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Subject: Re: Segfault with Faster MDCT patch and -fPIC

Re: Segfault with Faster MDCT patch and -fPIC

From: Thomas Martitz <thomas.martitz_at_student.htw-berlin.de>
Date: Sun, 10 Oct 2010 01:11:02 +0200

  On 17.09.2010 09:32, Slawomir Testowy wrote:
> Hi all!
>
> For some time I have been using tremor with fastermdct patch found on
> http://www.rockbox.org/wiki/FasterMDCT. This patch gives huge speedup on
> i686/x86_64 machines and smaller, but still significant, speedup on ARM.
>
> Everything works great until I compile tremor on ARM with -fPIC flag (I link
> tremor into a shared object). In such case it segfaults in macro TRANSFORM_EQUAL
> in file fft-ffmpeg_arm.h at line 158:
>
> asm volatile( "ldmia %[my_z], {%[t5],%[t6]}\n\t":[t5] "=r" (t5), [t6] "=r"
> (t6):[my_z] "r" (z));\
>
> When I compile tremor without -fPIC or with -U_ARM_ASSEM_, there is no segfault.
> Tremor without fastermdct.patch also doesn't have this problem.
>
> I tried debugging the code for a moment, but this code (MDCT +
> #defines + lots of
> assmebly) is completely incomprehensible to me.
>
> Unfortunately, I don't have gdb built for this platform (I managed to find the
> problematic line with `fprintf(stderr, ...)`, which itself could change the way
> it segfaults). At first sight all variables are initialized to proper values.
>
> The ARM processor I'm using is (according to /proc/cpuinfo):
>
> Processor : ARMv6-compatible processor rev 4 (v6l)
> CPU implementer : 0x41
> CPU architecture: 6TEJ
> CPU variant : 0x0
> CPU part : 0xb36
> CPU revision : 4
> Hardware : Freescale MX31/MX32 3-Stack Board
>
>
> Can you help me handle this issue?
>
> --
> regards,
> Slawomir Testowy

The android has the same problem (after enabling ASM optimzations),
crash/segfault immediately when trying to play a song. There's a number
of codecs that do not work and most (or all, I don't know exactly) of
them use the mdct: wma, vorbis, aac, a52, cook. I strongly suspect it's
the very same problem, since IIUC the NDKs gcc has -fPIC on by default
(as does the android gcc target in gcc 4.6).

I hope this can be resolved.

Best regards.
Received on 2010-10-10


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