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

[Benjamin Bellec]

Le 06/06/2011 17:34, Roland Scheidegger a écrit :
> Am 05.06.2011 03:55, schrieb Benjamin Bellec:
>> Le 05/06/2011 03:05, Matt Turner a écrit :
>>> On Sat, Jun 4, 2011 at 7:14 PM, Benjamin Bellec <b.bellec at gmail.com> wrote:
>>>> Le 03/06/2011 06:09, Matt Turner a écrit :
>>>>> Also, if you want to check if the value is already a power-of-two,
>>>>> instead of a case statement for every POT (including 0), just do the
>>>>> standard is-power-of-two check:
>>>>>
>>>>> (x & (x - 1)) == 0
>>>>
>>>> My own tests (on a Core2) shows that it's less efficient to do that, at
>>>> least with -O2 optimization enabled. With -O0, it's equal.
>>>
>>> For what input set? Powers of two?
>> Both, my test case loops with 29 POT and 6 NPOT.
>> I'm doing this because the OpenGL games that I have tested call the
>> function more often with "good" values.
>>
>>>
>>> Doesn't really matter, since the function isn't a hot path or
>>> anything, but I'd suppose that the Linux kernel has its
>>> is_power_of_2() function for a reason--that it's pretty ugly to have
>>> lots of case statements like powers of two.
>>>
>>> Matt
>> Ok, so here is a v3 patch which replace the switch statement.
> 
> I like this one better too.
> Do we actually need the x == 0 special case or would it be ok if we just
> remove that (which will return 0)?
> 
> Roland

If we delete this check, we need to specify (in a comment) that the
function have an undetermined behavior when passing 0 in parameters. Why
not, but then the most important thing to know is from where the
parameters came from: "mesa" or "GL apps"? If it's from GL apps it could
be dangerous to change the result of this function, even if 0 is a
stupid input (no?).

Benjamin