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

[Benjamin Bellec]

Le 06/06/2011 23:54, Roland Scheidegger a écrit :
> Am 06.06.2011 23:18, schrieb Tormod Volden:
>> On Sun, Jun 5, 2011 at 1:14 AM, Benjamin Bellec wrote:
>>> So here is a v2 patch with a builtin GCC optimization which is the
>>> fastest (thx Matt to point me to this solution).
>>>
>>
>> From patch:
>> +       return (1 << (32 - __builtin_clz(x - 1)));
>>
>> I don't know if the use of gcc guarantees that int will always be 32
>> bit, otherwise maybe use sizeof(int)*8 instead of 32? Or even
>> sizeof(int)*CHAR_BIT for good measures. Although probably the robots
>> have taken over before this becomes necessary :)
>>
> 
> Hmm I think a lot more things will break if that's not 32bit.
> There's another problem though, gcc docs say this:
> 
> — Built-in Function: int __builtin_clz (unsigned int x)
> 
>     Returns the number of leading 0-bits in x, starting at the most
> significant bit position. If x is 0, the result is undefined.
> 
> Which means it's now undefined for x == 1 too - not handling x == 0
> correctly might not be much of a problem in practice, but the same
> certainly cannot be said for x == 1. So that should probably be
> 
> +#if defined(PIPE_CC_GCC)
> +   if (x <= 1)
> +       return 1;
> +   else
> +       return (1 << (32 - __builtin_clz(x - 1)));
> 
> 
> Also I believe this builtin requires gcc 3.4 - not sure though if the
> rest of the code compiles on older gcc.
> 
> Roland
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Here is the v4 patch.

Benjamin
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