);
933 const uint64_t StructSize = SL->getSizeInBytes();
934 const uint64_t V1Off = SL->getElementOffset(C1->getZExtValue());
935 const uint64_t V2Off = SL->getElementOffset(C2->getZExtValue());
937 auto EltsDontOverlap = [StructSize](uint64_t V1Off, uint64_t V1Size,
938 uint64_t V2Off, uint64_t V2Size) {
939 return V1Off < V2Off && V1Off + V1Size <= V2Off &&
940 ((V2Off + V2Size <= StructSize) ||
941 (V2Off + V2Size - StructSize <= V1Off));
944 if (EltsDontOverlap(V1Off, V1Size, V2Off, V2Size) ||
945 EltsDontOverlap(V2Off, V2Size, V1Off, V1Size))
951 /// aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP instruction
952 /// against another pointer. We know that V1 is a GEP, but we don't know
953 /// anything about V2. UnderlyingV1 is GetUnderlyingObject(GEP1, DL),
954 /// UnderlyingV2 is the same for V2.
956 AliasResult BasicAliasAnalysis::aliasGEP(
957 const GEPOperator *GEP1, uint64_t V1Size, const AAMDNodes &V1AAInfo,
958 const Value *V2, uint64_t V2Size, const AAMDNodes &V2AAInfo,
959 const Value *UnderlyingV1, const Value *UnderlyingV2) {
960 int64_t GEP1BaseOffset;
961 bool GEP1MaxLookupReached;
962 SmallVector<VariableGEPIndex, 4> GEP1VariableIndices;
964 // We have to get two AssumptionCaches here because GEP1 and V2 may be from
965 // different functions.
966 // FIXME: This really doesn't make any sense. We get a dominator tree below
967 // that can only refer to a single function. But this function (aliasGEP) is
968 // a method on an immutable pass that can be called when there *isn't*
969 // a single function. The old pass management layer makes this "work", but
970 // this isn't really a clean solution.
971 AssumptionCacheTracker &ACT = getAnalysis<AssumptionCacheTracker>();
972 AssumptionCache *AC1 = nullptr, *AC2 = nullptr;
973 if (auto *GEP1I = dyn_cast<Instruction>(GEP1))
974 AC1 = &ACT.getAssumptionCache(
975 const_cast<Function &>(*GEP1I->getParent()->getParent()));
976 if (auto *I2 = dyn_cast<Instruction>(V2))
977 AC2 = &ACT.getAssumptionCache(
978 const_cast<Function &>(*I2->getParent()->getParent()));
980 DominatorTreeWrapperPass *DTWP =
981 getAnalysisIfAvailable<DominatorTreeWrapperPass>();
982 DominatorTree *DT = DTWP ? &DTWP->getDomTree() : nullptr;
984 // If we have two gep instructions with must-alias or not-alias'ing base
985 // pointers, figure out if the indexes to the GEP tell us anything about the
987 if (const GEPOperator *GEP2 = dyn_cast<GEPOperator>(V2)) {
988 // Do the base pointers alias?
989 AliasResult BaseAlias =
990 aliasCheck(UnderlyingV1, MemoryLocation::UnknownSize, AAMDNodes(),
991 UnderlyingV2, MemoryLocation::UnknownSize, AAMDNodes());
993 // Check for geps of non-aliasing underlying pointers where the offsets are
995 if ((BaseAlias == MayAlias) && V1Size == V2Size) {
996 // Do the base pointers alias assuming type and size.
997 AliasResult PreciseBaseAlias = aliasCheck(UnderlyingV1, V1Size,
998 V1AAInfo, UnderlyingV2,
1000 if (PreciseBaseAlias == NoAlias) {
1001 // See if the computed offset from the common pointer tells us about the
1002 // relation of the resulting pointer.
