[SDAG] Fold umul_lohi with 0 or 1 multiplicand

These can turn up during multiplication legalization. In principle
these should also apply to smul_lohi, but I wasn't able to figure
out how to produce those with the necessary operands.

Differential Revision: https://reviews.llvm.org/D66380

llvm-svn: 369864

1 files changed, 12 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index ec47f2ba2c4..75fb6559b9b 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -4364,6 +4364,18 @@ SDValue DAGCombiner::visitUMUL_LOHI(SDNode *N) {
   EVT VT = N->getValueType(0);
   SDLoc DL(N);
 
+  // (umul_lohi N0, 0) -> (0, 0)
+  if (isNullConstant(N->getOperand(1))) {
+    SDValue Zero = DAG.getConstant(0, DL, VT);
+    return CombineTo(N, Zero, Zero);
+  }
+
+  // (umul_lohi N0, 1) -> (N0, 0)
+  if (isOneConstant(N->getOperand(1))) {
+    SDValue Zero = DAG.getConstant(0, DL, VT);
+    return CombineTo(N, N->getOperand(0), Zero);
+  }
+
   // If the type is twice as wide is legal, transform the mulhu to a wider
   // multiply plus a shift.
   if (VT.isSimple() && !VT.isVector()) {