Cache dependence computation using FoldingSet.

This introduces an LDA-internal DependencePair class. The intention is,
that this is a place where dependence testers can store various results
such as SCEVs describing conflicting iterations, breaking conditions,
distance/direction vectors, etc.

llvm-svn: 76877

llvm/include/llvm/Analysis/LoopDependenceAnalysis.h

@@ -20,7 +20,9 @@
 #ifndef LLVM_ANALYSIS_LOOP_DEPENDENCE_ANALYSIS_H
 #define LLVM_ANALYSIS_LOOP_DEPENDENCE_ANALYSIS_H
 
+#include "llvm/ADT/FoldingSet.h"
 #include "llvm/Analysis/LoopPass.h"
+#include "llvm/Support/Allocator.h"
 #include "llvm/Support/raw_ostream.h"
 #include <iosfwd>
 
@@ -32,24 +34,58 @@ class ScalarEvolution;
 class Value;
 
 class LoopDependenceAnalysis : public LoopPass {
-  Loop *L;
   AliasAnalysis *AA;
   ScalarEvolution *SE;
 
+  /// L - The loop we are currently analysing.
+  Loop *L;
+
+  /// TODO: doc
+  enum DependenceResult { Independent = 0, Dependent = 1, Unknown = 2 };
+
+  /// DependencePair - Represents a data dependence relation between to memory
+  /// reference instructions.
+  ///
+  /// TODO: add subscripts vector
+  struct DependencePair : public FastFoldingSetNode {
+    Value *A;
+    Value *B;
+    DependenceResult Result;
+
+    DependencePair(const FoldingSetNodeID &ID, Value *a, Value *b) :
+        FastFoldingSetNode(ID), A(a), B(b), Result(Unknown) {}
+  };
+
+  /// findOrInsertDependencePair - Return true if a DependencePair for the
+  /// given Values already exists, false if a new DependencePair had to be
+  /// created. The third argument is set to the pair found or created.
+  bool findOrInsertDependencePair(Value*, Value*, DependencePair*&);
+
+  /// TODO: doc
+  void analysePair(DependencePair *P) const;
+
 public:
   static char ID; // Class identification, replacement for typeinfo
   LoopDependenceAnalysis() : LoopPass(&ID) {}
 
-  /// TODO: docs
+  /// isDependencePair - Check wether two values can possibly give rise to a
+  /// data dependence: that is the case if both are instructions accessing
+  /// memory and at least one of those accesses is a write.
   bool isDependencePair(const Value*, const Value*) const;
+
+  /// depends - Return a boolean indicating if there is a data dependence
+  /// between two instructions.
   bool depends(Value*, Value*);
 
   bool runOnLoop(Loop*, LPPassManager&);
-
+  virtual void releaseMemory();
   virtual void getAnalysisUsage(AnalysisUsage&) const;
-
   void print(raw_ostream&, const Module* = 0) const;
   virtual void print(std::ostream&, const Module* = 0) const;
+
+private:
+  FoldingSet<DependencePair> Pairs;
+  BumpPtrAllocator PairAllocator;
 }; // class LoopDependenceAnalysis
 
 

llvm/lib/Analysis/LoopDependenceAnalysis.cpp

@@ -81,36 +81,73 @@ bool LoopDependenceAnalysis::isDependencePair(const Value *A,
           cast<const Instruction>(B)->mayWriteToMemory());
 }
 
-bool LoopDependenceAnalysis::depends(Value *Src, Value *Dst) {
-  assert(isDependencePair(Src, Dst) && "Values form no dependence pair!");
-  DOUT << "== LDA test ==\n" << *Src << *Dst;
+bool LoopDependenceAnalysis::findOrInsertDependencePair(Value *X,
+                                                        Value *Y,
+                                                        DependencePair *&P) {
+  void *insertPos = 0;
+  FoldingSetNodeID id;
+  id.AddPointer(X);
+  id.AddPointer(Y);
 
