libav, neon and raspbian
I maintain a Debian derivative called raspbian. We rebuild debian armhf
with reduced minimum CPU requirements so it will run on all pi models.
Since it looks like raspbian will be continuing for the forseeable
future and being used on all pi models. I have been working on trying to
re-enable neon support in various packages (which we disabled in the
early days of raspbian because at the time it seemed pointless).
The first package I started to tackle was libav (i'm working on jessie
at the moment, I'll forward port efforts to stretch later). I modified
the source package to reenable the neon build and use appropriate
options for the different build.
To test whether neon is making a difference i'm disabling and reenabling
hwcaps by creating and removing /etc/ld.so.nohwcap . ldd and strace
confirm that this causes different libraries to be loaded. readelf shows
the libs in /usr/lib/arm-linux-gnueabihf/neon/vfp are tagged as neon and
armv7.
So far i've tried a couple of tests, converting a wav file from 44.1 kHz
to 48 kHz and converting a video file from mov to mpeg. Unfortunately
i'm not noticing any performance difference.
So I'd like some advice.
1. Is there any way to see in avconv whether it thinks the libav it's
using is built with neon support. I have the strace and readelf but I'd
like application level confirmation if it exists.
2. Does only certain functionality in libav benefit from neon? if so
what functionality?
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