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

Re: Segfault with Faster MDCT patch and -fPIC

From: Dave Hooper <dave_at_beermex.com>
Date: Fri, 17 Sep 2010 10:34:06 +0100

That's very interesting indeed. I too observed a segfault when trying to use
the fastermdct patch on top of a libtremor used by an MPD server but
couldn't pinpoint the reason. That's one of the reasons this patch hasn't
hit upstream Tremor.

I know the code well but that line seems very mysterious (I.e. harmless - it
should work unless something has corrupted the stack/address pointer, I
think).
Can any arm gurus see if I am misusing any registers in a way that prevents
shared library usage, or anything along those lines?
> 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
Received on 2010-09-17


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