Salut Marc !

> We have a variable of type uintmax_t and want to count the leading zeros. Can 
> we just call __builtin_clzll on it?
Yes.

> In particular, can uintmax_t be larger than unsigned long long in gcc?

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


> 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.


FX

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