[SDAG] Handle LowerOperation returning its input consistently

For almost all node types, if the target requested custom lowering, and
LowerOperation returned its input, we'd treat the original node as legal. This
did not work, however, for many loads and stores, because they follow
slightly different code paths, and we did not account for the possibility of
LowerOperation returning its input at those call sites.

I think that we now handle this consistently everywhere. At the call sites in
LegalizeDAG, we used to assert in this case, so there's no functional change
for any existing code there. For the call sites in LegalizeVectorOps, this
really only affects whether or not we set Changed = true, but I think makes the
semantics clearer.

No test case here, but it will be covered by an upcoming PowerPC commit adding
QPX support.

llvm-svn: 230332

1 files changed, 7 insertions, 3 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/LegalizeVectorOps.cpp b/llvm/lib/CodeGen/SelectionDAG/LegalizeVectorOps.cpp
index 9abbe738dfa..03c2734a9e4 100644
--- a/llvm/lib/CodeGen/SelectionDAG/LegalizeVectorOps.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/LegalizeVectorOps.cpp
@@ -207,6 +207,8 @@ SDValue VectorLegalizer::LegalizeOp(SDValue Op) {
         return TranslateLegalizeResults(Op, Result);
       case TargetLowering::Custom:
         if (SDValue Lowered = TLI.LowerOperation(Result, DAG)) {
+          if (Lowered == Result)
+            return TranslateLegalizeResults(Op, Lowered);
           Changed = true;
           if (Lowered->getNumValues() != Op->getNumValues()) {
             // This expanded to something other than the load. Assume the
@@ -232,9 +234,11 @@ SDValue VectorLegalizer::LegalizeOp(SDValue Op) {
       default: llvm_unreachable("This action is not supported yet!");
       case TargetLowering::Legal:
         return TranslateLegalizeResults(Op, Result);
-      case TargetLowering::Custom:
-        Changed = true;
-        return TranslateLegalizeResults(Op, TLI.LowerOperation(Result, DAG));
+      case TargetLowering::Custom: {
+        SDValue Lowered = TLI.LowerOperation(Result, DAG);
+        Changed = Lowered != Result;
+        return TranslateLegalizeResults(Op, Lowered);
+      }
       case TargetLowering::Expand:
         Changed = true;
         return LegalizeOp(ExpandStore(Op));