[X86][AVX] Use Zeroable mask to improve shuffle mask widening

[RKSimon]

Summary

Noticed while updating D42044, lowerV2X128VectorShuffle can improve the shuffle mask with the zeroable data to create a target shuffle mask to recognise more 'zero upper 128' patterns.

[RKSimon]

ping?

[andreadb]

LGTM

lib/Target/X86/X86ISelLowering.cpp

for (int i = 0, Size = Mask.size(); i < Size; i += 2) {
     return false;
   }
   assert(WidenedMask.size() == Mask.size() / 2 &&
          "Incorrect size of mask after widening the elements!");
 
   return true;
 }
 
+static bool canWidenShuffleElements(ArrayRef<int> Mask,
+                                    const APInt &Zeroable,
+                                    SmallVectorImpl<int> &WidenedMask) {
+  SmallVector<int, 32> TargetMask(Mask.begin(), Mask.end());
+  for (int i = 0, Size = TargetMask.size(); i < Size; ++i) {
+    if (TargetMask[i] == SM_SentinelUndef)
+      continue;
+    if (Zeroable[i])
+      TargetMask[i] = SM_SentinelZero;
+  }
+  return canWidenShuffleElements(TargetMask, WidenedMask);
+}
+
 /// Returns true if Elt is a constant zero or a floating point constant +0.0.
 bool X86::isZeroNode(SDValue Elt) {
   return isNullConstant(Elt) || isNullFPConstant(Elt);
 }
 
 // Build a vector of constants.
 // Use an UNDEF node if MaskElt == -1.
 // Split 64-bit constants in the 32-bit mode.
static SDValue lowerV2X128VectorShuffle(const SDLoc &DL, MVT VT, SDValue V1,
                                         const APInt &Zeroable,
                                         const X86Subtarget &Subtarget,
                                         SelectionDAG &DAG) {
   // With AVX2, use VPERMQ/VPERMPD for unary shuffles to allow memory folding.
   if (Subtarget.hasAVX2() && V2.isUndef())
     return SDValue();
 
   SmallVector<int, 4> WidenedMask;
-  if (!canWidenShuffleElements(Mask, WidenedMask))
+  if (!canWidenShuffleElements(Mask, Zeroable, WidenedMask))
     return SDValue();
 
   bool IsLowZero = (Zeroable & 0x3) == 0x3;
   bool IsHighZero = (Zeroable & 0xc) == 0xc;
 
   // Try to use an insert into a zero vector.
   if (WidenedMask[0] == 0 && IsHighZero) {
     MVT SubVT = MVT::getVectorVT(VT.getVectorElementType(), 2);
static SDValue lowerV2X128VectorShuffle(const SDLoc &DL, MVT VT, SDValue V1,
   // The immediate permute control byte looks like this:
   //    [1:0] - select 128 bits from sources for low half of destination
   //    [2]   - ignore
   //    [3]   - zero low half of destination
   //    [5:4] - select 128 bits from sources for high half of destination
   //    [6]   - ignore
   //    [7]   - zero high half of destination
 
-  assert(WidenedMask[0] >= 0 && WidenedMask[1] >= 0 && "Undef half?");
+  assert((WidenedMask[0] >= 0 || IsLowZero) &&
+         (WidenedMask[1] >= 0 || IsHighZero) && "Undef half?");
 
   unsigned PermMask = 0;
   PermMask |= IsLowZero  ? 0x08 : (WidenedMask[0] << 0);
   PermMask |= IsHighZero ? 0x80 : (WidenedMask[1] << 4);
 
   // Check the immediate mask and replace unused sources with undef.
   if ((PermMask & 0x0a) != 0x00 && (PermMask & 0xa0) != 0x00)
     V1 = DAG.getUNDEF(VT);

test/CodeGen/X86/avx-cast.ll

 ; AVX-NEXT:    retq
   %shuffle.i = shufflevector <4 x float> %m, <4 x float> zeroinitializer, <8 x i32> <i32 0, i32 1, i32 2, i32 3, i32 4, i32 4, i32 4, i32 4>
   ret <8 x float> %shuffle.i
 }
 
 define <4 x double> @castB(<2 x double> %m) nounwind uwtable readnone ssp {
 ; AVX-LABEL: castB:
 ; AVX:       ## %bb.0:
-; AVX-NEXT:    ## kill: def $xmm0 killed $xmm0 def $ymm0
-; AVX-NEXT:    vxorps %xmm1, %xmm1, %xmm1
-; AVX-NEXT:    vblendps {{.*#+}} ymm0 = ymm0[0,1,2,3],ymm1[4,5,6,7]
+; AVX-NEXT:    vmovaps %xmm0, %xmm0
 ; AVX-NEXT:    retq
   %shuffle.i = shufflevector <2 x double> %m, <2 x double> zeroinitializer, <4 x i32> <i32 0, i32 1, i32 2, i32 2>
   ret <4 x double> %shuffle.i
 }
 
 ; AVX2 is needed for integer types.
 
 define <4 x i64> @castC(<2 x i64> %m) nounwind uwtable readnone ssp {
 ; AVX-LABEL: castC:
 ; AVX:       ## %bb.0:
-; AVX-NEXT:    ## kill: def $xmm0 killed $xmm0 def $ymm0
-; AVX-NEXT:    vxorps %xmm1, %xmm1, %xmm1
-; AVX-NEXT:    vblendps {{.*#+}} ymm0 = ymm0[0,1,2,3],ymm1[4,5,6,7]
+; AVX-NEXT:    vmovaps %xmm0, %xmm0
 ; AVX-NEXT:    retq
   %shuffle.i = shufflevector <2 x i64> %m, <2 x i64> zeroinitializer, <4 x i32> <i32 0, i32 1, i32 2, i32 2>
   ret <4 x i64> %shuffle.i
 }
 
 ; The next three tests don't need any shuffling. There may or may not be a
 ; vzeroupper before the return, so just check for the absence of shuffles.