Add a fastpath to Load GVN to special case when we have exactly one dominating

load to avoid even messing around with SSAUpdate at all.  In this case (which
is very common, we can just use the input value directly).

This speeds up GVN time on gcc.c-torture/20001226-1.c from 36.4s to 16.3s,
which still isn't great, but substantially better and this is a simple speedup
that applies to lots of different cases.

llvm-svn: 91851

1 files changed, 10 insertions, 2 deletions

diff --git a/llvm/lib/Transforms/Scalar/GVN.cpp b/llvm/lib/Transforms/Scalar/GVN.cpp
index 927a3ac8a23..ec4d7cb579f 100644
--- a/llvm/lib/Transforms/Scalar/GVN.cpp
+++ b/llvm/lib/Transforms/Scalar/GVN.cpp
@@ -1311,7 +1311,15 @@ struct AvailableValueInBlock {
 static Value *ConstructSSAForLoadSet(LoadInst *LI, 
                          SmallVectorImpl<AvailableValueInBlock> &ValuesPerBlock,
                                      const TargetData *TD,
+                                     const DominatorTree &DT,
                                      AliasAnalysis *AA) {
+  // Check for the fully redundant, dominating load case.  In this case, we can
+  // just use the dominating value directly.
+  if (ValuesPerBlock.size() == 1 && 
+      DT.properlyDominates(ValuesPerBlock[0].BB, LI->getParent()))
+    return ValuesPerBlock[0].MaterializeAdjustedValue(LI->getType(), TD);
+
+  // Otherwise, we have to construct SSA form.
   SmallVector<PHINode*, 8> NewPHIs;
   SSAUpdater SSAUpdate(&NewPHIs);
   SSAUpdate.Initialize(LI);
@@ -1494,7 +1502,7 @@ bool GVN::processNonLocalLoad(LoadInst *LI,
     DEBUG(errs() << "GVN REMOVING NONLOCAL LOAD: " << *LI << '\n');
     
     // Perform PHI construction.
-    Value *V = ConstructSSAForLoadSet(LI, ValuesPerBlock, TD,
+    Value *V = ConstructSSAForLoadSet(LI, ValuesPerBlock, TD, *DT,
                                       VN.getAliasAnalysis());
     LI->replaceAllUsesWith(V);
 
@@ -1683,7 +1691,7 @@ bool GVN::processNonLocalLoad(LoadInst *LI,
   ValuesPerBlock.push_back(AvailableValueInBlock::get(UnavailablePred,NewLoad));
 
   // Perform PHI construction.
-  Value *V = ConstructSSAForLoadSet(LI, ValuesPerBlock, TD,
+  Value *V = ConstructSSAForLoadSet(LI, ValuesPerBlock, TD, *DT,
                                     VN.getAliasAnalysis());
   LI->replaceAllUsesWith(V);
   if (isa<PHINode>(V))