1003 int64_t GEP2BaseOffset;
1004 bool GEP2MaxLookupReached;
1005 SmallVector<VariableGEPIndex, 4> GEP2VariableIndices;
1006 const Value *GEP2BasePtr =
1007 DecomposeGEPExpression(GEP2, GEP2BaseOffset, GEP2VariableIndices,
1008 GEP2MaxLookupReached, *DL, AC2, DT);
1009 const Value *GEP1BasePtr =
1010 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices,
1011 GEP1MaxLookupReached, *DL, AC1, DT);
1012 // DecomposeGEPExpression and GetUnderlyingObject should return the
1013 // same result except when DecomposeGEPExpression has no DataLayout.
1014 if (GEP1BasePtr != UnderlyingV1 || GEP2BasePtr != UnderlyingV2) {
1016 "DecomposeGEPExpression and GetUnderlyingObject disagree!");
1019 // If the max search depth is reached the result is undefined
1020 if (GEP2MaxLookupReached || GEP1MaxLookupReached)
1024 if (GEP1BaseOffset == GEP2BaseOffset &&
1025 GEP1VariableIndices == GEP2VariableIndices)
1027 GEP1VariableIndices.clear();
1031 // If we get a No or May, then return it immediately, no amount of analysis
1032 // will improve this situation.
1033 if (BaseAlias != MustAlias) return BaseAlias;
1035 // Otherwise, we have a MustAlias. Since the base pointers alias each other
1036 // exactly, see if the computed offset from the common pointer tells us
1037 // about the relation of the resulting pointer.
1038 const Value *GEP1BasePtr =
1039 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices,
1040 GEP1MaxLookupReached, *DL, AC1, DT);
1042 int64_t GEP2BaseOffset;
1043 bool GEP2MaxLookupReached;
1044 SmallVector<VariableGEPIndex, 4> GEP2VariableIndices;
1045 const Value *GEP2BasePtr =
1046 DecomposeGEPExpression(GEP2, GEP2BaseOffset, GEP2VariableIndices,
1047 GEP2MaxLookupReached, *DL, AC2, DT);
1049 // DecomposeGEPExpression and GetUnderlyingObject should return the
1050 // same result except when DecomposeGEPExpression has no DataLayout.
1051 if (GEP1BasePtr != UnderlyingV1 || GEP2BasePtr != UnderlyingV2) {
1053 "DecomposeGEPExpression and GetUnderlyingObject disagree!");
1057 // If we know the two GEPs are based off of the exact same pointer (and not
1058 // just the same underlying object), see if that tells us anything about
1059 // the resulting pointers.
1060 if (DL && GEP1->getPointerOperand() == GEP2->getPointerOperand()) {
1061 AliasResult R = aliasSameBasePointerGEPs(GEP1, V1Size, GEP2, V2Size, *DL);
1062 // If we couldn't find anything interesting, don't abandon just yet.
1067 // If the max search depth is reached the result is undefined
1068 if (GEP2MaxLookupReached || GEP1MaxLookupReached)
1071 // Subtract the GEP2 pointer from the GEP1 pointer to find out their
1072 // symbolic difference.
1073 GEP1BaseOffset -= GEP2BaseOffset;
1074 GetIndexDifference(GEP1VariableIndices, GEP2VariableIndices);
1077 // Check to see if these two pointers are related by the getelementptr
1078 // instruction. If one pointer is a GEP with a non-zero index of the other
1079 // pointer, we know they cannot alias.
1081 // If both accesses are unknown size, we can't do anything useful here.
1082 if (V1Size == MemoryLocation::UnknownSize &&
1083 V2Size == MemoryLocation::UnknownSize)
1086 AliasResult R = aliasCheck(UnderlyingV1, MemoryLocation::UnknownSize,
1087 AAMDNodes(), V2, V2Size, V2AAInfo);
1089 // If V2 may alias GEP base pointer, conservatively returns MayAlias.
1090 // If V2 is known not to alias GEP base pointer, then the two values
1091 // cannot alias per GEP semantics: "A pointer value formed from a
1092 // getelementptr instruction is associated with the addresses associated
1093 // with the first operand of the getelementptr".
1096 const Value *GEP1BasePtr =
1097 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices,
1098 GEP1MaxLookupReached, *DL, AC1, DT);
1100 // DecomposeGEPExpression and GetUnderlyingObject should return the
1101 // same result except when DecomposeGEPExpression has no DataLayout.
1102 if (GEP1BasePtr != UnderlyingV1) {
1104 "DecomposeGEPExpression and GetUnderlyingObject disagree!");
1107 // If the max search depth is reached the result is undefined
1108 if (GEP1MaxLookupReached)
1112 // In the two GEP Case, if there is no difference in the offsets of the
1113 // computed pointers, the resultant pointers are a must alias. This
1114 // hapens when we have two lexically identical GEP's (for example).
1116 // In the other case, if we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2
1117 // must aliases the GEP, the end result is a must alias also.
1118 if (GEP1BaseOffset == 0 && GEP1VariableIndices.empty())
1121 // If there is a constant difference between the pointers, but the difference
1122 // is less than the size of the associated memory object, then we know
1123 // that the objects are partially overlapping. If the difference is
1124 // greater, we know they do not overlap.
1125 if (GEP1BaseOffset != 0 && GEP1VariableIndices.empty()) {
1126 if (GEP1BaseOffset >= 0) {
1127 if (V2Size != MemoryLocation::UnknownSize) {
1128 if ((uint64_t)GEP1BaseOffset < V2Size)
1129 return PartialAlias;
1133 // We have the situation where:
1136 // ---------------->|
1137 // |-->V1Size |-------> V2Size
1139 // We need to know that V2Size is not unknown, otherwise we might have
1140 // stripped a gep with negative index ('gep <ptr>, -1, ...).
1141 if (V1Size != MemoryLocation::UnknownSize &&
1142 V2Size != MemoryLocation::UnknownSize) {
1143 if (-(uint64_t)GEP1BaseOffset < V1Size)
1144 return PartialAlias;
1150 if (!GEP1VariableIndices.empty()) {
1151 uint64_t Modulo = 0;
1152 bool AllPositive = true;
1153 for (unsigned i = 0, e = GEP1VariableIndices.size(); i != e; ++i) {
1155 // Try to distinguish something like &A[i][1] against &A[42][0].
1156 // Grab the least significant bit set in any of the scales. We
1157 // don't need std::abs here (even if the scale's negative) as we'll
1158 // be ^'ing Modulo with itself later.
1159 Modulo |= (uint64_t) GEP1VariableIndices[i].Scale;
1162 // If the Value could change between cycles, then any reasoning about
1163 // the Value this cycle may not hold in the next cycle. We'll just
1164 // give up if we can't determine conditions that hold for every cycle:
1165 const Value *V = GEP1VariableIndices[i].V;
1167 bool SignKnownZero, SignKnownOne;
1168 ComputeSignBit(const_cast<Value *>(V), SignKnownZero, SignKnownOne, *DL,
1169 0, AC1, nullptr, DT);
1171 // Zero-extension widens the variable, and so forces the sign
1173 bool IsZExt = GEP1VariableIndices[i].Extension == EK_ZeroExt;
1174 SignKnownZero |= IsZExt;
1175 SignKnownOne &= !IsZExt;
1177 // If the variable begins with a zero then we know it's
1178 // positive, regardless of whether the value is signed or
1180 int64_t Scale = GEP1VariableIndices[i].Scale;
1182 (SignKnownZero && Scale >= 0) ||
1183 (SignKnownOne && Scale < 0);
1187 Modulo = Modulo ^ (Modulo & (Modulo - 1));
1189 // We can compute the difference between the two addresses
1190 // mod Modulo. Check whether that difference guarantees that the
1191 // two locations do not alias.
1192 uint64_t ModOffset = (uint64_t)GEP1BaseOffset & (Modulo - 1);
1193 if (V1Size != MemoryLocation::UnknownSize &&
1194 V2Size != MemoryLocation::UnknownSize && ModOffset >= V2Size &&
1195 V1Size <= Modulo - ModOffset)
1198 // If we know all the variables are positive, then GEP1 >= GEP1BasePtr.
1199 // If GEP1BasePtr > V2 (GEP1BaseOffset > 0) then we know the pointers
1200 // don't alias if V2Size can fit in the gap between V2 and GEP1BasePtr.
