[clang] Allow builtin addc/subc to be constant evaluated

Author: Bryce Wilson

clang/docs/LanguageExtensions.rst

@@ -5241,6 +5241,11 @@ Intrinsics Support within Constant Expressions
 
 The following builtin intrinsics can be used in constant expressions:
 
+* ``__builtin_addcb``
+* ``__builtin_addcs``
+* ``__builtin_addc``
+* ``__builtin_addcl``
+* ``__builtin_addcll``
 * ``__builtin_bitreverse8``
 * ``__builtin_bitreverse16``
 * ``__builtin_bitreverse32``
@@ -5287,6 +5292,11 @@ The following builtin intrinsics can be used in constant expressions:
 * ``__builtin_rotateright16``
 * ``__builtin_rotateright32``
 * ``__builtin_rotateright64``
+* ``__builtin_subcb``
+* ``__builtin_subcs``
+* ``__builtin_subc``
+* ``__builtin_subcl``
+* ``__builtin_subcll``
 
 The following x86-specific intrinsics can be used in constant expressions:
 

clang/include/clang/Basic/Builtins.td

@@ -4061,14 +4061,14 @@ class MPATemplate : Template<
 
 def Addc : Builtin, MPATemplate {
   let Spellings = ["__builtin_addc"];
-  let Attributes = [NoThrow];
+  let Attributes = [NoThrow, Constexpr];
   // FIXME: Why are these argumentes marked const?
   let Prototype = "T(T const, T const, T const, T*)";
 }
 
 def Subc : Builtin, MPATemplate {
   let Spellings = ["__builtin_subc"];
-  let Attributes = [NoThrow];
+  let Attributes = [NoThrow, Constexpr];
   // FIXME: Why are these argumentes marked const?
   let Prototype = "T(T const, T const, T const, T*)";
 }

clang/lib/AST/ExprConstant.cpp

@@ -12691,6 +12691,56 @@ 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, 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;
+
+    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
+    APSInt API{(uint32_t)(FirstOverflowed | SecondOverflowed)};
+    APValue APV{API};
+    if (!handleAssignment(Info, E, CarryOutLValue, ResultType, APV))
+      return false;
+    return Success(Result, E);
+  }
   case Builtin::BI__builtin_add_overflow:
   case Builtin::BI__builtin_sub_overflow:
   case Builtin::BI__builtin_mul_overflow: