[clang] [clang] Allow builtin addc/subc to be constant evaluated (PR #81656)

[Aaron Ballman via cfe-commits]

================
@@ -12696,6 +12696,59 @@ bool IntExprEvaluator::VisitBuiltinCallExpr(const CallExpr *E,
     return BuiltinOp == Builtin::BI__atomic_always_lock_free ?
         Success(0, E) : Error(E);
   }
+  case Builtin::BI__builtin_addcb:
+  case Builtin::BI__builtin_addcs:
+  case Builtin::BI__builtin_addc:
+  case Builtin::BI__builtin_addcl:
+  case Builtin::BI__builtin_addcll:
+  case Builtin::BI__builtin_subcb:
+  case Builtin::BI__builtin_subcs:
+  case Builtin::BI__builtin_subc:
+  case Builtin::BI__builtin_subcl:
+  case Builtin::BI__builtin_subcll: {
+    LValue CarryOutLValue;
+    APSInt LHS, RHS, CarryIn, CarryOut, Result;
+    QualType ResultType = E->getArg(0)->getType();
+    if (!EvaluateInteger(E->getArg(0), LHS, Info) ||
+        !EvaluateInteger(E->getArg(1), RHS, Info) ||
+        !EvaluateInteger(E->getArg(2), CarryIn, Info) ||
+        !EvaluatePointer(E->getArg(3), CarryOutLValue, Info))
+      return false;
+    // Copy the number of bits and sign
+    Result = LHS;
+    CarryOut = LHS;
+
+    bool FirstOverflowed = false;
+    bool SecondOverflowed = false;
+    switch (BuiltinOp) {
+    default:
+      llvm_unreachable("Invalid value for BuiltinOp");
+    case Builtin::BI__builtin_addcb:
+    case Builtin::BI__builtin_addcs:
+    case Builtin::BI__builtin_addc:
+    case Builtin::BI__builtin_addcl:
+    case Builtin::BI__builtin_addcll:
+      Result =
+          LHS.uadd_ov(RHS, FirstOverflowed).uadd_ov(CarryIn, SecondOverflowed);
+      break;
+    case Builtin::BI__builtin_subcb:
+    case Builtin::BI__builtin_subcs:
+    case Builtin::BI__builtin_subc:
+    case Builtin::BI__builtin_subcl:
+    case Builtin::BI__builtin_subcll:
+      Result =
+          LHS.usub_ov(RHS, FirstOverflowed).usub_ov(CarryIn, SecondOverflowed);
+      break;
+    }
+
+    // It is possible for both overflows to happen but CGBuiltin uses an OR so
+    // this is consistent
----------------
AaronBallman wrote:

```suggestion
    // this is consistent.
```

https://github.com/llvm/llvm-project/pull/81656