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Re: portable way to get highest bit set?

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Anton Shepelev <anton.txt@g{oogle}mail.com> writes:

> Tim Rentsch:
>
>> But the approach you are using requires knowing the size
>> of the type involved.  The challenge is to write code that
>> works without having to know the size in advance.
>
> Well, I could use macros, or the largest integer type, or:

Neither necessary nor sufficient.  The code should work for
the type T being __uint128_t, which is larger than uintmax_t.

> #include <stddef.h>
> #include <limits.h>
>
> [..examine value character-by-character, including
>    determining if little-endian encoding is used..]

You're making things more complicated than they need to be.
Looking at character values rather than just values of the type
involved is almost guaranteed to have potential representation
issues.  For example, little endian and big endian are not the
only choices possible for endianness.

Below is a proposed solution that mostly works.  What problems
does it have, and how can they be fixed?  And can we find
something better?

    /* The type T has been supplied somewhere upstream.  It */
    /* is an unsigned type, and possibly outside the realm  */
    /* of 'integer types' as defined by the C standard, as  */
    /* for example __uint128_t.                             */

    T
    highest_bit_set( T u ){
        T r = 1;
        while(  u>>1 > r  )  r <<= 1;
        return  r;
    }

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Re: portable way to get highest bit set? Anton Shepelev <anton.txt@g{oogle}mail.com> - 2023-10-12 11:11 +0300
  Re: portable way to get highest bit set? Michael S <already5chosen@yahoo.com> - 2023-10-12 02:27 -0700
  Re: portable way to get highest bit set? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-10-12 02:41 -0700
    Re: portable way to get highest bit set? Anton Shepelev <anton.txt@g{oogle}mail.com> - 2023-10-12 14:17 +0300
      Re: portable way to get highest bit set? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-10-12 07:01 -0700
        Re: portable way to get highest bit set? scott@slp53.sl.home (Scott Lurndal) - 2023-10-12 14:18 +0000
          Re: portable way to get highest bit set? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-10-12 07:58 -0700
        Re: portable way to get highest bit set? Michael S <already5chosen@yahoo.com> - 2023-10-12 17:30 +0300
          Re: portable way to get highest bit set? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-10-12 08:05 -0700
            Re: portable way to get highest bit set? Michael S <already5chosen@yahoo.com> - 2023-10-12 19:16 +0300
        Re: portable way to get highest bit set? Anton Shepelev <anton.txt@g{oogle}mail.com> - 2023-10-12 18:31 +0300

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