[Bug c/29440] New: 4.2 20061007 experiemental misscompiles libavcodec/h264.o

[Bug c/29440] New: 4.2 20061007 experiemental misscompiles libavcodec/h264.o

[poirierg at gmail dot com]

4.2 misscompiles libavcodec/h264.o from FFmpeg project.
The likely cause of this problem is lies in the file cabac.h (in attachment)
gcc-4.0 and 4.1.0 compile this file correctly.

Here's the commandline:
/home/guillaume/Prgm/gcc/bin/gcc   -DHAVE_AV_CONFIG_H -D_FILE_OFFSET_BITS=64
-D_LARGEFILE_SOURCE -D_ISOC9X_SOURCE -I.. -I.. -I../libavutil
-Wdeclaration-after-statement -fno-PIC -O4 -march=pentium-m -mtune=pentium-m
-pipe -ffast-math -fomit-frame-pointer -D_REENTRANT -D_LARGEFILE_SOURCE
-D_FILE_OFFSET_BITS=64 -D_LARGEFILE64_SOURCE -I/usr/include  -I/usr/include/SDL
-D_REENTRANT -I/usr/include/kde/artsc -pthread -I/usr/include/glib-2.0
-I/usr/lib/glib-2.0/include  -I/usr/include -I/usr/include/dvdnav 
-I/usr/include/freetype2  -c -o h264.o h264.c -v -save-temps
gcc: warning: -pipe ignored because -save-temps specified
Using built-in specs.
Target: i686-pc-linux-gnu
Configured with: ../gcc-4.2-20061007/configure
--prefix=/home/guillaume/Prgm/gcc/
Thread model: posix
gcc version 4.2.0 20061007 (experimental)
 /home/guillaume/Prgm/gcc/bin/../libexec/gcc/i686-pc-linux-gnu/4.2.0/cc1 -E
-quiet -v -I.. -I.. -I../libavutil -I/usr/include -I/usr/include/SDL
-I/usr/include/kde/artsc -I/usr/include/glib-2.0 -I/usr/lib/glib-2.0/include
-I/usr/include -I/usr/include/dvdnav -I/usr/include/freetype2 -iprefix
/home/guillaume/Prgm/gcc/bin/../lib/gcc/i686-pc-linux-gnu/4.2.0/ -D_REENTRANT
-DHAVE_AV_CONFIG_H -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE -D_ISOC9X_SOURCE
-D_REENTRANT -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 -D_LARGEFILE64_SOURCE
-D_REENTRANT h264.c -march=pentium-m -mtune=pentium-m
-Wdeclaration-after-statement -fno-PIC -ffast-math -fomit-frame-pointer -O4
-fpch-preprocess -o h264.i
ignoring nonexistent directory
"/home/guillaume/Prgm/gcc/bin/../lib/gcc/i686-pc-linux-gnu/4.2.0/../../../../i686-pc-linux-gnu/include"
ignoring duplicate directory
"/home/guillaume/Prgm/gcc//lib/gcc/i686-pc-linux-gnu/4.2.0/include"
ignoring nonexistent directory
"/home/guillaume/Prgm/gcc//lib/gcc/i686-pc-linux-gnu/4.2.0/../../../../i686-pc-linux-gnu/include"
ignoring duplicate directory ".."
ignoring duplicate directory "/usr/include"
  as it is a non-system directory that duplicates a system directory
ignoring duplicate directory "/usr/include"
  as it is a non-system directory that duplicates a system directory
#include "..." search starts here:
#include <...> search starts here:
 ..
 ../libavutil
 /usr/include/SDL
 /usr/include/kde/artsc
 /usr/include/glib-2.0
 /usr/lib/glib-2.0/include
 /usr/include/dvdnav
 /usr/include/freetype2
 /home/guillaume/Prgm/gcc/bin/../lib/gcc/i686-pc-linux-gnu/4.2.0/include
 /usr/local/include
 /home/guillaume/Prgm/gcc//include
 /usr/include
End of search list.
 /home/guillaume/Prgm/gcc/bin/../libexec/gcc/i686-pc-linux-gnu/4.2.0/cc1
-fpreprocessed h264.i -quiet -dumpbase h264.c -march=pentium-m -mtune=pentium-m
-auxbase-strip h264.o -O4 -Wdeclaration-after-statement -version -fno-PIC
-ffast-math -fomit-frame-pointer -o h264.s
GNU C version 4.2.0 20061007 (experimental) (i686-pc-linux-gnu)
        compiled by GNU C version 4.2.0 20061007 (experimental).
GGC heuristics: --param ggc-min-expand=30 --param ggc-min-heapsize=4096
Compiler executable checksum: 9d78c482f3158f545a4217a7c698d3a2
h264.c: In function 'decode_cabac_residual':
h264.c:6120: warning: initialization from incompatible pointer type
 as -V -Qy -o h264.o h264.s
GNU assembler version 2.16.91 (i486-linux-gnu) using BFD version 2.16.91
20060118 Debian GNU/Linux


