[llvm] [AArch64] fuse constant addition after sbb (PR #185117)

[David Green via llvm-commits]

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@@ -23346,6 +23346,36 @@ static SDValue performAddTruncShiftCombine(SDNode *N, SelectionDAG &DAG) {
   return DAG.getNode(ISD::ADD, DL, VT, Trunc, Shift);
 }
 
+// Fold ADD(SBC(Y, 0, W), C) -> SBC(Y, -C, W)
+// SBC(Y, 0, W) = Y - 0 - ~carry = Y + carry - 1
+// Adding C:  Y + carry - 1 + C = Y - (-C) - ~carry = SBC(Y, -C, W)
+static SDValue performAddWithSBCCombine(SDNode *N, SelectionDAG &DAG) {
+  if (N->getOpcode() != ISD::ADD)
+    return SDValue();
+  EVT VT = N->getValueType(0);
+  if (VT != MVT::i32 && VT != MVT::i64)
+    return SDValue();
+
+  SDValue SBC = N->getOperand(0);
+  SDValue C = N->getOperand(1);
+  // ADD is commutative; constant may be on either side.
+  if (SBC.getOpcode() != AArch64ISD::SBC)
+    std::swap(SBC, C);
+  if (SBC.getOpcode() != AArch64ISD::SBC || !SBC.hasOneUse())
+    return SDValue();
+  if (!isNullConstant(SBC.getOperand(1)))
+    return SDValue();
+  // AArch64 SBC (non-flag-setting) has only one output; no flags guard needed.
+  auto *CC = dyn_cast<ConstantSDNode>(C);
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davemgreen wrote:

Is it worth doing it for non-const too, to allow the neg to start earlier and create a possibly shorted critical path?

https://github.com/llvm/llvm-project/pull/185117