Home | History | Annotate | Download | only in Analysis 
      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 
     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.
     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 
    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)
    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 
    133 SCEVAAWrapperPass::SCEVAAWrapperPass() : FunctionPass(ID) {
    134   initializeSCEVAAWrapperPassPass(*PassRegistry::getPassRegistry());
    135 }
    136 
    137 bool SCEVAAWrapperPass::runOnFunction(Function &F) {
    138   Result.reset(
    139       new SCEVAAResult(getAnalysis<TargetLibraryInfoWrapperPass>().getTLI(),
    140                        getAnalysis<ScalarEvolutionWrapperPass>().getSE()));
    141   return false;
    142 }
    143 
    144 void SCEVAAWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const {
    145   AU.setPreservesAll();
    146   AU.addRequired<ScalarEvolutionWrapperPass>();
    147   AU.addRequired<TargetLibraryInfoWrapperPass>();
    148 }
    149