This optimization makes MANY phi nodes that all have the same incoming value.

If this happens, detect it early instead of relying on instcombine to notice
it later.  This can be a big speedup, because PHI nodes can have many
incoming values.

llvm-svn: 17741

1 files changed, 23 insertions, 7 deletions

diff --git a/llvm/lib/Transforms/Scalar/InstructionCombining.cpp b/llvm/lib/Transforms/Scalar/InstructionCombining.cpp
index eaadbb4801c..50a26c18c95 100644
--- a/llvm/lib/Transforms/Scalar/InstructionCombining.cpp
+++ b/llvm/lib/Transforms/Scalar/InstructionCombining.cpp
@@ -3456,21 +3456,37 @@ Instruction *InstCombiner::FoldPHIArgOpIntoPHI(PHINode &PN) {
   PHINode *NewPN = new PHINode(FirstInst->getOperand(0)->getType(),
                                PN.getName()+".in");
   NewPN->op_reserve(PN.getNumOperands());
-  InsertNewInstBefore(NewPN, PN);
+
+  Value *InVal = FirstInst->getOperand(0);
+  NewPN->addIncoming(InVal, PN.getIncomingBlock(0));
 
   // Add all operands to the new PHI.
-  for (unsigned i = 0, e = PN.getNumIncomingValues(); i != e; ++i)
-    NewPN->addIncoming(cast<Instruction>(PN.getIncomingValue(i))->getOperand(0),
-                       PN.getIncomingBlock(i));
+  for (unsigned i = 1, e = PN.getNumIncomingValues(); i != e; ++i) {
+    Value *NewInVal = cast<Instruction>(PN.getIncomingValue(i))->getOperand(0);
+    if (NewInVal != InVal)
+      InVal = 0;
+    NewPN->addIncoming(NewInVal, PN.getIncomingBlock(i));
+  }
+
+  Value *PhiVal;
+  if (InVal) {
+    // The new PHI unions all of the same values together.  This is really
+    // common, so we handle it intelligently here for compile-time speed.
+    PhiVal = InVal;
+    delete NewPN;
+  } else {
+    InsertNewInstBefore(NewPN, PN);
+    PhiVal = NewPN;
+  }
   
   // Insert and return the new operation.
   if (isa<CastInst>(FirstInst))
-    return new CastInst(NewPN, PN.getType());
+    return new CastInst(PhiVal, PN.getType());
   else if (BinaryOperator *BinOp = dyn_cast<BinaryOperator>(FirstInst))
-    return BinaryOperator::create(BinOp->getOpcode(), NewPN, ConstantOp);
+    return BinaryOperator::create(BinOp->getOpcode(), PhiVal, ConstantOp);
   else
     return new ShiftInst(cast<ShiftInst>(FirstInst)->getOpcode(),
-                         NewPN, ConstantOp);
+                         PhiVal, ConstantOp);
 }
 
 // PHINode simplification