[X86][AVX] lowerShuffleWithVTRUNC - pull out TRUNCATE/VTRUNC creation into helper code. NFCI.

Prep work toward adding v16i16/v32i8 support for lowerShuffleWithVTRUNC and improving lowerShuffleWithVPMOV.

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -11286,6 +11286,37 @@ static bool matchShuffleAsVTRUNC(MVT &SrcVT, MVT &DstVT, MVT VT,
   return false;
 }
 
+// Helper to create TRUNCATE/VTRUNC nodes, optionally with zero/undef upper
+// element padding to the final DstVT.
+static SDValue getAVX512TruncNode(const SDLoc &DL, MVT DstVT, SDValue Src,
+                                  const X86Subtarget &Subtarget,
+                                  SelectionDAG &DAG, bool ZeroUppers) {
+  MVT SrcVT = Src.getSimpleValueType();
+  unsigned NumDstElts = DstVT.getVectorNumElements();
+  unsigned NumSrcElts = SrcVT.getVectorNumElements();
+
+  // Perform a direct ISD::TRUNCATE if possible.
+  if (NumSrcElts == NumDstElts)
+    return DAG.getNode(ISD::TRUNCATE, DL, DstVT, Src);
+
+  if (NumSrcElts > NumDstElts) {
+    MVT TruncVT = MVT::getVectorVT(DstVT.getScalarType(), NumSrcElts);
+    SDValue Trunc = DAG.getNode(ISD::TRUNCATE, DL, TruncVT, Src);
+    return extractSubVector(Trunc, 0, DAG, DL, DstVT.getSizeInBits());
+  }
+
+  // Non-VLX targets must truncate from a 512-bit type, so we need to
+  // widen, truncate and then possibly extract the original subvector.
+  if (!Subtarget.hasVLX() && !SrcVT.is512BitVector()) {
+    SDValue NewSrc = widenSubVector(Src, ZeroUppers, Subtarget, DAG, DL, 512);
+    return getAVX512TruncNode(DL, DstVT, NewSrc, Subtarget, DAG, ZeroUppers);
+  }
+
+  // Fallback to a X86ISD::VTRUNC.
+  // TODO: Handle cases where we go from 512-bit vectors to sub-128-bit vectors.
+  return DAG.getNode(X86ISD::VTRUNC, DL, DstVT, Src);
+}
+
 static bool matchShuffleAsVPMOV(ArrayRef<int> Mask, bool SwappedOps,
                                 int Delta) {
   int Size = (int)Mask.size();
@@ -11388,7 +11419,7 @@ static SDValue lowerShuffleAsVTRUNC(const SDLoc &DL, MVT VT, SDValue V1,
 
   unsigned NumElts = VT.getVectorNumElements();
   unsigned EltSizeInBits = VT.getScalarSizeInBits();
-  unsigned MaxScale = 64 / VT.getScalarSizeInBits();
+  unsigned MaxScale = 64 / EltSizeInBits;
   for (unsigned Scale = 2; Scale <= MaxScale; Scale += Scale) {
     // TODO: Support non-BWI VPMOVWB truncations?
     unsigned SrcEltBits = EltSizeInBits * Scale;
@@ -11408,36 +11439,18 @@ static SDValue lowerShuffleAsVTRUNC(const SDLoc &DL, MVT VT, SDValue V1,
     if (UpperElts > 0 &&
         !Zeroable.extractBits(UpperElts, NumSrcElts).isAllOnesValue())
       continue;
+    bool UndefUppers =
+        UpperElts > 0 && isUndefInRange(Mask, NumSrcElts, UpperElts);
 
     // As we're using both sources then we need to concat them together
-    // and truncate from the 256-bit src.
+    // and truncate from the double-sized src.
     MVT ConcatVT = MVT::getVectorVT(VT.getScalarType(), NumElts * 2);
     SDValue Src = DAG.getNode(ISD::CONCAT_VECTORS, DL, ConcatVT, V1, V2);
 
     MVT SrcSVT = MVT::getIntegerVT(SrcEltBits);
     MVT SrcVT = MVT::getVectorVT(SrcSVT, NumSrcElts);
     Src = DAG.getBitcast(SrcVT, Src);
-
-    if (SrcVT.getVectorNumElements() == NumElts)
-      return DAG.getNode(ISD::TRUNCATE, DL, VT, Src);
-
-    if (!Subtarget.hasVLX()) {
-      // Non-VLX targets must truncate from a 512-bit type, so we need to
-      // widen, truncate and then possibly extract the original 128-bit
-      // vector.
-      bool UndefUppers = isUndefInRange(Mask, NumSrcElts, UpperElts);
-      Src = widenSubVector(Src, !UndefUppers, Subtarget, DAG, DL, 512);
-      unsigned NumWideSrcElts = Src.getValueType().getVectorNumElements();
-      if (NumWideSrcElts >= NumElts) {
-        // Widening means we can now use a regular TRUNCATE.
-        MVT WideVT = MVT::getVectorVT(VT.getScalarType(), NumWideSrcElts);
-        SDValue WideRes = DAG.getNode(ISD::TRUNCATE, DL, WideVT, Src);
-        if (!WideVT.is128BitVector())
-          WideRes = extract128BitVector(WideRes, 0, DAG, DL);
-        return WideRes;
-      }
-    }
-    return DAG.getNode(X86ISD::VTRUNC, DL, VT, Src);
+    return getAVX512TruncNode(DL, VT, Src, Subtarget, DAG, !UndefUppers);
   }
 
   return SDValue();