The C language doesn't, and has never, said anything about what the assembled code should look like. If you want performance, you have to use a good compiler. That's true with any language.
You're right, C11 doesn't have a 128-bit integer type; that should be remedied. Fortunately all major compilers (GCC, LLVM, VC++, and ICC) provide one.
Carry: "if (x + y < x)" compiles to code that uses the carry flag.
Overflow: "if ((int32_t) (x + y) < 0)" compiles to code that uses the overflow flag.
And the best part is, code written like this is portable to architectures such as TileGX that don't have status flags.
> Carry: "if (x + y < x)" compiles to code that uses the carry flag.
If x and y are signed, you're relying on UB, which the compiler might optimize out.
> "if ((int32_t) (x + y) < 0)" compiles to code that uses the overflow flag.
Again, which type are x and y? If signed, this code is UB. If unsigned, then the result of conversion to int32 may not fit into int32, in which case the result is implementation-defined, or and implementation-defined signal is raised.
You're right, C11 doesn't have a 128-bit integer type; that should be remedied. Fortunately all major compilers (GCC, LLVM, VC++, and ICC) provide one.
Carry: "if (x + y < x)" compiles to code that uses the carry flag.
Overflow: "if ((int32_t) (x + y) < 0)" compiles to code that uses the overflow flag.
And the best part is, code written like this is portable to architectures such as TileGX that don't have status flags.