1 //===- ScalarEvolutionAliasAnalysis.cpp - SCEV-based Alias Analysis -------===//
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
7 //===----------------------------------------------------------------------===//
9 // This file defines the ScalarEvolutionAliasAnalysis pass, which implements a
10 // simple alias analysis implemented in terms of ScalarEvolution queries.
12 // This differs from traditional loop dependence analysis in that it tests
13 // for dependencies within a single iteration of a loop, rather than
14 // dependencies between different iterations.
16 // ScalarEvolution has a more complete understanding of pointer arithmetic
17 // than BasicAliasAnalysis' collection of ad-hoc analyses.
19 //===----------------------------------------------------------------------===//
21 #include "llvm/Analysis/ScalarEvolutionAliasAnalysis.h"
22 #include "llvm/InitializePasses.h"
25 AliasResult SCEVAAResult::alias(const MemoryLocation &LocA,
26 const MemoryLocation &LocB, AAQueryInfo &AAQI) {
27 // If either of the memory references is empty, it doesn't matter what the
28 // pointer values are. This allows the code below to ignore this special
30 if (LocA.Size.isZero() || LocB.Size.isZero())
33 // This is SCEVAAResult. Get the SCEVs!
34 const SCEV *AS = SE.getSCEV(const_cast<Value *>(LocA.Ptr));
35 const SCEV *BS = SE.getSCEV(const_cast<Value *>(LocB.Ptr));
37 // If they evaluate to the same expression, it's a MustAlias.
41 // If something is known about the difference between the two addresses,
42 // see if it's enough to prove a NoAlias.
43 if (SE.getEffectiveSCEVType(AS->getType()) ==
44 SE.getEffectiveSCEVType(BS->getType())) {
45 unsigned BitWidth = SE.getTypeSizeInBits(AS->getType());
46 APInt ASizeInt(BitWidth, LocA.Size.hasValue()
47 ? LocA.Size.getValue()
48 : MemoryLocation::UnknownSize);
49 APInt BSizeInt(BitWidth, LocB.Size.hasValue()
50 ? LocB.Size.getValue()
51 : MemoryLocation::UnknownSize);
53 // Compute the difference between the two pointers.
54 const SCEV *BA = SE.getMinusSCEV(BS, AS);
56 // Test whether the difference is known to be great enough that memory of
57 // the given sizes don't overlap. This assumes that ASizeInt and BSizeInt
58 // are non-zero, which is special-cased above.
59 if (ASizeInt.ule(SE.getUnsignedRange(BA).getUnsignedMin()) &&
60 (-BSizeInt).uge(SE.getUnsignedRange(BA).getUnsignedMax()))
63 // Folding the subtraction while preserving range information can be tricky
64 // (because of INT_MIN, etc.); if the prior test failed, swap AS and BS
65 // and try again to see if things fold better that way.
67 // Compute the difference between the two pointers.
68 const SCEV *AB = SE.getMinusSCEV(AS, BS);
70 // Test whether the difference is known to be great enough that memory of
71 // the given sizes don't overlap. This assumes that ASizeInt and BSizeInt
72 // are non-zero, which is special-cased above.
73 if (BSizeInt.ule(SE.getUnsignedRange(AB).getUnsignedMin()) &&
74 (-ASizeInt).uge(SE.getUnsignedRange(AB).getUnsignedMax()))
78 // If ScalarEvolution can find an underlying object, form a new query.
79 // The correctness of this depends on ScalarEvolution not recognizing
80 // inttoptr and ptrtoint operators.
81 Value *AO = GetBaseValue(AS);
82 Value *BO = GetBaseValue(BS);
83 if ((AO && AO != LocA.Ptr) || (BO && BO != LocB.Ptr))
84 if (alias(MemoryLocation(AO ? AO : LocA.Ptr,
85 AO ? LocationSize::unknown() : LocA.Size,
86 AO ? AAMDNodes() : LocA.AATags),
87 MemoryLocation(BO ? BO : LocB.Ptr,
88 BO ? LocationSize::unknown() : LocB.Size,
89 BO ? AAMDNodes() : LocB.AATags),
93 // Forward the query to the next analysis.
94 return AAResultBase::alias(LocA, LocB, AAQI);
97 /// Given an expression, try to find a base value.
99 /// Returns null if none was found.
100 Value *SCEVAAResult::GetBaseValue(const SCEV *S) {
101 if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
102 // In an addrec, assume that the base will be in the start, rather
104 return GetBaseValue(AR->getStart());
105 } else if (const SCEVAddExpr *A = dyn_cast<SCEVAddExpr>(S)) {
106 // If there's a pointer operand, it'll be sorted at the end of the list.
107 const SCEV *Last = A->getOperand(A->getNumOperands() - 1);
108 if (Last->getType()->isPointerTy())
109 return GetBaseValue(Last);
110 } else if (const SCEVUnknown *U = dyn_cast<SCEVUnknown>(S)) {
111 // This is a leaf node.
112 return U->getValue();
114 // No Identified object found.
118 AnalysisKey SCEVAA::Key;
120 SCEVAAResult SCEVAA::run(Function &F, FunctionAnalysisManager &AM) {
121 return SCEVAAResult(AM.getResult<ScalarEvolutionAnalysis>(F));
124 char SCEVAAWrapperPass::ID = 0;
125 INITIALIZE_PASS_BEGIN(SCEVAAWrapperPass, "scev-aa",
126 "ScalarEvolution-based Alias Analysis", false, true)
127 INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
128 INITIALIZE_PASS_END(SCEVAAWrapperPass, "scev-aa",
129 "ScalarEvolution-based Alias Analysis", false, true)
131 FunctionPass *llvm::createSCEVAAWrapperPass() {
132 return new SCEVAAWrapperPass();
135 SCEVAAWrapperPass::SCEVAAWrapperPass() : FunctionPass(ID) {
136 initializeSCEVAAWrapperPassPass(*PassRegistry::getPassRegistry());
139 bool SCEVAAWrapperPass::runOnFunction(Function &F) {
141 new SCEVAAResult(getAnalysis<ScalarEvolutionWrapperPass>().getSE()));
145 void SCEVAAWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const {
146 AU.setPreservesAll();
147 AU.addRequired<ScalarEvolutionWrapperPass>();