[FFmpeg-devel] [RFC] abs vs FFABS

[Måns Rullgård]

"Ronald S. Bultje" <rsbultje at gmail.com> writes:

> Hi Uoti,
>
> On Sat, Jan 17, 2009 at 11:10 AM, Uoti Urpala <uoti.urpala at pp1.inet.fi> wrote:
>> The compiler should be able to optimize that whole function to
>> "return 0;", and current GCC does.
>
> The idea is that it doesn't because the input is (semi-)random. The
> abs()/FFABS() is done on a calculated number per cycle. I wish I could
> check assembler to ensure it does, but I don't think it can just be
> optimized to return 0, because every outcome is checked (see test_end)
> and it returns 1 if it is wrong.

The result is discarded, so there is no need to calculate it at all.
Why don't you check the assembler yourself and see that the loops are
gone.

-- 
M?ns Rullg?rd
mans at mansr.com