Re: portable way to get highest bit set?

Path	csiph.com!weretis.net!feeder8.news.weretis.net!eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail
From	Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups	comp.lang.c
Subject	Re: portable way to get highest bit set?
Date	Thu, 12 Oct 2023 02:41:25 -0700
Organization	A noiseless patient Spider
Lines	53
Message-ID	<86cyxkb2ka.fsf@linuxsc.com> (permalink)
References	<ug5gvh$1jkar$3@dont-email.me> <20231011102714.44a870af4dfe68f756974953@g{oogle}mail.com> <ug6huc$1rvp1$1@dont-email.me> <86h6mxawqq.fsf@linuxsc.com> <20231012111100.272c96b3209baad26a150e55@g{oogle}mail.com>
MIME-Version	1.0
Content-Type	text/plain; charset=us-ascii
Injection-Info	dont-email.me; posting-host="8a29b169449756014084ae3fe2a46ca6"; logging-data="2560063"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX185rh8UDkgyKWMicT8iBexYINHejVm3ec4="
User-Agent	Gnus/5.11 (Gnus v5.11) Emacs/22.4 (gnu/linux)
Cancel-Lock	sha1:M6SJag/U2cPSbeqNwsRwWnbxDSs= sha1:2S3LtOacVe7+wngz2jtb8/CudsU=
Xref	csiph.com comp.lang.c:236095

Show key headers only | View raw

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

> Tim Rentsch:
>
>> I've coded up a dozen[*] different solutions.
>> Here is my current avorite[**]:
>>
>>     typedef union {
>>         long double d;
>>         struct { unsigned long long s; unsigned e:15;} b;
>>     } X;
>>
>>     unsigned long long
>>     high_bit_set( unsigned long long u ){
>>         return  1ULL << (X){ .d = u }.b.e - 16383  & u;
>>     }
>
> And mine is quite anticlimactic.  It accepts a byte, because
> I wrote it yesterday, before learning that the OP needs a
> more generic solution:
>
>    unsigned char highest_bit( unsigned char byte )
>    {    int           i            ;
>         unsigned char mask = 1 << 7;
>         for( i = 0; i <= 8; i += 1 )
>         {    if( byte & mask )
>              {    break;  }
>              mask >>= 1;
>         }
>         return mask;
>    }
>
> It is the most straight-forward method that I could think
> of, and extensible to larger sizes of the variable.

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.
So something like the following, with type T being an
unknown unsigned integer type:

    T
    highest_bit_set( T u ){
        T r;
        /* set r to the highest bit set in u */
        return  r;
    }

I have written code for this form of the problem, but
won't be posting it before the weekend.  Try it!

P.S. My earlier "solution" was just for fun, not a serious
attempt to solve the problem.

Thread

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

csiph-web