1201 if (AllPositive && GEP1BaseOffset > 0 && V2Size <= (uint64_t) GEP1BaseOffset)
1205 // Statically, we can see that the base objects are the same, but the
1206 // pointers have dynamic offsets which we can't resolve. And none of our
1207 // little tricks above worked.
1209 // TODO: Returning PartialAlias instead of MayAlias is a mild hack; the
1210 // practical effect of this is protecting TBAA in the case of dynamic
1211 // indices into arrays of unions or malloc'd memory.
1212 return PartialAlias;
1215 static AliasResult MergeAliasResults(AliasResult A, AliasResult B) {
1216 // If the results agree, take it.
1219 // A mix of PartialAlias and MustAlias is PartialAlias.
1220 if ((A == PartialAlias && B == MustAlias) ||
1221 (B == PartialAlias && A == MustAlias))
1222 return PartialAlias;
1223 // Otherwise, we don't know anything.
1227 /// aliasSelect - Provide a bunch of ad-hoc rules to disambiguate a Select
1228 /// instruction against another.
1229 AliasResult BasicAliasAnalysis::aliasSelect(const SelectInst *SI,
1231 const AAMDNodes &SIAAInfo,
1232 const Value *V2, uint64_t V2Size,
1233 const AAMDNodes &V2AAInfo) {
1234 // If the values are Selects with the same condition, we can do a more precise
1235 // check: just check for aliases between the values on corresponding arms.
1236 if (const SelectInst *SI2 = dyn_cast<SelectInst>(V2))
1237 if (SI->getCondition() == SI2->getCondition()) {
1239 aliasCheck(SI->getTrueValue(), SISize, SIAAInfo,
1240 SI2->getTrueValue(), V2Size, V2AAInfo);
1241 if (Alias == MayAlias)
1243 AliasResult ThisAlias =
1244 aliasCheck(SI->getFalseValue(), SISize, SIAAInfo,
1245 SI2->getFalseValue(), V2Size, V2AAInfo);
1246 return MergeAliasResults(ThisAlias, Alias);
1249 // If both arms of the Select node NoAlias or MustAlias V2, then returns
1250 // NoAlias / MustAlias. Otherwise, returns MayAlias.
1252 aliasCheck(V2, V2Size, V2AAInfo, SI->getTrueValue(), SISize, SIAAInfo);
1253 if (Alias == MayAlias)
1256 AliasResult ThisAlias =
1257 aliasCheck(V2, V2Size, V2AAInfo, SI->getFalseValue(), SISize, SIAAInfo);
1258 return MergeAliasResults(ThisAlias, Alias);
1261 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI instruction
1263 AliasResult BasicAliasAnalysis::aliasPHI(const PHINode *PN, uint64_t PNSize,
1264 const AAMDNodes &PNAAInfo,
1265 const Value *V2, uint64_t V2Size,
1266 const AAMDNodes &V2AAInfo) {
1267 // Track phi nodes we have visited. We use this information when we determine
1268 // value equivalence.
1269 VisitedPhiBBs.insert(PN->getParent());
1271 // If the values are PHIs in the same block, we can do a more precise
1272 // as well as efficient check: just check for aliases between the values
1273 // on corresponding edges.
1274 if (const PHINode *PN2 = dyn_cast<PHINode>(V2))
1275 if (PN2->getParent() == PN->getParent()) {
1276 LocPair Locs(MemoryLocation(PN, PNSize, PNAAInfo),
1277 MemoryLocation(V2, V2Size, V2AAInfo));
1279 std::swap(Locs.first, Locs.second);
1280 // Analyse the PHIs' inputs under the assumption that the PHIs are
1282 // If the PHIs are May/MustAlias there must be (recursively) an input
1283 // operand from outside the PHIs' cycle that is MayAlias/MustAlias or
1284 // there must be an operation on the PHIs within the PHIs' value cycle
1285 // that causes a MayAlias.
1286 // Pretend the phis do not alias.
