[x264-devel] libx264: Illegal instruction on ARMv5 (Thecus N2100)

[Jason Garrett-Glaser]

On Sun, Dec 26, 2010 at 6:04 AM, Tobias Frost <tobi at frost.de> wrote:
> Hallo debian-multimedia list,
> Hallo x264 development list,
>
> Note: I'm not on the list! Please do not forget to CC me.
>
> When coding videos the codec fails immediatly with an illegal
> instruction.
> After browsing the code a bit, I think upstream did expect an
> ARM V6 or better, my Thecus has:
>
> $ uname -a
> Linux thecus 2.6.32-bpo.5-iop32x #1 Sun Sep 19 16:59:11 UTC 2010
> armv5tel GNU/Linux
>
> How to reproduce:
> ==================
>
> apt-get install x264
> wget http://samples.mplayerhq.hu/yuv4mpeg2/example.y4m.bz2
> bunzip example.y4m.bz2
> x264 -o example.mkv example.y4m
> Illegal instruction
>
> Update: Also:
> tobi at thecus:~$ x264
> Illegal instruction
>
>
> Obervations when compiling:
> ==============================
>
> ccache cc -Wshadow -O1 -g -g -O2 -Wall -I. -g -O2 -std=gnu99
> -mcpu=cortex-a8 -mfpu=neon -mfloat-abi=softfp    -fno-tree-vectorize
> -c -o common/mc.o common/mc.c
>
> I'm not sure if the -mcpu and -mfpu are right. The Thecus has
> an Intel XScale 255. This one has no NEON extension, nor it is a cortex
> A8.
> A brief check of the configure scripts seems to show that this
> is hardcoded as soon as ARM is set as ARCH, and it is only checked
> if the assembler supports the instruction, not if the hardware really
> does (would be hard for cross-compiling, I know...)
>
>
> How to fix?
> ===========
> A temporary fix is be to add "--disable-asm" to the
> configure script (via debian rules) if building for
> debian ARM. This is how I rebuild the packages to
> get a working solution.
>
> I assume that this should be really fixed on debian
> side, but currently debian cannot differenciate
> different CPU capabilites, ASFAIK.
>
> But a runtime-detection might work as well ?
> (Is a bad-instruction trapable?)
>
> --
> coldtobi
>
>
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>

x264 builds with NEON by default because x264 is so slow without NEON
(and on any non-NEON chip) as to be useless.  You can of course
compile with --disable-asm on such chips, but we don't do it by
default because we don't feel it's necessary to actively support chips
on which x264 would be basically useless to begin with.

The ARM people (of which I'm not one, I don't maintain the ARM part)
also strongly discouraged us from using too much runtime detection...

Jason