[DAGcombine] Do not replace truncate node by itself when doing constant folding, this trigger needless extra rounds of combine for nothing. NFC

llvm-svn: 317926

1 files changed, 9 insertions, 3 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 557cf6d5212..dd8d613dcf7 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -8360,12 +8360,18 @@ SDValue DAGCombiner::visitTRUNCATE(SDNode *N) {
   // noop truncate
   if (N0.getValueType() == N->getValueType(0))
     return N0;
-  // fold (truncate c1) -> c1
-  if (DAG.isConstantIntBuildVectorOrConstantInt(N0))
-    return DAG.getNode(ISD::TRUNCATE, SDLoc(N), VT, N0);
+
   // fold (truncate (truncate x)) -> (truncate x)
   if (N0.getOpcode() == ISD::TRUNCATE)
     return DAG.getNode(ISD::TRUNCATE, SDLoc(N), VT, N0.getOperand(0));
+
+  // fold (truncate c1) -> c1
+  if (DAG.isConstantIntBuildVectorOrConstantInt(N0)) {
+    SDValue C = DAG.getNode(ISD::TRUNCATE, SDLoc(N), VT, N0);
+    if (C.getNode() != N)
+      return C;
+  }
+
   // fold (truncate (ext x)) -> (ext x) or (truncate x) or x
   if (N0.getOpcode() == ISD::ZERO_EXTEND ||
       N0.getOpcode() == ISD::SIGN_EXTEND ||