1287 AliasResult Alias = NoAlias;
1288 assert(AliasCache.count(Locs) &&
1289 "There must exist an entry for the phi node");
1290 AliasResult OrigAliasResult = AliasCache[Locs];
1291 AliasCache[Locs] = NoAlias;
1293 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
1294 AliasResult ThisAlias =
1295 aliasCheck(PN->getIncomingValue(i), PNSize, PNAAInfo,
1296 PN2->getIncomingValueForBlock(PN->getIncomingBlock(i)),
1298 Alias = MergeAliasResults(ThisAlias, Alias);
1299 if (Alias == MayAlias)
1303 // Reset if speculation failed.
1304 if (Alias != NoAlias)
1305 AliasCache[Locs] = OrigAliasResult;
1310 SmallPtrSet<Value*, 4> UniqueSrc;
1311 SmallVector<Value*, 4> V1Srcs;
1312 bool isRecursive = false;
1313 for (Value *PV1 : PN->incoming_values()) {
1314 if (isa<PHINode>(PV1))
1315 // If any of the source itself is a PHI, return MayAlias conservatively
1316 // to avoid compile time explosion. The worst possible case is if both
1317 // sides are PHI nodes. In which case, this is O(m x n) time where 'm'
1318 // and 'n' are the number of PHI sources.
1321 if (EnableRecPhiAnalysis)
1322 if (GEPOperator *PV1GEP = dyn_cast<GEPOperator>(PV1)) {
1323 // Check whether the incoming value is a GEP that advances the pointer
1324 // result of this PHI node (e.g. in a loop). If this is the case, we
1325 // would recurse and always get a MayAlias. Handle this case specially
1327 if (PV1GEP->getPointerOperand() == PN && PV1GEP->getNumIndices() == 1 &&
1328 isa<ConstantInt>(PV1GEP->idx_begin())) {
1334 if (UniqueSrc.insert(PV1).second)
1335 V1Srcs.push_back(PV1);
1338 // If this PHI node is recursive, set the size of the accessed memory to
1339 // unknown to represent all the possible values the GEP could advance the
1342 PNSize = MemoryLocation::UnknownSize;
1344 AliasResult Alias = aliasCheck(V2, V2Size, V2AAInfo,
1345 V1Srcs[0], PNSize, PNAAInfo);
1347 // Early exit if the check of the first PHI source against V2 is MayAlias.
1348 // Other results are not possible.
1349 if (Alias == MayAlias)
1352 // If all sources of the PHI node NoAlias or MustAlias V2, then returns
1353 // NoAlias / MustAlias. Otherwise, returns MayAlias.
1354 for (unsigned i = 1, e = V1Srcs.size(); i != e; ++i) {
1355 Value *V = V1Srcs[i];
1357 AliasResult ThisAlias = aliasCheck(V2, V2Size, V2AAInfo,
1358 V, PNSize, PNAAInfo);
1359 Alias = MergeAliasResults(ThisAlias, Alias);
1360 if (Alias == MayAlias)
1367 // aliasCheck - Provide a bunch of ad-hoc rules to disambiguate in common cases,
1368 // such as array references.
1370 AliasResult BasicAliasAnalysis::aliasCheck(const Value *V1, uint64_t V1Size,
1371 AAMDNodes V1AAInfo, const Value *V2,
1373 AAMDNodes V2AAInfo) {
1374 // If either of the memory references is empty, it doesn't matter what the
1375 // pointer values are.
1376 if (V1Size == 0 || V2Size == 0)
1379 // Strip off any casts if they exist.
1380 V1 = V1->stripPointerCasts();
1381 V2 = V2->stripPointerCasts();
1383 // If V1 or V2 is undef, the result is NoAlias because we can always pick a
1384 // value for undef that aliases nothing in the program.
1385 if (isa<UndefValue>(V1) || isa<UndefValue>(V2))
1388 // Are we checking for alias of the same value?
1389 // Because we look 'through' phi nodes we could look at "Value" pointers from
1390 // different iterations. We must therefore make sure that this is not the
1391 // case. The function isValueEqualInPotentialCycles ensures that this cannot
1392 // happen by looking at the visited phi nodes and making sure they cannot
1394 if (isValueEqualInPotentialCycles(V1, V2))
1397 if (!V1->getType()->isPointerTy() || !V2->getType()->isPointerTy())
1398 return NoAlias; // Scalars cannot alias each other
1400 // Figure out what objects these things are pointing to if we can.
