[PATCH] D41119: [X86][SSE] MOVMSK only uses the sign bit from each vector element

[Simon Pilgrim via Phabricator via llvm-commits]

RKSimon created this revision.
RKSimon added reviewers: spatel, craig.topper, zvi, andreadb.

Pass the input vector through SimplifyDemandedBits as we only need the sign bit from each vector element of MOVMSK

We'd probably get more hits if SimplifyDemandedBits was better at handling vectors...


Repository:
  rL LLVM

https://reviews.llvm.org/D41119

Files:
  lib/Target/X86/X86ISelLowering.cpp
  test/CodeGen/X86/setcc-lowering.ll


Index: test/CodeGen/X86/setcc-lowering.ll
===================================================================
--- test/CodeGen/X86/setcc-lowering.ll
+++ test/CodeGen/X86/setcc-lowering.ll
@@ -57,7 +57,6 @@
 ; AVX-NEXT:    vpand %xmm0, %xmm3, %xmm3
 ; AVX-NEXT:    vpsllw $7, %xmm3, %xmm3
 ; AVX-NEXT:    vpand %xmm2, %xmm3, %xmm3
-; AVX-NEXT:    vpcmpgtb %xmm3, %xmm1, %xmm3
 ; AVX-NEXT:    vpmovmskb %xmm3, %eax
 ; AVX-NEXT:    testw %ax, %ax
 ; AVX-NEXT:    jne .LBB1_1
Index: lib/Target/X86/X86ISelLowering.cpp
===================================================================
--- lib/Target/X86/X86ISelLowering.cpp
+++ lib/Target/X86/X86ISelLowering.cpp
@@ -36170,6 +36170,27 @@
   return SDValue();
 }
 
+static SDValue combineMOVMSK(SDNode *N, SelectionDAG &DAG,
+                             TargetLowering::DAGCombinerInfo &DCI) {
+  SDValue Src = N->getOperand(0);
+  MVT SrcVT = Src.getSimpleValueType();
+
+  const TargetLowering &TLI = DAG.getTargetLoweringInfo();
+  TargetLowering::TargetLoweringOpt TLO(DAG, !DCI.isBeforeLegalize(),
+                                        !DCI.isBeforeLegalizeOps());
+
+  // MOVMSK only uses the MSB from each vector element.
+  KnownBits Known;
+  APInt DemandedMask(APInt::getSignMask(SrcVT.getScalarSizeInBits()));
+  if (TLI.SimplifyDemandedBits(Src, DemandedMask, Known, TLO)) {
+    DCI.AddToWorklist(Src.getNode());
+    DCI.CommitTargetLoweringOpt(TLO);
+    return SDValue(N, 0);
+  }
+
+  return SDValue();
+}
+
 static SDValue combineGatherScatter(SDNode *N, SelectionDAG &DAG,
                                     TargetLowering::DAGCombinerInfo &DCI,
                                     const X86Subtarget &Subtarget) {
@@ -37341,6 +37362,7 @@
   case X86ISD::FMSUBADD_RND:
   case X86ISD::FMADDSUB:
   case X86ISD::FMSUBADD:    return combineFMADDSUB(N, DAG, Subtarget);
+  case X86ISD::MOVMSK:      return combineMOVMSK(N, DAG, DCI);
   case X86ISD::MGATHER:
   case X86ISD::MSCATTER:
   case ISD::MGATHER:


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