Re: GCC 4.4.1 bug? need help isolating problem

[Ryan Hill <dirtyepic at gentoo dot org>]

On Mon, 28 Sep 2009 16:41:57 -0700
Corey Hickey <bugfood-ml@fatooh.org> wrote:

> Hello,
> 
> I'm running into a segfault with MPlayer on MKV files. I think it might
> be a bug in some aspect of GCC 4.4's optimization, but I'm having
> trouble narrowing it down. All I know for sure is:
> 
> Works:
> * compiled by gcc 4.4 with -O2
> * compiled by gcc 3.3, 3.4, 4.0, 4.1, 4.2, 4.3 (any optimization level)
> * compiled by gcc svn trunk (any optimization level)
> 
> Crashes:
> * compiled by gcc 4.4 with -O3
> 
> 
> Here's a full list of steps to reproduce this bug.
> ------------------------------------------------------------------------
> $ svn co -r '{2009-09-26}' svn://svn.mplayerhq.hu/mplayer/trunk mplayer
> $ cd mplayer
> $ CFLAGS=-O3 ./configure --cc=gcc-4.4
> $ make
> $ wget http://samples.mplayerhq.hu/Matroska/theora.mkv
> $ ./mplayer theora.mkv
> ------------------------------------------------------------------------
> 
> I'm seeing mplayer segfault here. I don't get a segfault if I compile
> with a lower optimization level:
> 
> $ make distclean && CFLAGS=-O2 ./configure --cc=gcc-4.4 && make
> 
> ...or with a different version of GCC.
> 
> $ make distclean && CFLAGS=-O3 ./configure --cc=gcc-4.3 && make
> 
> I understand that messing with CFLAGS is contra-indicated when compiling
> MPlayer, but I'm just trying to isolate a problem that occurs with the
> default CFLAGS anyway.
> 
> GCC's optimization levels, if I understand correctly, enable groups of
> compiler flags. I tried to determine if the segfault was caused by a
> particular flag; however, it seems that there are several flags where
> removing any one of them makes the crash go away. In other words, the
> following set of flags causes a segfault:
> 
> -O1
> -fgcse
> -finline-small-functions
> -fschedule-insns2
> -fstrict-aliasing
> -finline-functions
> 
> I wasn't able to narrow down the "-O1" any further; when I replace it
> with the group of flags supposedly enabled by "-O1", the crash goes
> away. I don't know what I'm doing wrong. I'm getting the list by running:
> 
> $ gcc-4.4 -c -Q -O0 --help=optimizers > /tmp/O0
> $ gcc-4.4 -c -Q -O1 --help=optimizers > /tmp/O1
> $ diff /tmp/O0 /tmp/O1 | awk '/enabled/ {print $2}'
> 
> At this point I'm kind of stuck. I can use GDB to determine the point at
> which the segfault occurs:
> ------------------------------------------------------------------------
> Program received signal SIGSEGV, Segmentation fault.
> 0x0000000000584039 in add_cluster_position (demuxer=<value optimized
> out>, ds=<value optimized out>)
>     at libmpdemux/demux_mkv.c:240
> 240       mkv_d->cluster_positions[mkv_d->num_cluster_pos++] = position;
> (gdb) print mkv_d
> $1 = <value optimized out>
> ------------------------------------------------------------------------
> 
> ...but with the optimizations enabled I can't figure out what's going on
> with the code. I started to try some "printf debugging" but stopped when
> I discovered that any printf usage makes the crash go away (see attached
> mask_bug.diff).
> 
> One other item worth mentioning is that the line where the crash occurs
> is right after a probable realloc(). I can't determine that for sure
> though--the optimizations throw off my GDB usage.
> 
> I discovered this bug with Debian's gcc 4.4.1 and verified the same
> behavior with unmodified svn from "gcc-4_4-branch". My gcc is compiled
> without any special options. Just:
> ------------------------------------------------------------------------
> $ mkdir build
> $ cd build
> $ ../gcc-4_4-branch/configure --prefix=/home/bugfood/gcc4
> $ make
> $ make install
> ------------------------------------------------------------------------
> 
> Most of my testing of this bug is on amd64, but it seems to happen the
> same on i386 as well.
> 
> 
> Does anybody have advice for how I should proceed? I'd like to be able
> to write an actual bug report but at this point I don't think I have
> enough precise information.
> 
> Thanks,
> Corey
> 
We've seen the same problem and I think it was narrowed down to
-finline-functions.  See http://bugs.gentoo.org/269975.  Unfortunately the
rest of the bug report is just noise.  Good luck!


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