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