[FFmpeg-devel] [PATCH] avutil/mathematics: speed up av_gcd by using Stein's binary GCD algorithm

[Ganesh Ajjanagadde]

On Sat, Oct 10, 2015 at 5:45 PM, Ganesh Ajjanagadde <gajjanag at mit.edu> wrote:
> On Sat, Oct 10, 2015 at 5:32 PM, Henrik Gramner <henrik at gramner.com> wrote:
>> On Sat, Oct 10, 2015 at 11:06 PM, Ganesh Ajjanagadde
>> <gajjanagadde at gmail.com> wrote:
>>> This uses Stein's binary GCD algorithm:
>>> https://en.wikipedia.org/wiki/Binary_GCD_algorithm
>>> to get a reported 1.7-2.1x speedup over Euclidean GCD on standard architectures.
>>> Have not benchmarked, so I can't comment
>>
>> Before you submit a patch that's supposed to make something faster,
>> you should benchmark it to verify that it is in fact faster. Do this
>> with inputs of various sizes on both 32- and 64-bit architectures and
>> both with and without compilers that support __builtin_ctzll(v).
>
> I did not do this because I don't know how to write a benchmark in C
> easily: for instance, printing out can lead to I/O time loss, and on
> O3, unless there is I/O, a function may not be evaluated at all due to
> the "as if" optimization rules. Another problem is that the function
> is small, so I can't just time a single call, there are cache issues
> with arrays, etc etc. Yes, I can do it - but it will take some effort.
>
> Also, the only compilers I have are clang and gcc, both of which have
> the builtin. I figured someone with a better array of tools and more
> knowledgable in writing benchmarks can trivially do this. Furthermore,
> at the moment, comparisons are incomplete: I don't care about Windows
> myself but one could trivially add the lines if one so desired (see my
> commit message).
>
> Lastly, I can't find any links on how to do benchmarks cleanly,
> efficiently, and accurately. I hoped that FFmpeg has some clean way of
> doing this, but all the methods I see are terribly ad-hoc.

So I hacked up a quick and dirty benchmark. I fill up two input arrays
with 10^8 entries each via random() calls. I then start a timer, and
compute using the old method the gcd, inputs drawn from the arrays.
Then I used the new method, repeating the above.
Results (on my Intel Haswell, GCC 5.2):
Old method: 22.755636
New method: 6.495941

a substantial improvement.

>
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