[llvm] [RISCV][GISel] Lower G_SADDE (PR #156865)

[Matt Arsenault via llvm-commits]

================
@@ -9288,6 +9290,40 @@ LegalizerHelper::lowerSADDO_SSUBO(MachineInstr &MI) {
   return Legalized;
 }
 
+LegalizerHelper::LegalizeResult LegalizerHelper::lowerSADDE(MachineInstr &MI) {
+  auto [Res, OvOut, LHS, RHS, CarryIn] = MI.getFirst5Regs();
+  const LLT Ty = MRI.getType(Res);
+  const LLT BoolTy = MRI.getType(OvOut);
+
+  Register NewRes = MRI.cloneVirtualRegister(Res);
+
+  // Step 1: tmp = LHS + RHS
+  auto Tmp = MIRBuilder.buildAdd(Ty, LHS, RHS);
+
+  // ov0 = (tmp < lhs) XOR (rhs < 0)
+  auto TmpLtLHS = MIRBuilder.buildICmp(CmpInst::ICMP_SLT, BoolTy, Tmp, LHS);
+  auto Zero = MIRBuilder.buildConstant(Ty, 0);
+  auto RHSLt0 = MIRBuilder.buildICmp(CmpInst::ICMP_SLT, BoolTy, RHS, Zero);
+  auto Ov0 = MIRBuilder.buildXor(BoolTy, TmpLtLHS, RHSLt0);
+
+  // Step 2: sum = tmp + zext(CarryIn)
+  auto CarryInZ = MIRBuilder.buildZExt(Ty, CarryIn);
+  MIRBuilder.buildAdd(NewRes, Tmp, CarryInZ);
+
+  // ov1 = CarryIn & (sum < tmp)
+  auto SumLtTmp = MIRBuilder.buildICmp(CmpInst::ICMP_SLT, BoolTy, NewRes, Tmp);
+  auto Ov1 = MIRBuilder.buildAnd(BoolTy, SumLtTmp, CarryIn);
+
+  // ov = ov0 | ov1
+  auto Ov = MIRBuilder.buildOr(BoolTy, Ov0, Ov1);
+
+  MIRBuilder.buildCopy(OvOut, Ov);
+  MIRBuilder.buildCopy(Res, NewRes);
----------------
arsenm wrote:

This case should be simple to just directly use the original result registers for the new instructions without a new copy 

https://github.com/llvm/llvm-project/pull/156865