Check in LLVM r95781.

1 files changed, 139 insertions, 0 deletions

diff --git a/lib/Analysis/ScalarEvolutionAliasAnalysis.cpp b/lib/Analysis/ScalarEvolutionAliasAnalysis.cpp
new file mode 100644
index 0000000..498c4a8
--- /dev/null
+++ b/lib/Analysis/ScalarEvolutionAliasAnalysis.cpp
@@ -0,0 +1,139 @@
+//===- 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"
+using namespace llvm;
+
+namespace {
+  /// ScalarEvolutionAliasAnalysis - This is a simple alias analysis
+  /// implementation that uses ScalarEvolution to answer queries.
+  class ScalarEvolutionAliasAnalysis : public FunctionPass,
+                                       public AliasAnalysis {
+    ScalarEvolution *SE;
+
+  public:
+    static char ID; // Class identification, replacement for typeinfo
+    ScalarEvolutionAliasAnalysis() : FunctionPass(&ID), SE(0) {}
+
+    /// getAdjustedAnalysisPointer - This method is used when a pass implements
+    /// an analysis interface through multiple inheritance.  If needed, it
+    /// should override this to adjust the this pointer as needed for the
+    /// specified pass info.
+    virtual void *getAdjustedAnalysisPointer(const PassInfo *PI) {
+      if (PI->isPassID(&AliasAnalysis::ID))
+        return (AliasAnalysis*)this;
+      return this;
+    }
+                                         
+  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 *GetBaseValue(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;
+}
+
+/// GetBaseValue - Given an expression, try to find a
+/// base value. Return null is none was found.
+Value *
+ScalarEvolutionAliasAnalysis::GetBaseValue(const SCEV *S) {
+  if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
+    // In an addrec, assume that the base will be in the start, rather
+    // than the step.
+    return GetBaseValue(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 GetBaseValue(Last);
+  } else if (const SCEVUnknown *U = dyn_cast<SCEVUnknown>(S)) {
+    // This is a leaf node.
+    return U->getValue();
+  }
+  // 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 = GetBaseValue(AS);
+  Value *BO = GetBaseValue(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);
+}