Somehow I was getting reasonable results for the test cases I was interested

in despite not ever incrementing any path costs, so that the only nonzero costs
arose from the all-left path in the first column.  Anyway.  Perform the diff
starting from the beginning of the block to avoid capturing (say) loads of
allocas.

Vastly improves diff results on code that hasn't been mem2reg'ed.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@109741 91177308-0d34-0410-b5e6-96231b3b80d8

tools/llvm-diff/DifferenceEngine.cpp

@@ -124,7 +124,7 @@ class FunctionDifferenceEngine {
   /// The current mapping from old blocks to new blocks.
   DenseMap<BasicBlock*, BasicBlock*> Blocks;
 
-  DenseSet<std::pair<Value*, Value*> > TentativeValuePairs;
+  DenseSet<std::pair<Value*, Value*> > TentativeValues;
 
   unsigned getUnprocPredCount(BasicBlock *Block) const {
     unsigned Count = 0;
@@ -186,22 +186,32 @@ class FunctionDifferenceEngine {
     BasicBlock::iterator LI = L->begin(), LE = L->end();
     BasicBlock::iterator RI = R->begin(), RE = R->end();
 
+    llvm::SmallVector<std::pair<Instruction*,Instruction*>, 20> TentativePairs;
+
     do {
       assert(LI != LE && RI != RE);
       Instruction *LeftI = &*LI, *RightI = &*RI;
 
       // If the instructions differ, start the more sophisticated diff
-      // algorithm here.
+      // algorithm at the start of the block.
-      if (diff(LeftI, RightI, false, true))
+      if (diff(LeftI, RightI, false, true)) {
-        return runBlockDiff(LI, RI);
+        TentativeValues.clear();
+        return runBlockDiff(L->begin(), R->begin());
+      }
 
-      // Otherwise, unify them.
+      // Otherwise, tentatively unify them.
       if (!LeftI->use_empty())
-        Values[LeftI] = RightI;
+        TentativeValues.insert(std::make_pair(LeftI, RightI));
 
       ++LI, ++RI;
     } while (LI != LE); // This is sufficient: we can't get equality of
                         // terminators if there are residual instructions.
+
+    // Make all the tentative pairs solid.
+    for (llvm::DenseSet<std::pair<Value*,Value*> >::iterator
+           I = TentativeValues.begin(), E = TentativeValues.end(); I != E; ++I)
+      Values[I->first] = I->second;
+    TentativeValues.clear();
   }
 
   bool matchForBlockDiff(Instruction *L, Instruction *R);
@@ -417,7 +427,7 @@ class FunctionDifferenceEngine {
       return equivalentAsOperands(cast<Constant>(L), cast<Constant>(R));
 
     if (isa<Instruction>(L))
-      return Values[L] == R || TentativeValuePairs.count(std::make_pair(L, R));
+      return Values[L] == R || TentativeValues.count(std::make_pair(L, R));
 
     if (isa<Argument>(L))
       return Values[L] == R;
@@ -477,11 +487,15 @@ void FunctionDifferenceEngine::runBlockDiff(BasicBlock::iterator LStart,
   DiffEntry *Cur = Paths1.data();
   DiffEntry *Next = Paths2.data();
 
-  assert(TentativeValuePairs.empty());
+  const unsigned LeftCost = 2;
+  const unsigned RightCost = 2;
+  const unsigned MatchCost = 0;
+
+  assert(TentativeValues.empty());
 
   // Initialize the first column.
   for (unsigned I = 0; I != NL+1; ++I) {
-    Cur[I].Cost = I;
+    Cur[I].Cost = I * LeftCost;
     for (unsigned J = 0; J != I; ++J)
       Cur[I].Path.push_back(DifferenceEngine::DC_left);
   }
@@ -489,19 +503,23 @@ void FunctionDifferenceEngine::runBlockDiff(BasicBlock::iterator LStart,
   for (BasicBlock::iterator RI = RStart; RI != RE; ++RI) {
     // Initialize the first row.
     Next[0] = Cur[0];
+    Next[0].Cost += RightCost;
     Next[0].Path.push_back(DifferenceEngine::DC_right);
 
     unsigned Index = 1;
     for (BasicBlock::iterator LI = LStart; LI != LE; ++LI, ++Index) {
       if (matchForBlockDiff(&*LI, &*RI)) {
         Next[Index] = Cur[Index-1];
+        Next[Index].Cost += MatchCost;
         Next[Index].Path.push_back(DifferenceEngine::DC_match);
-        TentativeValuePairs.insert(std::make_pair(&*LI, &*RI));
+        TentativeValues.insert(std::make_pair(&*LI, &*RI));
       } else if (Next[Index-1].Cost <= Cur[Index].Cost) {
         Next[Index] = Next[Index-1];
+        Next[Index].Cost += LeftCost;
         Next[Index].Path.push_back(DifferenceEngine::DC_left);
       } else {
         Next[Index] = Cur[Index];
+        Next[Index].Cost += RightCost;
         Next[Index].Path.push_back(DifferenceEngine::DC_right);
       }
     }
@@ -518,15 +536,21 @@ void FunctionDifferenceEngine::runBlockDiff(BasicBlock::iterator LStart,
   while (Path.back() == DifferenceEngine::DC_match)
     Path.pop_back();
 
-  for (SmallVectorImpl<char>::iterator
+  // Skip leading matches.
-         PI = Path.begin(), PE = Path.end(); PI != PE; ++PI) {
+  SmallVectorImpl<char>::iterator
+    PI = Path.begin(), PE = Path.end();
+  while (PI != PE && *PI == DifferenceEngine::DC_match)
+    ++PI, ++LI, ++RI;
+
+  for (; PI != PE; ++PI) {
     switch (static_cast<DifferenceEngine::DiffChange>(*PI)) {
     case DifferenceEngine::DC_match:
       assert(LI != LE && RI != RE);
       {
         Instruction *L = &*LI, *R = &*RI;
         DifferenceEngine::Context C(Engine, L, R);
-        diff(L, R, false, true);
+        diff(L, R, false, true); // unify successors
+        Values[L] = R; // make non-tentative
         Diff.addMatch(L, R);
       }
       ++LI; ++RI;
@@ -546,7 +570,7 @@ void FunctionDifferenceEngine::runBlockDiff(BasicBlock::iterator LStart,
     }
   }
 
-  TentativeValuePairs.clear();
+  TentativeValues.clear();
 }
 
 }