Re: [PATCH 2/2] use __builtin_add_overflow() in st_add() with Clang

[Junio C Hamano <gitster@pobox.com>]

René Scharfe <l.s.r@web.de> writes:

> Provide a variant of st_add() that wraps __builtin_add_overflow() to
> help Clang optimize it.  Use it on all platforms for simplicity.
> ...
> +/* Help Clang; GCC generates the same code for both variants. */
> +#if defined(__clang__)
> +static inline size_t st_add(size_t a, size_t b)
> +{
> +	size_t sum;
> +	if (__builtin_add_overflow(a, b, &sum))
> +		die("size_t overflow: %"PRIuMAX" + %"PRIuMAX,
> +		    (uintmax_t)a, (uintmax_t)b);
> +	return sum;
> +}
> +#else
>  static inline size_t st_add(size_t a, size_t b)
>  {
>  	if (unsigned_add_overflows(a, b))
> @@ -621,6 +632,7 @@ static inline size_t st_add(size_t a, size_t b)
>  		    (uintmax_t)a, (uintmax_t)b);
>  	return a + b;
>  }
> +#endif

Makes me wonder if we tweaked unsigned_add_overflows() to take an
extra *dst parameter to match __builtin_add_overflow(), which of
course requires us to all of 18 callsites, it might make the whole
thing a bit simpler.  New uses of unsigned_add_overflows(), if we
ever add them, would automatically benefit, right?