allow ffs & co. a binary search

[Richard Yao]

On 06/11/2015 10:52 AM, Ian Lepore wrote:
> On Thu, 2015-06-11 at 10:49 -0400, Richard Yao wrote:
>> On 06/06/2015 08:13 PM, Erich Dollansky wrote:
> [...]
>>
>> The fastest portable way of calculating highest bit set is to use a
>> debrujin sequence:
>>
>> https://graphics.stanford.edu/~seander/bithacks.html#IntegerLogDeBruijn
>>
>> The binary approach is inferior, especially on Pentium 4 processors.
> 
> And of course it's crucial that we optimize for Pentium 4 processors in
> 2015.

The debrujin approach is fast regardless of the architecture. Anyway,
this is a solved problem. There is no need to reinvent the wheel when
the portable solutions are listed on that page at Stanford.

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