[llvm] [GlobalIsel] Combine ADDO (PR #82927)

[Sergei Barannikov via llvm-commits]

================
@@ -6918,3 +6920,199 @@ bool CombinerHelper::matchOr(MachineInstr &MI, BuildFnTy &MatchInfo) {
 
   return false;
 }
+
+bool CombinerHelper::matchAddOverflow(MachineInstr &MI, BuildFnTy &MatchInfo) {
+  GAddCarryOut *Add = cast<GAddCarryOut>(&MI);
+
+  // Addo has no flags
+  Register Dst = Add->getReg(0);
+  Register Carry = Add->getReg(1);
+  Register LHS = Add->getLHSReg();
+  Register RHS = Add->getRHSReg();
+  bool IsSigned = Add->isSigned();
+  LLT DstTy = MRI.getType(Dst);
+  LLT CarryTy = MRI.getType(Carry);
+
+  // We want do fold the [u|s]addo.
+  if (!MRI.hasOneNonDBGUse(Dst))
+    return false;
+
+  // Fold addo, if the carry is dead -> add, undef.
+  if (MRI.use_nodbg_empty(Carry) &&
+      isLegalOrBeforeLegalizer({TargetOpcode::G_ADD, {DstTy}})) {
+    MatchInfo = [=](MachineIRBuilder &B) {
+      B.buildAdd(Dst, LHS, RHS);
+      B.buildUndef(Carry);
+    };
+    return true;
+  }
+
+  // We want do fold the [u|s]addo.
+  if (!MRI.hasOneNonDBGUse(Carry))
+    return false;
+
+  // Canonicalize constant to RHS.
+  if (isConstantOrConstantVectorI(LHS) && !isConstantOrConstantVectorI(RHS)) {
+    if (IsSigned) {
+      MatchInfo = [=](MachineIRBuilder &B) {
+        B.buildSAddo(Dst, Carry, RHS, LHS);
+      };
+      return true;
+    } else {
+      MatchInfo = [=](MachineIRBuilder &B) {
+        B.buildUAddo(Dst, Carry, RHS, LHS);
+      };
+      return true;
+    }
+  }
+
+  std::optional<APInt> MaybeLHS = getConstantOrConstantSplatVector(LHS);
+  std::optional<APInt> MaybeRHS = getConstantOrConstantSplatVector(RHS);
+
+  // Fold addo(c1, c2) -> c3, carry.
+  if (MaybeLHS && MaybeRHS && isConstantLegalOrBeforeLegalizer(DstTy) &&
+      isConstantLegalOrBeforeLegalizer(CarryTy)) {
+    // They must both have the same bitwidth. Otherwise APInt might
+    // assert. Pre legalization, they may have widely different bitwidths.
----------------
s-barannikov wrote:

> Pre legalization, they may have widely different bitwidths.

How is that possible? Is this because `getConstantOrConstantSplatVector` peeks through exts/truncs?
There doesn't seem to be a test for this.


https://github.com/llvm/llvm-project/pull/82927