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

[Craig Topper via llvm-commits]

================
@@ -12846,21 +12846,43 @@ 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.)
----------------
topperc wrote:

how are we guaranteeing power of 2 here?

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