[FFmpeg-devel] [PATCH] lavc/cook: get rid of wasteful pow in init_pow2table

[Ganesh Ajjanagadde]

On Tue, Dec 29, 2015 at 1:12 PM, Ganesh Ajjanagadde <gajjanag at mit.edu> wrote:
> On Tue, Dec 29, 2015 at 11:29 AM, Clément Bœsch <u at pkh.me> wrote:
>> On Tue, Dec 29, 2015 at 09:28:34AM -0800, Ganesh Ajjanagadde wrote:
>>> The table is highly structured, so pow (or exp2 for that matter) can entirely
>>> be avoided, yielding a ~ 40x speedup with no loss of accuracy.
>>>
>>> sample benchmark (Haswell, GNU/Linux):
>>> new:
>>> 4449 decicycles in init_pow2table(loop 1000),     254 runs,      2 skips
>>> 4411 decicycles in init_pow2table(loop 1000),     510 runs,      2 skips
>>> 4391 decicycles in init_pow2table(loop 1000),    1022 runs,      2 skips
>>>
>>> old:
>>> 183673 decicycles in init_pow2table(loop 1000),     256 runs,      0 skips
>>> 182142 decicycles in init_pow2table(loop 1000),     512 runs,      0 skips
>>> 182104 decicycles in init_pow2table(loop 1000),    1024 runs,      0 skips
>>>
>>> Signed-off-by: Ganesh Ajjanagadde <gajjanagadde at gmail.com>
>>> ---
>>>  libavcodec/cook.c | 11 +++++++++--
>>>  1 file changed, 9 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/libavcodec/cook.c b/libavcodec/cook.c
>>> index d8fb736..aa434a2 100644
>>> --- a/libavcodec/cook.c
>>> +++ b/libavcodec/cook.c
>>> @@ -166,10 +166,17 @@ static float rootpow2tab[127];
>>>  /* table generator */
>>>  static av_cold void init_pow2table(void)
>>>  {
>>> +    /* fast way of computing 2^i and 2^(0.5*i) for -63 <= i < 64 */
>>>      int i;
>>> +    static const float exp2_tab[2] = {1, M_SQRT2};
>>
>>> +    float exp2_val = 1.0842021724855044e-19; /* 2^(-63) */
>>> +    float root_val = 2.3283064365386963e-10; /* 2^(-32) */
>>
>> I'm pretty sure you can do
>>     float exp2_val = pow(2, -63);
>>     float root_val = pow(2, -32);
>> and compilers will inline them
>
> Any decent compiler would.

Confirmed on gcc that they are identical binaries. Pushed, thanks.

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