[Mesa-dev] [PATCH] faster util_next_power_of_two() function

[Brian Paul]

On Sun, Jun 5, 2011 at 11:34 AM, Ian Romanick <idr at freedesktop.org> wrote:
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> On 06/02/2011 04:40 PM, Brian Paul wrote:
>> On 06/02/2011 03:18 PM, Benjamin Bellec wrote:
>>> It's me again, with a new minor optimization for the
>>> util_next_power_of_two() function.
>>>
>>> This patch implements a faster algorithm, still taken from Wikipedia (
>>> http://en.wikipedia.org/wiki/Power_of_two#Algorithm_to_round_up_to_power_of_two
>>>
>>> ).
>>> But especially, I added the most common values used by this function. I
>>> noted that ETQW, TA Spring and ExtremeTuxRacer call often "good" values.
>>> I can think that most OpenGL apps uses also these values.
>>>
>>> I have benchmarked this. With -O2 optimization, this new function is
>>> twice faster than the former. Without GCC optimization the difference is
>>> even bigger.
>>>
>>> There is no piglit regressions.
>>>
>>> But there is an issue, during compilation there is an error (-Wall)
>>> "warning: right shift count>= width of type [enabled by default]".
>>
>> I'd like to avoid the warning if at all possible.  If you replace (val
>>>> 32) with (val >> (sizeof(unsigned) * 4)) does that silence it?
>
> It will because it will evaluate to val >> 16, which is not what's wanted.

It would evaluate to 32 (8*4) when relevant.  Please note the conditional.

-Brian