[Libav-user] Runtime ARM Neon detection?

[Reuben Scratton]

>> If I compile with -mfpu=neon then FFmpeg blows up
>> with SIGILL (ILLOPTPC) when run on a non-Neon CPU.
>> If I compile with -mfpu=vfvp3 then FFmpeg runs fine,
>> but presumably without the optimized code

> Did you test this? (with a debugger)
> I don't know but it seems very unlikely to me that a
> compiler option ("-mfpu=vfvp3") affects FFmpeg asm
> optimizations.

Not with a debugger - I'm not too handy with gdb - just by turning compiler
options on and off in the hope of squeezing better performance from slower
devices. I could be wrong but since -mfpu=neon always results in ILLOPTPC
and other -mfpu options don't, it seemed logical to connect the two.

Here's my configuration and build output for Android's armeabi-v7a ABI ...
perhaps I'm using a wrong combination of options?


# ./configure --target-os=linux --arch=arm
--cross-prefix=/Users/reuben/android/ndk/toolchains/arm-linux-androideabi-4.7/prebuilt/darwin-x86_64/bin/arm-linux-androideabi-
--sysroot=/Users/reuben/android/ndk/platforms/android-8/arch-arm
--disable-everything --enable-shared --disable-symver --enable-static
--enable-gpl --enable-nonfree --disable-ffmpeg --disable-ffplay
--disable-ffserver --disable-ffprobe --disable-encoders --disable-muxers
--disable-devices --disable-protocols --enable-asm --arch=arm5te
--enable-armv5te --enable-avfilter --enable-parser=mpeg4video
--enable-parser=mpegaudio --enable-parser=mpegvideo --enable-demuxer=m4v
--enable-parser=mov --enable-demuxer=avi --enable-demuxer=mov
--enable-decoder=h264 --enable-parser=h264 --enable-demuxer=h264
--enable-decoder=aac --enable-parser=aac --enable-demuxer=aac
--enable-decoder=mp3 --enable-demuxer=mp3 --enable-avresample
--enable-filter=aconvert --enable-filter=atempo --enable-neon
--prefix=/Users/reuben/work/pluralsight/android/app/jni/build/ffmpeg/armeabi-v7a
--disable-doc --extra-cflags='-fPIC -DANDROID -DANDROID_NDK -std=c99
-march=armv7-a -mtune=cortex-a8 -mfpu=vfpv3 -O3 -Os -Wno-sign-compare
-Wno-switch -Wno-pointer-sign' --extra-ldflags=


C compiler
 /Users/reuben/android/ndk/toolchains/arm-linux-androideabi-4.7/prebuilt/darwin-x86_64/bin/arm-linux-androideabi-gcc
ARCH                      arm (armv7-a)
big-endian                no
runtime cpu detection     yes
ARMv5TE enabled           yes
ARMv6 enabled             yes
ARMv6T2 enabled           yes
VFP enabled               yes
NEON enabled              yes
THUMB enabled             no
debug symbols             yes
strip symbols             yes
optimize for size         no
optimizations             yes
static                    yes
shared                    yes
postprocessing support    yes
new filter support        yes
network support           no
threading support         pthreads
safe bitstream reader     yes
SDL support               no
texi2html enabled         no
perl enabled              yes
pod2man enabled           yes
makeinfo enabled          yes

External libraries:
zlib

Enabled decoders:
aac h264 mp3

Enabled encoders:

Enabled hwaccels:

Enabled parsers:
aac mpeg4video mpegvideo
h264 mpegaudio

Enabled demuxers:
aac h264 mov
avi m4v mp3

Enabled muxers:

Enabled protocols:

Enabled filters:
aconvert atempo

Enabled bsfs:

Enabled indevs:

Enabled outdevs:

License: nonfree and unredistributable
Creating config.mak and config.h...
libavutil/avconfig.h is unchanged

WARNING:
/Users/reuben/android/ndk/toolchains/arm-linux-androideabi-4.7/prebuilt/darwin-x86_64/bin/arm-linux-androideabi-pkg-config
not found, library detection may fail.
CC libavdevice/alldevices.o

...




On Thu, Apr 11, 2013 at 5:48 PM, Carl Eugen Hoyos <cehoyos at ag.or.at> wrote:

> Reuben Scratton <reuben.scratton at ...> writes:
>
> > If I compile with -mfpu=neon then FFmpeg blows up
> > with SIGILL (ILLOPTPC) when run on a non-Neon CPU.
>
> > If I compile with -mfpu=vfvp3 then FFmpeg runs fine,
> > but presumably without the optimized code
>
> Did you test this? (with a debugger)
> I don't know but it seems very unlikely to me that a
> compiler option ("-mfpu=vfvp3") affects FFmpeg asm
> optimizations.
>
> [...]
>
> > Would it be possible for FFmpeg to detect Neon at runtime?
>
> Looking at ff_dsputil_init_arm() in
> libavcodec/arm/dsputil_init_arm.c, that is exactly what it
> currently does.
>
> Carl Eugen
>
> _______________________________________________
> Libav-user mailing list
> Libav-user at ffmpeg.org
> http://ffmpeg.org/mailman/listinfo/libav-user
>
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