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.

llvm-svn: 46306

1 files changed, 8 insertions, 17 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 788c8c04a82..6ce29fb89fb 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/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 {