1 //===- ScalarEvolutionAliasAnalysis.cpp - SCEV-based Alias Analysis -------===//
2 //
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
6 //
7 //===----------------------------------------------------------------------===//
8 //
9 // This file defines the ScalarEvolutionAliasAnalysis pass, which implements a
10 // simple alias analysis implemented in terms of ScalarEvolution queries.
11 //
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.
15 //
16 // ScalarEvolution has a more complete understanding of pointer arithmetic
17 // than BasicAliasAnalysis' collection of ad-hoc analyses.
18 //
19 //===----------------------------------------------------------------------===//
20 
21 #include "llvm/Analysis/ScalarEvolutionAliasAnalysis.h"
22 #include "llvm/Analysis/ScalarEvolution.h"
23 #include "llvm/Analysis/ScalarEvolutionExpressions.h"
24 #include "llvm/InitializePasses.h"
25 using namespace llvm;
26 
27 static bool canComputePointerDiff(ScalarEvolution &SE,
28                                   const SCEV *A, const SCEV *B) {
29   if (SE.getEffectiveSCEVType(A->getType()) !=
30       SE.getEffectiveSCEVType(B->getType()))
31     return false;
32 
33   return SE.instructionCouldExistWitthOperands(A, B);
34 }
35 
36 AliasResult SCEVAAResult::alias(const MemoryLocation &LocA,
37                                 const MemoryLocation &LocB, AAQueryInfo &AAQI,
38                                 const Instruction *) {
39   // If either of the memory references is empty, it doesn't matter what the
40   // pointer values are. This allows the code below to ignore this special
41   // case.
42   if (LocA.Size.isZero() || LocB.Size.isZero())
43     return AliasResult::NoAlias;
44 
45   // This is SCEVAAResult. Get the SCEVs!
46   const SCEV *AS = SE.getSCEV(const_cast<Value *>(LocA.Ptr));
47   const SCEV *BS = SE.getSCEV(const_cast<Value *>(LocB.Ptr));
48 
49   // If they evaluate to the same expression, it's a MustAlias.
50   if (AS == BS)
51     return AliasResult::MustAlias;
52 
53   // If something is known about the difference between the two addresses,
54   // see if it's enough to prove a NoAlias.
55   if (canComputePointerDiff(SE, AS, BS)) {
56     unsigned BitWidth = SE.getTypeSizeInBits(AS->getType());
57     APInt ASizeInt(BitWidth, LocA.Size.hasValue()
58                                  ? LocA.Size.getValue()
59                                  : MemoryLocation::UnknownSize);
60     APInt BSizeInt(BitWidth, LocB.Size.hasValue()
61                                  ? LocB.Size.getValue()
62                                  : MemoryLocation::UnknownSize);
63 
64     // Compute the difference between the two pointers.
65     const SCEV *BA = SE.getMinusSCEV(BS, AS);
66 
67     // Test whether the difference is known to be great enough that memory of
68     // the given sizes don't overlap. This assumes that ASizeInt and BSizeInt
69     // are non-zero, which is special-cased above.
70     if (!isa<SCEVCouldNotCompute>(BA) &&
71         ASizeInt.ule(SE.getUnsignedRange(BA).getUnsignedMin()) &&
72         (-BSizeInt).uge(SE.getUnsignedRange(BA).getUnsignedMax()))
73       return AliasResult::NoAlias;
74 
75     // Folding the subtraction while preserving range information can be tricky
76     // (because of INT_MIN, etc.); if the prior test failed, swap AS and BS
77     // and try again to see if things fold better that way.
78 
79     // Compute the difference between the two pointers.
80     const SCEV *AB = SE.getMinusSCEV(AS, BS);
81 
82     // Test whether the difference is known to be great enough that memory of
83     // the given sizes don't overlap. This assumes that ASizeInt and BSizeInt
84     // are non-zero, which is special-cased above.
