From c20e24524f6dcfb423b9c6d9d223ec11f13d3c84 Mon Sep 17 00:00:00 2001
From: Chris Lattner <sabre@nondot.org>
Date: Thu, 22 Aug 2002 20:22:01 +0000
Subject: [PATCH] Instcombine PHI's of the form  %PN = phi PN, X into X  and
 %PN = phi PN, PN, PN into 0 (because the phi must not be reachable)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@3470 91177308-0d34-0410-b5e6-96231b3b80d8
---
 lib/Transforms/Scalar/InstructionCombining.cpp | 17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/lib/Transforms/Scalar/InstructionCombining.cpp b/lib/Transforms/Scalar/InstructionCombining.cpp
index 30e143dec46..0f8ae0c6bdb 100644
--- a/lib/Transforms/Scalar/InstructionCombining.cpp
+++ b/lib/Transforms/Scalar/InstructionCombining.cpp
@@ -610,10 +610,19 @@ Instruction *InstCombiner::visitPHINode(PHINode &PN) {
   // Otherwise if all of the incoming values are the same for the PHI, replace
   // the PHI node with the incoming value.
   //
-  Value *InVal = PN.getIncomingValue(0);
-  for (unsigned i = 1, e = PN.getNumIncomingValues(); i != e; ++i)
-    if (PN.getIncomingValue(i) != InVal)
-      return 0;  // Not the same, bail out.
+  Value *InVal = 0;
+  for (unsigned i = 0, e = PN.getNumIncomingValues(); i != e; ++i)
+    if (PN.getIncomingValue(i) != &PN)  // Not the PHI node itself...
+      if (InVal && PN.getIncomingValue(i) != InVal)
+        return 0;  // Not the same, bail out.
+      else
+        InVal = PN.getIncomingValue(i);
+
+  // The only case that could cause InVal to be null is if we have a PHI node
+  // that only has entries for itself.  In this case, there is no entry into the
+  // loop, so kill the PHI.
+  //
+  if (InVal == 0) InVal = Constant::getNullValue(PN.getType());
 
   // All of the incoming values are the same, replace the PHI node now.
   return ReplaceInstUsesWith(PN, InVal);