Create a ScalarEvolution-based AliasAnalysis implementation.

This is a simple AliasAnalysis implementation which works by making
ScalarEvolution queries. ScalarEvolution has a more complete understanding
of arithmetic than BasicAA's collection of ad-hoc checks, so it handles
some cases that BasicAA misses, for example p[i] and p[i+1] within the
same iteration of a loop.

This is currently experimental. It may be that the main use for this pass
will be to help find cases where BasicAA can be profitably extended, or
to help in the development of the overall AliasAnalysis infrastructure,
however it's also possible that it could grow up to become a directly
useful pass.

llvm-svn: 80098

1 files changed, 128 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/ScalarEvolutionAliasAnalysis.cpp b/llvm/lib/Analysis/ScalarEvolutionAliasAnalysis.cpp
new file mode 100644
index 00000000000..381ea204864
--- /dev/null
+++ b/llvm/lib/Analysis/ScalarEvolutionAliasAnalysis.cpp
@@ -0,0 +1,128 @@
+//===- ScalarEvolutionAliasAnalysis.cpp - SCEV-based Alias Analysis -------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file defines the ScalarEvolutionAliasAnalysis pass, which implements a
+// simple alias analysis implemented in terms of ScalarEvolution queries.
+//
+// ScalarEvolution has a more complete understanding of pointer arithmetic
+// than BasicAliasAnalysis' collection of ad-hoc analyses.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Analysis/AliasAnalysis.h"
+#include "llvm/Analysis/ScalarEvolutionExpressions.h"
+#include "llvm/Analysis/Passes.h"
+#include "llvm/Pass.h"
+#include "llvm/Support/Compiler.h"
+using namespace llvm;
+
+namespace {
+  /// ScalarEvolutionAliasAnalysis - This is a simple alias analysis
+  /// implementation that uses ScalarEvolution to answer queries.
+  class VISIBILITY_HIDDEN ScalarEvolutionAliasAnalysis : public FunctionPass,
+                                                         public AliasAnalysis {
+    ScalarEvolution *SE;
+
+  public:
+    static char ID; // Class identification, replacement for typeinfo
+    ScalarEvolutionAliasAnalysis() : FunctionPass(&ID), SE(0) {}
+
+  private:
+    virtual void getAnalysisUsage(AnalysisUsage &AU) const;
+    virtual bool runOnFunction(Function &F);
+    virtual AliasResult alias(const Value *V1, unsigned V1Size,
+                              const Value *V2, unsigned V2Size);
+
+    Value *GetUnderlyingIdentifiedObject(const SCEV *S);
+  };
+}  // End of anonymous namespace
+
+// Register this pass...
+char ScalarEvolutionAliasAnalysis::ID = 0;
+static RegisterPass<ScalarEvolutionAliasAnalysis>
+X("scev-aa", "ScalarEvolution-based Alias Analysis", false, true);
+
+// Declare that we implement the AliasAnalysis interface
+static RegisterAnalysisGroup<AliasAnalysis> Y(X);
+
+FunctionPass *llvm::createScalarEvolutionAliasAnalysisPass() {
+  return new ScalarEvolutionAliasAnalysis();
+}
+
+void
+ScalarEvolutionAliasAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
+  AU.addRequiredTransitive<ScalarEvolution>();
+  AU.setPreservesAll();
+  AliasAnalysis::getAnalysisUsage(AU);
+}
+
+bool
+ScalarEvolutionAliasAnalysis::runOnFunction(Function &F) {
+  InitializeAliasAnalysis(this);
+  SE = &getAnalysis<ScalarEvolution>();
+  return false;
+}
+
+Value *
+ScalarEvolutionAliasAnalysis::GetUnderlyingIdentifiedObject(const SCEV *S) {
+  if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
+    return GetUnderlyingIdentifiedObject(AR->getStart());
+  } else if (const SCEVAddExpr *A = dyn_cast<SCEVAddExpr>(S)) {
+    // If there's a pointer operand, it'll be sorted at the end of the list.
+    const SCEV *Last = A->getOperand(A->getNumOperands()-1);
+    if (isa<PointerType>(Last->getType()))
+      return GetUnderlyingIdentifiedObject(Last);
+  } else if (const SCEVUnknown *U = dyn_cast<SCEVUnknown>(S)) {
+    // Determine if we've found an Identified object.
+    Value *V = U->getValue();
+    if (isIdentifiedObject(V))
+      return V;
+  }
+  // No Identified object found.
+  return 0;
+}
+
+AliasAnalysis::AliasResult
+ScalarEvolutionAliasAnalysis::alias(const Value *A, unsigned ASize,
+                                    const Value *B, unsigned BSize) {
+  // This is ScalarEvolutionAliasAnalysis. Get the SCEVs!
+  const SCEV *AS = SE->getSCEV(const_cast<Value *>(A));
+  const SCEV *BS = SE->getSCEV(const_cast<Value *>(B));
+
+  // If they evaluate to the same expression, it's a MustAlias.
+  if (AS == BS) return MustAlias;
+
+  // If something is known about the difference between the two addresses,
+  // see if it's enough to prove a NoAlias.
+  if (SE->getEffectiveSCEVType(AS->getType()) ==
+      SE->getEffectiveSCEVType(BS->getType())) {
+    unsigned BitWidth = SE->getTypeSizeInBits(AS->getType());
+    APInt AI(BitWidth, ASize);
+    const SCEV *BA = SE->getMinusSCEV(BS, AS);
+    if (AI.ule(SE->getUnsignedRange(BA).getUnsignedMin())) {
+      APInt BI(BitWidth, BSize);
+      const SCEV *AB = SE->getMinusSCEV(AS, BS);
+      if (BI.ule(SE->getUnsignedRange(AB).getUnsignedMin()))
+        return NoAlias;
+    }
+  }
+
+  // If ScalarEvolution can find an underlying object, form a new query.
+  // The correctness of this depends on ScalarEvolution not recognizing
+  // inttoptr and ptrtoint operators.
+  Value *AO = GetUnderlyingIdentifiedObject(AS);
+  Value *BO = GetUnderlyingIdentifiedObject(BS);
+  if ((AO && AO != A) || (BO && BO != B))
+    if (alias(AO ? AO : A, AO ? ~0u : ASize,
+              BO ? BO : B, BO ? ~0u : BSize) == NoAlias)
+      return NoAlias;
+
+  // Forward the query to the next analysis.
+  return AliasAnalysis::alias(A, ASize, B, BSize);
+}