85     if (!isa<SCEVCouldNotCompute>(AB) &&
86         BSizeInt.ule(SE.getUnsignedRange(AB).getUnsignedMin()) &&
87         (-ASizeInt).uge(SE.getUnsignedRange(AB).getUnsignedMax()))
88       return AliasResult::NoAlias;
89   }
90 
91   // If ScalarEvolution can find an underlying object, form a new query.
92   // The correctness of this depends on ScalarEvolution not recognizing
93   // inttoptr and ptrtoint operators.
94   Value *AO = GetBaseValue(AS);
95   Value *BO = GetBaseValue(BS);
96   if ((AO && AO != LocA.Ptr) || (BO && BO != LocB.Ptr))
97     if (alias(MemoryLocation(AO ? AO : LocA.Ptr,
98                              AO ? LocationSize::beforeOrAfterPointer()
99                                 : LocA.Size,
100                              AO ? AAMDNodes() : LocA.AATags),
101               MemoryLocation(BO ? BO : LocB.Ptr,
102                              BO ? LocationSize::beforeOrAfterPointer()
103                                 : LocB.Size,
104                              BO ? AAMDNodes() : LocB.AATags),
105               AAQI, nullptr) == AliasResult::NoAlias)
106       return AliasResult::NoAlias;
107 
108   // Forward the query to the next analysis.
109   return AAResultBase::alias(LocA, LocB, AAQI, nullptr);
110 }
111 
112 /// Given an expression, try to find a base value.
113 ///
114 /// Returns null if none was found.
115 Value *SCEVAAResult::GetBaseValue(const SCEV *S) {
116   if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
117     // In an addrec, assume that the base will be in the start, rather
118     // than the step.
119     return GetBaseValue(AR->getStart());
120   } else if (const SCEVAddExpr *A = dyn_cast<SCEVAddExpr>(S)) {
121     // If there's a pointer operand, it'll be sorted at the end of the list.
122     const SCEV *Last = A->getOperand(A->getNumOperands() - 1);
123     if (Last->getType()->isPointerTy())
124       return GetBaseValue(Last);
125   } else if (const SCEVUnknown *U = dyn_cast<SCEVUnknown>(S)) {
126     // This is a leaf node.
127     return U->getValue();
128   }
129   // No Identified object found.
130   return nullptr;
131 }
132 
133 bool SCEVAAResult::invalidate(Function &Fn, const PreservedAnalyses &PA,
134                               FunctionAnalysisManager::Invalidator &Inv) {
135   // We don't care if this analysis itself is preserved, it has no state. But
136   // we need to check that the analyses it depends on have been.
137   return Inv.invalidate<ScalarEvolutionAnalysis>(Fn, PA);
138 }
139 
140 AnalysisKey SCEVAA::Key;
141 
142 SCEVAAResult SCEVAA::run(Function &F, FunctionAnalysisManager &AM) {
143   return SCEVAAResult(AM.getResult<ScalarEvolutionAnalysis>(F));
144 }
145 
146 char SCEVAAWrapperPass::ID = 0;
147 INITIALIZE_PASS_BEGIN(SCEVAAWrapperPass, "scev-aa",
148                       "ScalarEvolution-based Alias Analysis", false, true)
149 INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
150 INITIALIZE_PASS_END(SCEVAAWrapperPass, "scev-aa",
151                     "ScalarEvolution-based Alias Analysis", false, true)
152 
153 FunctionPass *llvm::createSCEVAAWrapperPass() {
154   return new SCEVAAWrapperPass();
155 }
156 
157 SCEVAAWrapperPass::SCEVAAWrapperPass() : FunctionPass(ID) {
158   initializeSCEVAAWrapperPassPass(*PassRegistry::getPassRegistry());
159 }
160 
161 bool SCEVAAWrapperPass::runOnFunction(Function &F) {
162   Result.reset(
163       new SCEVAAResult(getAnalysis<ScalarEvolutionWrapperPass>().getSE()));
164   return false;
165 }
166 
167 void SCEVAAWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const {
168   AU.setPreservesAll();
169   AU.addRequired<ScalarEvolutionWrapperPass>();
170 }
171