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1 //===- ScalarEvolutionAliasAnalysis.cpp - SCEV-based Alias Analysis -------===//
2 //
3 //                     The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
10 // This file defines the ScalarEvolutionAliasAnalysis pass, which implements a
11 // simple alias analysis implemented in terms of ScalarEvolution queries.
12 //
13 // This differs from traditional loop dependence analysis in that it tests
14 // for dependencies within a single iteration of a loop, rather than
15 // dependencies between different iterations.
16 //
17 // ScalarEvolution has a more complete understanding of pointer arithmetic
18 // than BasicAliasAnalysis' collection of ad-hoc analyses.
19 //
20 //===----------------------------------------------------------------------===//
21 
22 #include "llvm/Analysis/ScalarEvolutionAliasAnalysis.h"
23 #include "llvm/Analysis/TargetLibraryInfo.h"
24 using namespace llvm;
25 
alias(const MemoryLocation & LocA,const MemoryLocation & LocB)26 AliasResult SCEVAAResult::alias(const MemoryLocation &LocA,
27                                 const MemoryLocation &LocB) {
28   // If either of the memory references is empty, it doesn't matter what the
29   // pointer values are. This allows the code below to ignore this special
30   // case.
31   if (LocA.Size == 0 || LocB.Size == 0)
32     return NoAlias;
33 
34   // This is SCEVAAResult. Get the SCEVs!
35   const SCEV *AS = SE.getSCEV(const_cast<Value *>(LocA.Ptr));
36   const SCEV *BS = SE.getSCEV(const_cast<Value *>(LocB.Ptr));
37 
38   // If they evaluate to the same expression, it's a MustAlias.
39   if (AS == BS)
40     return MustAlias;
41 
42   // If something is known about the difference between the two addresses,
43   // see if it's enough to prove a NoAlias.
44   if (SE.getEffectiveSCEVType(AS->getType()) ==
45       SE.getEffectiveSCEVType(BS->getType())) {
46     unsigned BitWidth = SE.getTypeSizeInBits(AS->getType());
47     APInt ASizeInt(BitWidth, LocA.Size);
48     APInt BSizeInt(BitWidth, LocB.Size);
49 
50     // Compute the difference between the two pointers.
51     const SCEV *BA = SE.getMinusSCEV(BS, AS);
52 
53     // Test whether the difference is known to be great enough that memory of
54     // the given sizes don't overlap. This assumes that ASizeInt and BSizeInt
55     // are non-zero, which is special-cased above.
56     if (ASizeInt.ule(SE.getUnsignedRange(BA).getUnsignedMin()) &&
57         (-BSizeInt).uge(SE.getUnsignedRange(BA).getUnsignedMax()))
58       return NoAlias;
59 
60     // Folding the subtraction while preserving range information can be tricky
61     // (because of INT_MIN, etc.); if the prior test failed, swap AS and BS
62     // and try again to see if things fold better that way.
63 
64     // Compute the difference between the two pointers.
65     const SCEV *AB = SE.getMinusSCEV(AS, BS);
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 (BSizeInt.ule(SE.getUnsignedRange(AB).getUnsignedMin()) &&
71         (-ASizeInt).uge(SE.getUnsignedRange(AB).getUnsignedMax()))
72       return NoAlias;
73   }
74 
75   // If ScalarEvolution can find an underlying object, form a new query.
76   // The correctness of this depends on ScalarEvolution not recognizing
77   // inttoptr and ptrtoint operators.
78   Value *AO = GetBaseValue(AS);
79   Value *BO = GetBaseValue(BS);
80   if ((AO && AO != LocA.Ptr) || (BO && BO != LocB.Ptr))
81     if (alias(MemoryLocation(AO ? AO : LocA.Ptr,
82                              AO ? +MemoryLocation::UnknownSize : LocA.Size,
83                              AO ? AAMDNodes() : LocA.AATags),
84               MemoryLocation(BO ? BO : LocB.Ptr,
85                              BO ? +MemoryLocation::UnknownSize : LocB.Size,
86                              BO ? AAMDNodes() : LocB.AATags)) == NoAlias)
87       return NoAlias;
88 
89   // Forward the query to the next analysis.
90   return AAResultBase::alias(LocA, LocB);
91 }
92 
93 /// Given an expression, try to find a base value.
94 ///
95 /// Returns null if none was found.
GetBaseValue(const SCEV * S)96 Value *SCEVAAResult::GetBaseValue(const SCEV *S) {
97   if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
98     // In an addrec, assume that the base will be in the start, rather
99     // than the step.
100     return GetBaseValue(AR->getStart());
101   } else if (const SCEVAddExpr *A = dyn_cast<SCEVAddExpr>(S)) {
102     // If there's a pointer operand, it'll be sorted at the end of the list.
103     const SCEV *Last = A->getOperand(A->getNumOperands() - 1);
104     if (Last->getType()->isPointerTy())
105       return GetBaseValue(Last);
106   } else if (const SCEVUnknown *U = dyn_cast<SCEVUnknown>(S)) {
107     // This is a leaf node.
108     return U->getValue();
109   }
110   // No Identified object found.
111   return nullptr;
112 }
113 
run(Function & F,AnalysisManager<Function> * AM)114 SCEVAAResult SCEVAA::run(Function &F, AnalysisManager<Function> *AM) {
115   return SCEVAAResult(AM->getResult<TargetLibraryAnalysis>(F),
116                       AM->getResult<ScalarEvolutionAnalysis>(F));
117 }
118 
119 char SCEVAA::PassID;
120 
121 char SCEVAAWrapperPass::ID = 0;
122 INITIALIZE_PASS_BEGIN(SCEVAAWrapperPass, "scev-aa",
123                       "ScalarEvolution-based Alias Analysis", false, true)
INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)124 INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
125 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
126 INITIALIZE_PASS_END(SCEVAAWrapperPass, "scev-aa",
127                     "ScalarEvolution-based Alias Analysis", false, true)
128 
129 FunctionPass *llvm::createSCEVAAWrapperPass() {
130   return new SCEVAAWrapperPass();
131 }
132 
SCEVAAWrapperPass()133 SCEVAAWrapperPass::SCEVAAWrapperPass() : FunctionPass(ID) {
134   initializeSCEVAAWrapperPassPass(*PassRegistry::getPassRegistry());
135 }
136 
runOnFunction(Function & F)137 bool SCEVAAWrapperPass::runOnFunction(Function &F) {
138   Result.reset(
139       new SCEVAAResult(getAnalysis<TargetLibraryInfoWrapperPass>().getTLI(),
140                        getAnalysis<ScalarEvolutionWrapperPass>().getSE()));
141   return false;
142 }
143 
getAnalysisUsage(AnalysisUsage & AU) const144 void SCEVAAWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const {
145   AU.setPreservesAll();
146   AU.addRequired<ScalarEvolutionWrapperPass>();
147   AU.addRequired<TargetLibraryInfoWrapperPass>();
148 }
149