1401 const Value *O1 = GetUnderlyingObject(V1, *DL, MaxLookupSearchDepth);
1402 const Value *O2 = GetUnderlyingObject(V2, *DL, MaxLookupSearchDepth);
1404 // Null values in the default address space don't point to any object, so they
1405 // don't alias any other pointer.
1406 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O1))
1407 if (CPN->getType()->getAddressSpace() == 0)
1409 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O2))
1410 if (CPN->getType()->getAddressSpace() == 0)
1414 // If V1/V2 point to two different objects we know that we have no alias.
1415 if (isIdentifiedObject(O1) && isIdentifiedObject(O2))
1418 // Constant pointers can't alias with non-const isIdentifiedObject objects.
1419 if ((isa<Constant>(O1) && isIdentifiedObject(O2) && !isa<Constant>(O2)) ||
1420 (isa<Constant>(O2) && isIdentifiedObject(O1) && !isa<Constant>(O1)))
1423 // Function arguments can't alias with things that are known to be
1424 // unambigously identified at the function level.
1425 if ((isa<Argument>(O1) && isIdentifiedFunctionLocal(O2)) ||
1426 (isa<Argument>(O2) && isIdentifiedFunctionLocal(O1)))
1429 // Most objects can't alias null.
1430 if ((isa<ConstantPointerNull>(O2) && isKnownNonNull(O1)) ||
1431 (isa<ConstantPointerNull>(O1) && isKnownNonNull(O2)))
1434 // If one pointer is the result of a call/invoke or load and the other is a
1435 // non-escaping local object within the same function, then we know the
1436 // object couldn't escape to a point where the call could return it.
1438 // Note that if the pointers are in different functions, there are a
1439 // variety of complications. A call with a nocapture argument may still
1440 // temporary store the nocapture argument's value in a temporary memory
1441 // location if that memory location doesn't escape. Or it may pass a
1442 // nocapture value to other functions as long as they don't capture it.
1443 if (isEscapeSource(O1) && isNonEscapingLocalObject(O2))
1445 if (isEscapeSource(O2) && isNonEscapingLocalObject(O1))
1449 // If the size of one access is larger than the entire object on the other
1450 // side, then we know such behavior is undefined and can assume no alias.
1452 if ((V1Size != MemoryLocation::UnknownSize &&
1453 isObjectSmallerThan(O2, V1Size, *DL, *TLI)) ||
1454 (V2Size != MemoryLocation::UnknownSize &&
1455 isObjectSmallerThan(O1, V2Size, *DL, *TLI)))
1458 // Check the cache before climbing up use-def chains. This also terminates
1459 // otherwise infinitely recursive queries.
1460 LocPair Locs(MemoryLocation(V1, V1Size, V1AAInfo),
1461 MemoryLocation(V2, V2Size, V2AAInfo));
1463 std::swap(Locs.first, Locs.second);
1464 std::pair<AliasCacheTy::iterator, bool> Pair =
1465 AliasCache.insert(std::make_pair(Locs, MayAlias));
1467 return Pair.first->second;
1469 // FIXME: This isn't aggressively handling alias(GEP, PHI) for example: if the
1470 // GEP can't simplify, we don't even look at the PHI cases.
