[X86] Change the behavior of canWidenShuffleElements used by lowerV2X128Shuffle to match the behavior in lowerVectorShuffle with regards to zeroable elements.

Previously we marked zeroable elements in a way that prevented
the widening check from recognizing that it could widen. Now
we only mark them zeroable if V2 is an all zeros vector. This
matches what we do for widening elements in lowerVectorShuffle.

Fixes PR43866.

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -5324,15 +5324,18 @@ static bool canWidenShuffleElements(ArrayRef<int> Mask,
 
 static bool canWidenShuffleElements(ArrayRef<int> Mask,
                                     const APInt &Zeroable,
+                                    bool V2IsZero,
                                     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;
+  // Create an alternative mask with info about zeroable elements.
+  // Here we do not set undef elements as zeroable.
+  SmallVector<int, 64> ZeroableMask(Mask.begin(), Mask.end());
+  if (V2IsZero) {
+    assert(!Zeroable.isNullValue() && "V2's non-undef elements are used?!");
+    for (int i = 0, Size = Mask.size(); i != Size; ++i)
+      if (Mask[i] != SM_SentinelUndef && Zeroable[i])
+        ZeroableMask[i] = SM_SentinelZero;
   }
-  return canWidenShuffleElements(TargetMask, WidenedMask);
+  return canWidenShuffleElements(ZeroableMask, WidenedMask);
 }
 
 static bool canWidenShuffleElements(ArrayRef<int> Mask) {
@@ -14817,8 +14820,10 @@ static SDValue lowerV2X128Shuffle(const SDLoc &DL, MVT VT, SDValue V1,
   if (Subtarget.hasAVX2() && V2.isUndef())
     return SDValue();
 
+  bool V2IsZero = !V2.isUndef() && ISD::isBuildVectorAllZeros(V2.getNode());
+
   SmallVector<int, 4> WidenedMask;
-  if (!canWidenShuffleElements(Mask, Zeroable, WidenedMask))
+  if (!canWidenShuffleElements(Mask, Zeroable, V2IsZero, WidenedMask))
     return SDValue();
 
   bool IsLowZero = (Zeroable & 0x3) == 0x3;
@@ -17095,23 +17100,13 @@ static SDValue lowerVectorShuffle(SDValue Op, const X86Subtarget &Subtarget,
 
   bool V2IsZero = !V2IsUndef && ISD::isBuildVectorAllZeros(V2.getNode());
 
-  // Create an alternative mask with info about zeroable elements.
-  // Here we do not set undef elements as zeroable.
-  SmallVector<int, 64> ZeroableMask(OrigMask.begin(), OrigMask.end());
-  if (V2IsZero) {
-    assert(!Zeroable.isNullValue() && "V2's non-undef elements are used?!");
-    for (int i = 0; i != NumElements; ++i)
-      if (OrigMask[i] != SM_SentinelUndef && Zeroable[i])
-        ZeroableMask[i] = SM_SentinelZero;
-  }
-
   // Try to collapse shuffles into using a vector type with fewer elements but
   // wider element types. We cap this to not form integers or floating point
   // elements wider than 64 bits, but it might be interesting to form i128
   // integers to handle flipping the low and high halves of AVX 256-bit vectors.
   SmallVector<int, 16> WidenedMask;
   if (VT.getScalarSizeInBits() < 64 && !Is1BitVector &&
-      canWidenShuffleElements(ZeroableMask, WidenedMask)) {
+      canWidenShuffleElements(OrigMask, Zeroable, V2IsZero, WidenedMask)) {
     // Shuffle mask widening should not interfere with a broadcast opportunity
     // by obfuscating the operands with bitcasts.
     // TODO: Avoid lowering directly from this top-level function: make this

llvm/test/CodeGen/X86/pr43866.ll

@@ -0,0 +1,37 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc < %s -mtriple=x86_64-unknown-unknown -mattr=avx | FileCheck %s
+
+@v2_0 = global <2 x i32> zeroinitializer, align 8
+
+define void @test()  {
+; CHECK-LABEL: test:
+; CHECK:       # %bb.0: # %entry
+; CHECK-NEXT:    pushq %rbp
+; CHECK-NEXT:    .cfi_def_cfa_offset 16
+; CHECK-NEXT:    .cfi_offset %rbp, -16
+; CHECK-NEXT:    movq %rsp, %rbp
+; CHECK-NEXT:    .cfi_def_cfa_register %rbp
+; CHECK-NEXT:    andq $-32, %rsp
+; CHECK-NEXT:    subq $64, %rsp
+; CHECK-NEXT:    vmovsd {{.*#+}} xmm0 = mem[0],zero
+; CHECK-NEXT:    vpcmpeqd %xmm1, %xmm1, %xmm1
+; CHECK-NEXT:    vinsertf128 $1, %xmm1, %ymm0, %ymm0
+; CHECK-NEXT:    vxorps %xmm2, %xmm2, %xmm2
+; CHECK-NEXT:    vcmptrueps %ymm2, %ymm2, %ymm2
+; CHECK-NEXT:    vinsertf128 $1, %xmm1, %ymm2, %ymm1
+; CHECK-NEXT:    vshufps {{.*#+}} ymm1 = ymm1[1,0],ymm0[1,0],ymm1[5,4],ymm0[5,4]
+; CHECK-NEXT:    vshufps {{.*#+}} ymm0 = ymm1[2,0],ymm0[0,0],ymm1[6,4],ymm0[4,4]
+; CHECK-NEXT:    vmovaps %ymm0, (%rsp)
+; CHECK-NEXT:    movq %rbp, %rsp
+; CHECK-NEXT:    popq %rbp
+; CHECK-NEXT:    .cfi_def_cfa %rsp, 8
+; CHECK-NEXT:    vzeroupper
+; CHECK-NEXT:    retq
+entry:
+  %v8_0 = alloca <8 x i32>, align 32
+  %v8_0.0.v8_0.0..sroa_cast = bitcast <8 x i32>* %v8_0 to i8*
+  %0 = load <2 x i32>, <2 x i32>* @v2_0, align 8
+  %shuffle = shufflevector <2 x i32> %0, <2 x i32> <i32 -1, i32 -1>, <8 x i32> <i32 1, i32 3, i32 0, i32 0, i32 3, i32 3, i32 2, i32 2>
+  store volatile <8 x i32> %shuffle, <8 x i32>* %v8_0, align 32
+  ret void
+}