[DAGCombiner] Add use check for VSCALE in visitSUB.

[Jimerlife]

Summary

If there are multiple uses of VSCALE, then it may be more beneficial to keep origin form.

[RKSimon]

test case?

[Jimerlife]

In D132115#3731453, @RKSimon wrote:

test case?

I add precommit tests D132116. After tests landed, I will rebase this patch.

[Jimerlife]

rebase test

[david-arm]

LGTM! Seems like a sensible change.

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

if (N1.getOpcode() == ISD::SIGN_EXTEND_INREG) {
     if (TN->getVT() == MVT::i1) {
       SDValue ZExt = DAG.getNode(ISD::AND, DL, VT, N1.getOperand(0),
                                  DAG.getConstant(1, DL, VT));
       return DAG.getNode(ISD::ADD, DL, VT, N0, ZExt);
     }
   }
 
   // canonicalize (sub X, (vscale * C)) to (add X, (vscale * -C))
-  if (N1.getOpcode() == ISD::VSCALE) {
+  if (N1.getOpcode() == ISD::VSCALE && N1.hasOneUse()) {
     const APInt &IntVal = N1.getConstantOperandAPInt(0);
     return DAG.getNode(ISD::ADD, DL, VT, N0, DAG.getVScale(DL, VT, -IntVal));
   }
 
   // canonicalize (sub X, step_vector(C)) to (add X, step_vector(-C))
   if (N1.getOpcode() == ISD::STEP_VECTOR && N1.hasOneUse()) {
     APInt NewStep = -N1.getConstantOperandAPInt(0);
     return DAG.getNode(ISD::ADD, DL, VT, N0,

llvm/test/CodeGen/AArch64/sve-vscale-combine.ll

  ret i32 %sub
 }
 
 ; Tests of multiple uses of vscale when canonicalize
 ; (sub X, (vscale * C)) to (add X,  (vscale * -C))
 define i64 @multiple_uses_sub_vscale_i64(i64 %x, i64 %y) nounwind {
 ; CHECK-LABEL: multiple_uses_sub_vscale_i64:
 ; CHECK-NEXT:  rdvl x8, #1
-; CHECK-NEXT:  rdvl x9, #-1
 ; CHECK-NEXT:  lsr  x8, x8, #4
-; CHECK-NEXT:  asr  x9, x9, #4
-; CHECK-NEXT:  add  x9, x0, x9
+; CHECK-NEXT:  sub  x9, x0, x8
 ; CHECK-NEXT:  add  x8, x1, x8
 ; CHECK-NEXT:  mul  x0, x9, x8
 ; CHECK-NEXT:  ret
  %vscale = call i64 @llvm.vscale.i64()
  %sub = sub i64 %x, %vscale
  %add = add i64 %y, %vscale
  %res = mul i64 %sub, %add
  ret i64 %res