include: Implement N2867.

[des]

Summary

This adds macros for checked addition, subtraction, and multiplication with semantics similar to the builtins gcc and clang have had for years.

[emaste]

Once this goes in I will redo 20bd59416dcacbd2b776fe49dfa193900f303287 to use the new macros.

[kib]

include/stdckdint.h

8

These names pollute user namespace. Typically we prepend the underscore to upper-case symbols to avoid that.

[des]

include/stdckdint.h

8

I was considering just using __STDC_VERSION_STDCKDINT_H__ here, can you see any reason not to?

[des]

Use the feature macro as include guard.

[imp]

include/stdckdint.h

8

I forget: can we use #pragma once yet?

8

Though this is a good way to do it too

[des]

include/stdckdint.h

8

I forget: can we use #pragma once yet?

Rejected by WG14 because it was deemed too hard to describe portably. All the compilers we care about support it, though, so we could just decide to use it regardless. I wouldn't be surprised if it speeds up the build noticeably.

[kib]

include/stdckdint.h

19

Isn't __has_builtin clang-specific? Did you tested this with gcc? I suspect that gcc would always hit _Static_assert()s despite providing the built-ins.

[emaste]

include/stdckdint.h

19

(recent enough) GCC supports __has_builtin
https://gcc.gnu.org/onlinedocs/cpp/_005f_005fhas_005fbuiltin.html

In 20bd59416dca (when an earlier GCC version was relevant) I used:

#if __GNUC__ >= 5 || \
    (defined(__has_builtin) && __has_builtin(__builtin_add_overflow))
        if (__builtin_add_overflow(a, b, &result))
                errx(1, "Corrupt patch");
#else
        if ((b > 0 && a > OFF_MAX - b) || (b < 0 && a < OFF_MIN - b))
                errx(1, "Corrupt patch");
        result = a + b;
#endif

[des]

support gcc < 10

[des]

include/stdckdint.h

19

GCC has had __has_builtin() since 10.1, but I'm not 100% satisfied that we no longer care about GCC 9. I've regenerated the patch.

[emaste]

similar to my comment in the other review would be nice to briefly mention what N2867 is in the commit subject like include: Implement N2867, checked addition subtraction and multiplication or overflow-checked math or so

[des]

In D41734#951824, @emaste wrote:

similar to my comment in the other review would be nice to briefly mention what N2867 is in the commit subject