[llvm] [AArch64][SVE] Rework VECTOR_COMPRESS lowering (PR #171162)

[Benjamin Maxwell via llvm-commits]

================
@@ -7445,60 +7448,43 @@ static SDValue convertFromSVEContainerType(SDLoc DL, SDValue Vec, EVT VecVT,
   return Vec;
 }
 
-SDValue AArch64TargetLowering::LowerVECTOR_COMPRESS(SDValue Op,
-                                                    SelectionDAG &DAG) const {
+SDValue AArch64TargetLowering::LowerFixedLengthVectorCompressToSVE(
+    SDValue Op, SelectionDAG &DAG) const {
   SDLoc DL(Op);
-  SDValue Vec = Op.getOperand(0);
-  SDValue Mask = Op.getOperand(1);
-  SDValue Passthru = Op.getOperand(2);
-  EVT VecVT = Vec.getValueType();
-  EVT MaskVT = Mask.getValueType();
-  EVT ElmtVT = VecVT.getVectorElementType();
-  const bool IsFixedLength = VecVT.isFixedLengthVector();
-  const bool HasPassthru = !Passthru.isUndef();
-  unsigned MinElmts = VecVT.getVectorElementCount().getKnownMinValue();
-  EVT FixedVecVT = MVT::getVectorVT(ElmtVT.getSimpleVT(), MinElmts);
+  EVT VT = Op.getValueType();
 
-  assert(VecVT.isVector() && "Input to VECTOR_COMPRESS must be vector.");
+  EVT ContainerVT = getContainerForFixedLengthVector(DAG, VT);
+  SDValue Vec = convertToScalableVector(DAG, ContainerVT, Op.getOperand(0));
+  SDValue Mask = convertFixedMaskToScalableVector(Op.getOperand(1), DAG);
+  SDValue Passthru =
+      convertToScalableVector(DAG, ContainerVT, Op.getOperand(2));
 
-  if (!Subtarget->isSVEAvailable())
-    return SDValue();
+  SDValue Result =
+      DAG.getNode(ISD::VECTOR_COMPRESS, DL, ContainerVT, Vec, Mask, Passthru);
+  return convertFromScalableVector(DAG, VT, Result);
+}
 
-  if (IsFixedLength && VecVT.getSizeInBits().getFixedValue() > 128)
-    return SDValue();
+SDValue AArch64TargetLowering::LowerVECTOR_COMPRESS(SDValue Op,
+                                                    SelectionDAG &DAG) const {
+  SDLoc DL(Op);
+  EVT VT = Op.getValueType();
+  if (VT.isFixedLengthVector())
+    return LowerFixedLengthVectorCompressToSVE(Op, DAG);
----------------
MacDue wrote:

Yes, this is a common pattern for these lowerings. This converts the arguments to scalable vectors, which produces another `VECTOR_COMPRESS` node, which then is custom lowered again.  

https://github.com/llvm/llvm-project/pull/171162