[DAGCombiner] Replace gathers with a zero mask with the passthru value

These can be created by the legalizer when splitting a larger gather.

See https://llvm.org/PR42055 for a motivating example.

Differential Revision: https://reviews.llvm.org/D62613

llvm-svn: 362015

1 files changed, 7 insertions, 3 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 48c918051bb..1518efd1256 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -8084,13 +8084,17 @@ SDValue DAGCombiner::visitMSTORE(SDNode *N) {
 }
 
 SDValue DAGCombiner::visitMGATHER(SDNode *N) {
-  if (Level >= AfterLegalizeTypes)
-    return SDValue();
-
   MaskedGatherSDNode *MGT = cast<MaskedGatherSDNode>(N);
   SDValue Mask = MGT->getMask();
   SDLoc DL(N);
 
+  // Zap gathers with a zero mask.
+  if (ISD::isBuildVectorAllZeros(Mask.getNode()))
+    return CombineTo(N, MGT->getPassThru(), MGT->getChain());
+
+  if (Level >= AfterLegalizeTypes)
+    return SDValue();
+
   // If the MGATHER result requires splitting and the mask is provided by a
   // SETCC, then split both nodes and its operands before legalization. This
   // prevents the type legalizer from unrolling SETCC into scalar comparisons