From b58934484b51d908b660c6b915d8df3bb37c7055 Mon Sep 17 00:00:00 2001
From: Chris Lattner <sabre@nondot.org>
Date: Sun, 14 Nov 2004 19:29:34 +0000
Subject: [PATCH] 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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@17741 91177308-0d34-0410-b5e6-96231b3b80d8
---
 .../Scalar/InstructionCombining.cpp           | 30 ++++++++++++++-----
 1 file changed, 23 insertions(+), 7 deletions(-)

diff --git a/lib/Transforms/Scalar/InstructionCombining.cpp b/lib/Transforms/Scalar/InstructionCombining.cpp
index eaadbb4801c..50a26c18c95 100644
--- a/lib/Transforms/Scalar/InstructionCombining.cpp
+++ b/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