1471 if (!isa<GEPOperator>(V1) && isa<GEPOperator>(V2)) {
1473 std::swap(V1Size, V2Size);
1475 std::swap(V1AAInfo, V2AAInfo);
1477 if (const GEPOperator *GV1 = dyn_cast<GEPOperator>(V1)) {
1478 AliasResult Result = aliasGEP(GV1, V1Size, V1AAInfo, V2, V2Size, V2AAInfo, O1, O2);
1479 if (Result != MayAlias) return AliasCache[Locs] = Result;
1482 if (isa<PHINode>(V2) && !isa<PHINode>(V1)) {
1484 std::swap(V1Size, V2Size);
1485 std::swap(V1AAInfo, V2AAInfo);
1487 if (const PHINode *PN = dyn_cast<PHINode>(V1)) {
1488 AliasResult Result = aliasPHI(PN, V1Size, V1AAInfo,
1489 V2, V2Size, V2AAInfo);
1490 if (Result != MayAlias) return AliasCache[Locs] = Result;
1493 if (isa<SelectInst>(V2) && !isa<SelectInst>(V1)) {
1495 std::swap(V1Size, V2Size);
1496 std::swap(V1AAInfo, V2AAInfo);
1498 if (const SelectInst *S1 = dyn_cast<SelectInst>(V1)) {
1499 AliasResult Result = aliasSelect(S1, V1Size, V1AAInfo,
1500 V2, V2Size, V2AAInfo);
1501 if (Result != MayAlias) return AliasCache[Locs] = Result;
1504 // If both pointers are pointing into the same object and one of them
1505 // accesses is accessing the entire object, then the accesses must
1506 // overlap in some way.
1508 if ((V1Size != MemoryLocation::UnknownSize &&
1509 isObjectSize(O1, V1Size, *DL, *TLI)) ||
1510 (V2Size != MemoryLocation::UnknownSize &&
1511 isObjectSize(O2, V2Size, *DL, *TLI)))
1512 return AliasCache[Locs] = PartialAlias;
1514 AliasResult Result =
1515 AliasAnalysis::alias(MemoryLocation(V1, V1Size, V1AAInfo),
1516 MemoryLocation(V2, V2Size, V2AAInfo));
1517 return AliasCache[Locs] = Result;
1520 bool BasicAliasAnalysis::isValueEqualInPotentialCycles(const Value *V,
1525 const Instruction *Inst = dyn_cast<Instruction>(V);
1529 if (VisitedPhiBBs.empty())
1532 if (VisitedPhiBBs.size() > MaxNumPhiBBsValueReachabilityCheck)
1535 // Use dominance or loop info if available.
1536 DominatorTreeWrapperPass *DTWP =
1537 getAnalysisIfAvailable<DominatorTreeWrapperPass>();
1538 DominatorTree *DT = DTWP ? &DTWP->getDomTree() : nullptr;
1539 auto *LIWP = getAnalysisIfAvailable<LoopInfoWrapperPass>();
1540 LoopInfo *LI = LIWP ? &LIWP->getLoopInfo() : nullptr;
1542 // Make sure that the visited phis cannot reach the Value. This ensures that
1543 // the Values cannot come from different iterations of a potential cycle the
1544 // phi nodes could be involved in.
1545 for (auto *P : VisitedPhiBBs)
1546 if (isPotentiallyReachable(P->begin(), Inst, DT, LI))
1552 /// GetIndexDifference - Dest and Src are the variable indices from two
1553 /// decomposed GetElementPtr instructions GEP1 and GEP2 which have common base
1554 /// pointers. Subtract the GEP2 indices from GEP1 to find the symbolic
1555 /// difference between the two pointers.
1556 void BasicAliasAnalysis::GetIndexDifference(
1557 SmallVectorImpl<VariableGEPIndex> &Dest,
1558 const SmallVectorImpl<VariableGEPIndex> &Src) {
1562 for (unsigned i = 0, e = Src.size(); i != e; ++i) {
1563 const Value *V = Src[i].V;
1564 ExtensionKind Extension = Src[i].Extension;
1565 int64_t Scale = Src[i].Scale;
1567 // Find V in Dest. This is N^2, but pointer indices almost never have more
1568 // than a few variable indexes.
1569 for (unsigned j = 0, e = Dest.size(); j != e; ++j) {
1570 if (!isValueEqualInPotentialCycles(Dest[j].V, V) ||
1571 Dest[j].Extension != Extension)
1574 // If we found it, subtract off Scale V's from the entry in Dest. If it
1575 // goes to zero, remove the entry.
1576 if (Dest[j].Scale != Scale)
1577 Dest[j].Scale -= Scale;
1579 Dest.erase(Dest.begin() + j);
1584 // If we didn't consume this entry, add it to the end of the Dest list.
1586 VariableGEPIndex Entry = { V, Extension, -Scale };
1587 Dest.push_back(Entry);