Re: portable way to get highest bit set?

From	Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups	comp.lang.c
Subject	Re: portable way to get highest bit set?
Date	2023-10-15 01:44 -0700
Organization	A noiseless patient Spider
Message-ID	<86lec48ebs.fsf@linuxsc.com> (permalink)
References	<ug5gvh$1jkar$3@dont-email.me> <20231011143809.748@kylheku.com> <20231014222046.551@kylheku.com>

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Kaz Kylheku <864-117-4973@kylheku.com> writes:

> On 2023-10-11, Kaz Kylheku <864-117-4973@kylheku.com> wrote:
>
>> E.g. 32 bit code:
>>
>>    uint32_t fill_mask_down(uint32_t x)
>>    {
>>      x |= x >> 1;    // e.g.   1000...0000 -> 1100...0000
>>      x |= x >> 2;    // e.g.   1100...0000 -> 1111...0000
>>      x |= x >> 4;    // e.g.   11110000...  -> 11111111...
>>      x |= x >> 8;
>>      x |= x >> 16;
>>
>>      return x;
>>    }
>>
>> Thus:
>>
>>   uint32_t isolate_highest_bit(uint32_t x)
>>   {
>>      uint32_t m = fill_mask_down(x);
>>      return m ^ (m >> 1);
>>   }
>
> I guess this went over people's heads?

Not at all.  It's a well-known technique.  But it doesn't
address the central ask for an approach that works no
matter how wide the value type is.

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Re: portable way to get highest bit set? Kaz Kylheku <864-117-4973@kylheku.com> - 2023-10-15 05:26 +0000
  Re: portable way to get highest bit set? Michael S <already5chosen@yahoo.com> - 2023-10-15 00:28 -0700
  Re: portable way to get highest bit set? Michael S <already5chosen@yahoo.com> - 2023-10-15 11:35 +0300
    Re: portable way to get highest bit set? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-10-15 02:52 -0700
  Re: portable way to get highest bit set? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-10-15 01:44 -0700

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