[X86][SSE] LowerBUILD_VECTORAsVariablePermute - fix PSHUFB source/index operand ordering

[RKSimon]

Summary

As detailed in rL317463, PSHUFB (like most variable shuffle instructions) uses Op[0] for the source vector and Op[1] for the shuffle index vector, VPERMV works in reverse which is probably where the confusion comes from.

[craig.topper]

LGTM

[zvi]

Thanks for the fix.

lib/Target/X86/X86ISelLowering.cpp

if (VT.isFloatingPoint())
     IndicesVT = MVT::getVectorVT(MVT::getIntegerVT(VT.getScalarSizeInBits()),
                                  VT.getVectorNumElements());
   IndicesVec = DAG.getZExtOrTrunc(IndicesVec, SDLoc(IndicesVec), IndicesVT);
   if (SrcVec.getValueSizeInBits() < IndicesVT.getSizeInBits()) {
     SrcVec =
         DAG.getNode(ISD::INSERT_SUBVECTOR, SDLoc(SrcVec), VT, DAG.getUNDEF(VT),
                     SrcVec, DAG.getIntPtrConstant(0, SDLoc(SrcVec)));
   }
-  return DAG.getNode(VT == MVT::v16i8 ? X86ISD::PSHUFB : X86ISD::VPERMV,
-                     SDLoc(V), VT, IndicesVec, SrcVec);
+  if (VT == MVT::v16i8)
+    return DAG.getNode(X86ISD::PSHUFB, SDLoc(V), VT, SrcVec, IndicesVec);
+  return DAG.getNode(X86ISD::VPERMV, SDLoc(V), VT, IndicesVec, SrcVec);
 }
 
 SDValue
 X86TargetLowering::LowerBUILD_VECTOR(SDValue Op, SelectionDAG &DAG) const {
   SDLoc dl(Op);
 
   MVT VT = Op.getSimpleValueType();
   MVT EltVT = VT.getVectorElementType();

test/CodeGen/X86/var-permute-128.ll

; AVX512VLBW-NEXT:    retq
   %ret6 = insertelement <8 x i16> %ret5, i16 %v6, i32 6
   %ret7 = insertelement <8 x i16> %ret6, i16 %v7, i32 7
   ret <8 x i16> %ret7
 }
 
 define <16 x i8> @var_shuffle_v16i8(<16 x i8> %v, <16 x i8> %indices) nounwind {
 ; SSSE3-LABEL: var_shuffle_v16i8:
 ; SSSE3:       # %bb.0:
-; SSSE3-NEXT:    pshufb %xmm0, %xmm1
-; SSSE3-NEXT:    movdqa %xmm1, %xmm0
+; SSSE3-NEXT:    pshufb %xmm1, %xmm0
 ; SSSE3-NEXT:    retq
 ;
 ; AVX-LABEL: var_shuffle_v16i8:
 ; AVX:       # %bb.0:
-; AVX-NEXT:    vpshufb %xmm0, %xmm1, %xmm0
+; AVX-NEXT:    vpshufb %xmm1, %xmm0, %xmm0
 ; AVX-NEXT:    retq
   %index0 = extractelement <16 x i8> %indices, i32 0
   %index1 = extractelement <16 x i8> %indices, i32 1
   %index2 = extractelement <16 x i8> %indices, i32 2
   %index3 = extractelement <16 x i8> %indices, i32 3
   %index4 = extractelement <16 x i8> %indices, i32 4
   %index5 = extractelement <16 x i8> %indices, i32 5
   %index6 = extractelement <16 x i8> %indices, i32 6