Re: __builtin_clzll and uintmax_t

[Marc Glisse <marc dot glisse at inria dot fr>]

Coucou FX,

On Sat, 5 Mar 2011, FX wrote:

> uintmax_t is the largest of the standard unsigned C types, so it cannot be larger than unsigned long long.

That's a gcc property then. The C99 standard only guarantees that 
uintmax_t is at least as large as unsigned long long, but it is allowed to 
be some other larger type:

"The following type designates an unsigned integer type capable of 
representing any value of any unsigned integer type: uintmax_t"


> On x86_64, for example:
>
> > #include <stdio.h>
> > #include <stdint.h>
> >
> > int main (void)
> > {
> >   printf ("%lu ", sizeof (uintmax_t));
> >   printf ("%lu ", sizeof (int));
> >   printf ("%lu ", sizeof (long int));
> >   printf ("%lu ", sizeof (long long int));
> >   printf ("%lu\n", sizeof (__int128));
> > }
>
> gives : 8 4 8 8 16

I am not sure how legal that is. __int128 is an extended signed integer 
type, and thus the statement about intmax_t should apply to it as well. So 
gcc is just pretending that __int128 is not really there.

> > Is __builtin_clzll available on all platforms?
>
> Yes, we emit calls to this built-in unconditionally in the Fortran 
> front-end, and it has caused no trouble.

Thank you, that's the best guarantee I could ask for about the existence 
of __builtin_clzll.

--
Marc Glisse