[X86] Fold broadcast(scalar) -> scalar_to_vector(scalar) if only the lowest element is demanded.

Author: RKSimon

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -42863,15 +42863,18 @@ bool X86TargetLowering::SimplifyDemandedVectorEltsForTargetNode(
   case X86ISD::VBROADCAST: {
     SDValue Src = Op.getOperand(0);
     MVT SrcVT = Src.getSimpleValueType();
-    if (!SrcVT.isVector())
-      break;
     // Don't bother broadcasting if we just need the 0'th element.
     if (DemandedElts == 1) {
+      if (!SrcVT.isVector())
+        return TLO.CombineTo(
+            Op, TLO.DAG.getNode(ISD::SCALAR_TO_VECTOR, SDLoc(Op), VT, Src));
       if (Src.getValueType() != VT)
         Src = widenSubVector(VT.getSimpleVT(), Src, false, Subtarget, TLO.DAG,
                              SDLoc(Op));
       return TLO.CombineTo(Op, Src);
     }
+    if (!SrcVT.isVector())
+      break;
     APInt SrcUndef, SrcZero;
     APInt SrcElts = APInt::getOneBitSet(SrcVT.getVectorNumElements(), 0);
     if (SimplifyDemandedVectorElts(Src, SrcElts, SrcUndef, SrcZero, TLO,

llvm/test/CodeGen/X86/pr29222.ll

@@ -54,9 +54,7 @@ define i32 @PR29222(i32) nounwind {
 ; X64-AVX-NEXT:    movd %edi, %mm0
 ; X64-AVX-NEXT:    pshufw $68, %mm0, %mm0 # mm0 = mm0[0,1,0,1]
 ; X64-AVX-NEXT:    packsswb %mm0, %mm0
-; X64-AVX-NEXT:    movq %mm0, %rax
-; X64-AVX-NEXT:    vmovq %rax, %xmm0
-; X64-AVX-NEXT:    vpbroadcastq %xmm0, %xmm0
+; X64-AVX-NEXT:    movq2dq %mm0, %xmm0
 ; X64-AVX-NEXT:    vpacksswb %xmm0, %xmm0, %xmm0
 ; X64-AVX-NEXT:    vmovd %xmm0, %eax
 ; X64-AVX-NEXT:    retq

llvm/test/CodeGen/X86/shuffle-half.ll

@@ -225,7 +225,7 @@ define <32 x half> @build_vec(ptr %p, <32 x i1> %mask) {
 ; CHECK-NEXT:    testl $65536, %eax # imm = 0x10000
 ; CHECK-NEXT:    je .LBB1_35
 ; CHECK-NEXT:  .LBB1_34: # %cond.load46
-; CHECK-NEXT:    vpbroadcastw 32(%rdi), %xmm1
+; CHECK-NEXT:    vpinsrw $0, 32(%rdi), %xmm0, %xmm1
 ; CHECK-NEXT:    vextracti64x4 $1, %zmm0, %ymm2
 ; CHECK-NEXT:    vpblendw {{.*#+}} xmm1 = xmm1[0],xmm2[1,2,3,4,5,6,7]
 ; CHECK-NEXT:    vpblendd {{.*#+}} ymm1 = ymm1[0,1,2,3],ymm2[4,5,6,7]