-  // We only analyse loads and stores; for possible memory accesses by e.g.
-  // free, call, or invoke instructions we conservatively assume dependence.
-  if (!IsLoadOrStoreInst(Src) || !IsLoadOrStoreInst(Dst))
-    return true;
+  P = Pairs.FindNodeOrInsertPos(id, insertPos);
+  if (P) return true;
 
-  Value *srcPtr = GetPointerOperand(Src);
-  Value *dstPtr = GetPointerOperand(Dst);
-  const Value *srcObj = srcPtr->getUnderlyingObject();
-  const Value *dstObj = dstPtr->getUnderlyingObject();
+  P = PairAllocator.Allocate<DependencePair>();
+  new (P) DependencePair(id, X, Y);
+  Pairs.InsertNode(P, insertPos);
+  return false;
+}
+
+void LoopDependenceAnalysis::analysePair(DependencePair *P) const {
+  DOUT << "Analysing:\n" << *P->A << "\n" << *P->B << "\n";
+
+  // Our default answer: we don't know anything, i.e. we failed to analyse this
+  // pair to get a more specific answer (dependent, independent).
+  P->Result = Unknown;
+
+  // We only analyse loads and stores but no possible memory accesses by e.g.
+  // free, call, or invoke instructions.
+  if (!IsLoadOrStoreInst(P->A) || !IsLoadOrStoreInst(P->B)) {
+    DOUT << "--> [?] no load/store\n";
+    return;
+  }
+
+  Value *aptr = GetPointerOperand(P->A);
+  Value *bptr = GetPointerOperand(P->B);
+  const Value *aobj = aptr->getUnderlyingObject();
+  const Value *bobj = bptr->getUnderlyingObject();
   AliasAnalysis::AliasResult alias = AA->alias(
-      srcObj, AA->getTargetData().getTypeStoreSize(srcObj->getType()),
-      dstObj, AA->getTargetData().getTypeStoreSize(dstObj->getType()));
+      aobj, AA->getTargetData().getTypeStoreSize(aobj->getType()),
+      bobj, AA->getTargetData().getTypeStoreSize(bobj->getType()));
 
-  // If we don't know whether or not the two objects alias, assume dependence.
-  if (alias == AliasAnalysis::MayAlias)
-    return true;
+  // We can not analyse objects if we do not know about their aliasing.
+  if (alias == AliasAnalysis::MayAlias) {
+    DOUT << "---> [?] may alias\n";
+    return;
+  }
 
   // If the objects noalias, they are distinct, accesses are independent.
-  if (alias == AliasAnalysis::NoAlias)
-    return false;
+  if (alias == AliasAnalysis::NoAlias) {
+    DOUT << "---> [I] no alias\n";
+    P->Result = Independent;
+    return;
+  }
 
   // TODO: the underlying objects MustAlias, test for dependence
 
-  // We couldn't establish a more precise result, so we have to conservatively
-  // assume full dependence.
-  return true;
+  DOUT << "---> [?] cannot analyse\n";
+  return;
+}
+
+bool LoopDependenceAnalysis::depends(Value *A, Value *B) {
+  assert(isDependencePair(A, B) && "Values form no dependence pair!");
+
+  DependencePair *p;
+  if (!findOrInsertDependencePair(A, B, p)) {
+    // The pair is not cached, so analyse it.
+    analysePair(p);
+  }
+  return p->Result != Independent;
 }
 
 //===----------------------------------------------------------------------===//
@@ -124,6 +161,11 @@ bool LoopDependenceAnalysis::runOnLoop(Loop *L, LPPassManager &) {
   return false;
 }
 
+void LoopDependenceAnalysis::releaseMemory() {
+  Pairs.clear();
+  PairAllocator.Reset();
+}
+
 void LoopDependenceAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
   AU.setPreservesAll();
   AU.addRequiredTransitive<AliasAnalysis>();