[llvm] [RISCV] Vector sub (zext, zext) -> sext (sub (zext, zext)) (PR #82455)

[Philip Reames via llvm-commits]

================
@@ -12846,21 +12846,44 @@ static SDValue performSUBCombine(SDNode *N, SelectionDAG &DAG,
   if (SDValue V = combineSubOfBoolean(N, DAG))
     return V;
 
+  EVT VT = N->getValueType(0);
   SDValue N0 = N->getOperand(0);
   SDValue N1 = N->getOperand(1);
   // fold (sub 0, (setcc x, 0, setlt)) -> (sra x, xlen - 1)
   if (isNullConstant(N0) && N1.getOpcode() == ISD::SETCC && N1.hasOneUse() &&
       isNullConstant(N1.getOperand(1))) {
     ISD::CondCode CCVal = cast<CondCodeSDNode>(N1.getOperand(2))->get();
     if (CCVal == ISD::SETLT) {
-      EVT VT = N->getValueType(0);
       SDLoc DL(N);
       unsigned ShAmt = N0.getValueSizeInBits() - 1;
       return DAG.getNode(ISD::SRA, DL, VT, N1.getOperand(0),
                          DAG.getConstant(ShAmt, DL, VT));
     }
   }
 
+  // sub (zext, zext) -> sext (sub (zext, zext))
+  //   where the sum of the extend widths match, and the inner zexts
+  //   add at least one bit.  (For profitability on rvv, we use a
+  //   power of two for both inner and outer extend.)
+  if (VT.isVector() && Subtarget.getTargetLowering()->isTypeLegal(VT) &&
+      N0.getOpcode() == N1.getOpcode() && N0.hasOneUse() && N1.hasOneUse() &&
+      N0.getOpcode() == ISD::ZERO_EXTEND) {
+    SDValue Src0 = N0.getOperand(0);
+    SDValue Src1 = N1.getOperand(0);
+    EVT SrcVT = Src0.getValueType();
+    if (Subtarget.getTargetLowering()->isTypeLegal(SrcVT) &&
+        SrcVT == Src1.getValueType() && SrcVT.getScalarSizeInBits() >= 8 &&
+        SrcVT.getScalarSizeInBits() < VT.getScalarSizeInBits() / 2) {
+      LLVMContext &C = *DAG.getContext();
+      EVT ElemVT = VT.getVectorElementType().getHalfSizedIntegerVT(C);
----------------
preames wrote:

The combiner will iterate won't it?  So first we'd produce  (i64 (sext (i32 (sub (zext i8), (zext i8)))) and then the transform would run again producing  (i64 (sext (i16 (sub (zext i8), (zext i8)))).  (There'd be one extra step to fold the two sext together.)

https://github.com/llvm/llvm-project/pull/82455