From 125991a1f62db419fc4a504986a998d85fbef699 Mon Sep 17 00:00:00 2001
From: Chris Lattner <sabre@nondot.org>
Date: Thu, 24 Jan 2008 07:57:06 +0000
Subject: [PATCH] Fix this buggy transformation.  Two observations: 1. we
 already know the value is dead, so don't bother replacing    it with undef.
 2. The very case the comment describes actually makes the load    live which
 asserts in deletenode.  If we do the replacement    and the node becomes
 live, just treat it as new.  This fixes    a failure on
 X86/2008-01-16-InvalidDAGCombineXform.ll with    some local changes in my
 tree.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@46306 91177308-0d34-0410-b5e6-96231b3b80d8
---
 lib/CodeGen/SelectionDAG/DAGCombiner.cpp | 25 ++++++++----------------
 1 file changed, 8 insertions(+), 17 deletions(-)

diff --git a/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 788c8c04a82..6ce29fb89fb 100644
--- a/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -4090,28 +4090,19 @@ SDOperand DAGCombiner::visitLOAD(SDNode *N) {
         // v1, chain2 = load chain1, loc
         // v2, chain3 = load chain2, loc
         // v3         = add v2, c
-        // Now we replace use of v1 with undef, use of chain2 with chain1.
-        // ReplaceAllUsesWith() will iterate through uses of the first load and
-        // update operands:
-        // v1, chain2 = load chain1, loc
-        // v2, chain3 = load chain1, loc
-        // v3         = add v2, c
-        // Now the second load is the same as the first load, SelectionDAG cse
-        // will ensure the use of second load is replaced with the first load.
-        // v1, chain2 = load chain1, loc
-        // v3         = add v1, c
-        // Then v1 is replaced with undef and bad things happen.
+        // Now we replace use of chain2 with chain1.  This makes the second load
+        // isomorphic to the one we are deleting, and thus makes this load live.
         std::vector<SDNode*> NowDead;
-        SDOperand Undef = DAG.getNode(ISD::UNDEF, N->getValueType(0));
         DOUT << "\nReplacing.6 "; DEBUG(N->dump(&DAG));
-        DOUT << "\nWith: "; DEBUG(Undef.Val->dump(&DAG));
-        DOUT << " and 1 other value\n";
-        DAG.ReplaceAllUsesOfValueWith(SDOperand(N, 0), Undef, &NowDead);
+        DOUT << "\nWith chain: "; DEBUG(Chain.Val->dump(&DAG));
+        DOUT << "\n";
         DAG.ReplaceAllUsesOfValueWith(SDOperand(N, 1), Chain, &NowDead);
-        removeFromWorkList(N);
         for (unsigned i = 0, e = NowDead.size(); i != e; ++i)
           removeFromWorkList(NowDead[i]);
-        DAG.DeleteNode(N);
+        if (N->use_empty()) {
+          removeFromWorkList(N);
+          DAG.DeleteNode(N);
+        }
         return SDOperand(N, 0);   // Return N so it doesn't get rechecked!
       }
     } else {