[DAG] SimplifyDemandedVectorElts - adjust demanded elements for selection mask for known zero results

If an element is known zero from both selections then it shouldn't matter what the selection mask element is.

Author: RKSimon

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -11053,6 +11053,9 @@ SDValue DAGCombiner::visitVSELECT(SDNode *N) {
     if (SDValue V = foldVSelectToSignBitSplatMask(N, DAG))
       return V;
 
+  if (SimplifyDemandedVectorElts(SDValue(N, 0)))
+    return SDValue(N, 0);
+
   return SDValue();
 }
 

llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp

@@ -3052,31 +3052,40 @@ bool TargetLowering::SimplifyDemandedVectorElts(
     break;
   }
   case ISD::VSELECT: {
+    SDValue Sel = Op.getOperand(0);
+    SDValue LHS = Op.getOperand(1);
+    SDValue RHS = Op.getOperand(2);
+
     // Try to transform the select condition based on the current demanded
     // elements.
-    // TODO: If a condition element is undef, we can choose from one arm of the
-    //       select (and if one arm is undef, then we can propagate that to the
-    //       result).
-    // TODO - add support for constant vselect masks (see IR version of this).
-    APInt UnusedUndef, UnusedZero;
-    if (SimplifyDemandedVectorElts(Op.getOperand(0), DemandedElts, UnusedUndef,
-                                   UnusedZero, TLO, Depth + 1))
+    APInt UndefSel, UndefZero;
+    if (SimplifyDemandedVectorElts(Sel, DemandedElts, UndefSel, UndefZero, TLO,
+                                   Depth + 1))
       return true;
 
     // See if we can simplify either vselect operand.
     APInt DemandedLHS(DemandedElts);
     APInt DemandedRHS(DemandedElts);
     APInt UndefLHS, ZeroLHS;
     APInt UndefRHS, ZeroRHS;
-    if (SimplifyDemandedVectorElts(Op.getOperand(1), DemandedLHS, UndefLHS,
-                                   ZeroLHS, TLO, Depth + 1))
+    if (SimplifyDemandedVectorElts(LHS, DemandedLHS, UndefLHS, ZeroLHS, TLO,
+                                   Depth + 1))
       return true;
-    if (SimplifyDemandedVectorElts(Op.getOperand(2), DemandedRHS, UndefRHS,
-                                   ZeroRHS, TLO, Depth + 1))
+    if (SimplifyDemandedVectorElts(RHS, DemandedRHS, UndefRHS, ZeroRHS, TLO,
+                                   Depth + 1))
       return true;
 
     KnownUndef = UndefLHS & UndefRHS;
     KnownZero = ZeroLHS & ZeroRHS;
+
+    // If we know that the selected element is always zero, we don't need the
+    // select value element.
+    APInt DemandedSel = DemandedElts & ~KnownZero;
+    if (DemandedSel != DemandedElts)
+      if (SimplifyDemandedVectorElts(Sel, DemandedSel, UndefSel, UndefZero, TLO,
+                                     Depth + 1))
+        return true;
+
     break;
   }
   case ISD::VECTOR_SHUFFLE: {

llvm/test/CodeGen/X86/vselect-constants.ll

@@ -282,24 +282,17 @@ define i32 @wrong_min_signbits(<2 x i16> %x) {
 ; SSE-NEXT:    pxor %xmm1, %xmm1
 ; SSE-NEXT:    pcmpeqw %xmm0, %xmm1
 ; SSE-NEXT:    movdqa {{.*#+}} xmm0 = [1,0,0,0]
-; SSE-NEXT:    pandn %xmm0, %xmm1
-; SSE-NEXT:    psllw $15, %xmm1
-; SSE-NEXT:    psraw $15, %xmm1
-; SSE-NEXT:    movdqa %xmm1, %xmm2
-; SSE-NEXT:    pandn %xmm0, %xmm2
-; SSE-NEXT:    pand {{\.?LCPI[0-9]+_[0-9]+}}(%rip), %xmm1
-; SSE-NEXT:    por %xmm2, %xmm1
+; SSE-NEXT:    pand %xmm1, %xmm0
+; SSE-NEXT:    pandn {{\.?LCPI[0-9]+_[0-9]+}}(%rip), %xmm1
+; SSE-NEXT:    por %xmm0, %xmm1
 ; SSE-NEXT:    movd %xmm1, %eax
 ; SSE-NEXT:    retq
 ;
 ; AVX-LABEL: wrong_min_signbits:
 ; AVX:       # %bb.0:
 ; AVX-NEXT:    vpxor %xmm1, %xmm1, %xmm1
 ; AVX-NEXT:    vpcmpeqw %xmm1, %xmm0, %xmm0
-; AVX-NEXT:    vmovdqa {{.*#+}} xmm1 = [1,0,0,0]
-; AVX-NEXT:    vpandn %xmm1, %xmm0, %xmm0
-; AVX-NEXT:    vpsllw $15, %xmm0, %xmm0
-; AVX-NEXT:    vpsraw $15, %xmm0, %xmm0
+; AVX-NEXT:    vmovdqa {{.*#+}} xmm1 = [2,0,0,0]
 ; AVX-NEXT:    vpblendvb %xmm0, {{\.?LCPI[0-9]+_[0-9]+}}(%rip), %xmm1, %xmm0
 ; AVX-NEXT:    vmovd %xmm0, %eax
 ; AVX-NEXT:    retq