Re: [PATCH v2 3/4] libav.inc: duplicate armv7a over-rides for armv7ve

Khem Raj
2016-01-11T22:58:13Z

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> On Jan 7, 2016, at 3:40 PM, Phil Blundell <pb@pbcl.net> wrote:
>
> On Thu, 2016-01-07 at 14:29 -0800, Andre McCurdy wrote:
>> FULL_OPTIMIZATION_armv7a = "-fexpensive-optimizations -fomit-frame-pointer -O4 -ffast-math"
>> +FULL_OPTIMIZATION_armv7ve = "-fexpensive-optimizations -fomit-frame-pointer -O4 -ffast-math"
>

O4 ? may be just replace this whole thing with -Ofast now a days
-fexpensive-optimizations is enabled at -O2,-O3 anyway same for fomit-frame-pointer and -Ofast turns on
feast-match on top of -O3

> I wonder if this would be better as just FULL_OPTIMIZATION_arm. That
> said, the OPTIMIZATION variables are primarily distro knobs and it's not
> entirely clear that individual packages ought to be overriding them at
> all.
>
> Does anybody know whether there is any actual science behind the use of
> those flags on armv7a specifically?

it came into ffmpeg circa 2008 and proliferated into related components primarily to support vectorization and neon
which was quite nascent for ARM architecture in those days in gcc. Today if you were to configure libav with default options
then it uses mainly -fomit-frame-pointer-O3 -fno-math-errno -fno-signed-zeros -fno-tree-vectorize

which is primarily -Ofast -fno-tree-vectorize
>
> p.
>
>
> --
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