Re: [PATCH v2] _Generic.3: New page documenting _Generic()

[Alejandro Colomar <alx.manpages@gmail.com>]

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On 9/4/22 23:16, Alejandro Colomar wrote:
> Hi Florian,
> 
> On 8/23/22 09:58, Florian Weimer wrote:
>> Note that this approach does not really work that well in practice
>> because macros using _Generic expand all the alternatives (in current
>> implementations; doing this differently requires deviating from the
>> layered implementation strategy suggested in the C standard).  This
>> means that _Generic-using macros can only be nested maybe three or four
>> levels deep, depending on the number of _Generic alternatives on each
>> level.  For <tgmath.h>, this is really not enough, so a high-quality
>> implementation of <tgmath.h> using _Generic is not feasible.  GCC
>> provides __builtin_tgmath, which is designed in such a way that when
>> used in a macro, the macro argument is only expanded once.
>>
>> Maybe mention this under BUGS?
>>
>> C++ templates do not suffer from this particular problem.
> 
> 
> Heh, I don't know how this didn't oocur to me before.  Well, maybe it's 
> because it's non-standard (but the standard might very well benefit from 
> adding this, IMO).
> 
> An always_inline function with no extern definition behaves as if it 
> were a macro (trying to take a pointer to it, or something that needs a 
> linker symbol, will result in linker errors, which is not the most 
> readable error, but good enough), in the sense that it doesn't have ABI 
> issues, but has the benefit of not creating code exponentially.
> 
> 
> Cheers,
> 
> Alex
> 
> ---
> 
> diff --git a/man3/_Generic.3 b/man3/_Generic.3
> index f3daf98c1..3bd5f306c 100644
> --- a/man3/_Generic.3
> +++ b/man3/_Generic.3
> @@ -30,7 +30,9 @@ C11 and later.
>   The following program demonstrates how to write
>   a replacement for the standard
>   .BR imaxabs (3)
> -function, which being a function can't really provide what it promises:
> +function, which being an
> +.I extern
> +function can't really provide what it promises:
>   seamlessly upgrading to the widest available type.
>   .PP
>   .\" SRC BEGIN (_Generic.c)
> @@ -39,11 +41,16 @@ seamlessly upgrading to the widest available type.
>   #include <stdio.h>
>   #include <stdlib.h>
> 
> -#define my_imaxabs(j)  _Generic(INTMAX_C(0),  \e
> -    long:           labs(j),                  \e
> -    long long:      llabs(j)                  \e
> - /* long long long: lllabs(j) */              \e
> -)
> +[[gnu::always_inline]]
> +inline intmax_t
> +my_imaxabs(intmax_t j)
> +{
> +    return _Generic(j,
> +        long:           labs(j),
> +        long long:      llabs(j)
> +     /* long long long: lllabs(j) */
> +    )
> +}
> 
>   int
>   main(void)
> 

Huh, although this goes back again to macros, and potentially 
exponential code, I think it's quite clean and minimal, and has the 
benefit that it allows one to take pointers:

$ cat gen.c
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>

#define my_imaxabs  _Generic(INTMAX_C(0),  \
         long:           labs,              \
         long long:      llabs              \
      /* long long long: lllabs */          \
)

int
main(void)
{
         off_t  a;

         a = -42;
         printf("imaxabs(%jd) == %jd\n", (intmax_t) a, my_imaxabs(a));
         printf("&imaxabs == %p\n", &my_imaxabs);
         printf("&labs    == %p\n", &labs);
         printf("&llabs   == %p\n", &llabs);

         exit(EXIT_SUCCESS);
}

$ cc -Wall -Wextra gen.c
$ ./a.out
imaxabs(-42) == 42
&imaxabs == 0x7ff5aa2407a0
&labs    == 0x7ff5aa2407a0
&llabs   == 0x7ff5aa2407b0


Nice experiment :)

Cheers,

Alex


-- 
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