-- 
           Summary: 4.2 20061007 experiemental misscompiles
                    libavcodec/h264.o
           Product: gcc
           Version: 4.2.0
            Status: UNCONFIRMED
          Severity: major
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: poirierg at gmail dot com
 GCC build triplet: (no options)
  GCC host triplet: i686, Linux, debian
GCC target triplet: i686, Linux, debian


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29440

[Bug c/29440] 4.2 20061007 experiemental misscompiles libavcodec/h264.o

[poirierg at gmail dot com]

------- Comment #1 from poirierg at gmail dot com  2006-10-12 14:25 -------
Created an attachment (id=12418)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=12418&action=view)
the offending file (results in a grey picture)


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29440

[Bug c/29440] 4.2 20061007 experiemental misscompiles libavcodec/h264.o

[poirierg at gmail dot com]

------- Comment #2 from poirierg at gmail dot com  2006-10-12 14:28 -------
Created an attachment (id=12419)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=12419&action=view)
The header that seems to hold the code that's misscompiled


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29440

[Bug middle-end/29440] 4.2 20061007 experimental misscompiles libavcodec/h264.o

[pinskia at gcc dot gnu dot org]

-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|major                       |normal


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29440

[Bug middle-end/29440] 4.2 20061007 experimental misscompiles libavcodec/h264.o

[pinskia at gcc dot gnu dot org]

------- Comment #3 from pinskia at gcc dot gnu dot org  2006-10-12 14:34 -------
Does it work when compiled with -O0 instead of -O4?  How about -O1?

Besies above, I noticed that the asm in get_cabac looks to be clobbering memory
but is not marked as such.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29440

[Bug middle-end/29440] 4.2 20061007 experimental misscompiles libavcodec/h264.o

[pluto at agmk dot net]

------- Comment #4 from pluto at agmk dot net  2006-10-12 14:40 -------
(In reply to comment #3)
> Does it work when compiled with -O0 instead of -O4?  How about -O1?
> 
> Besies above, I noticed that the asm in get_cabac looks to be clobbering memory
> but is not marked as such.
> 

Andrew, this testcase violates aliasing rules.

h264.c: In function 'filter_mb_fast':
h264.c:7074: warning: dereferencing type-punned pointer will break
                      strict-aliasing rules


-- 

pluto at agmk dot net changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |pluto at agmk dot net


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29440

[Bug middle-end/29440] 4.2 20061007 experimental misscompiles libavcodec/h264.o

[poirierg at gmail dot com]

------- Comment #5 from poirierg at gmail dot com  2006-10-12 14:53 -------
Hello,

(In reply to comment #3)
> Does it work when compiled with -O0 instead of -O4?  How about -O1?

The code compiles and produces the expected result with -O1, O2, but can't be
compiled with -O0:
In file included from h264.c:36:
cabac.h: In function 'get_cabac':
cabac.h:454: error: can't find a register in class 'GENERAL_REGS' while
reloading 'asm'
cabac.h:454: error: 'asm' operand has impossible constraints
h264.c: In function 'decode_cabac_residual':
h264.c:6120: warning: initialization from incompatible pointer type

> Besies above, I noticed that the asm in get_cabac looks to be clobbering memory
> but is not marked as such.

I can't really comment on that as I'm not too inline-asm fluent... however, I
can say that this code can't be compiled without -fomit-frame-pointer.
Is GCC supposed to produce valid code with this source to begin with?

Guillaume


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29440

[Bug middle-end/29440] 4.2 20061007 experimental misscompiles libavcodec/h264.o

[poirierg at gmail dot com]

------- Comment #6 from poirierg at gmail dot com  2006-10-12 15:03 -------
(In reply to comment #4)
> (In reply to comment #3)
> > Does it work when compiled with -O0 instead of -O4?  How about -O1?
> > 
> > Besies above, I noticed that the asm in get_cabac looks to be clobbering memory
> > but is not marked as such.
> > 
> 
> Andrew, this testcase violates aliasing rules.
> 
> h264.c: In function 'filter_mb_fast':
> h264.c:7074: warning: dereferencing type-punned pointer will break
>                       strict-aliasing rules

For what it's worth, the code around line 7074 compiled correctly with snapshot
4.2-20060909 and has been in FFmpeg since Mon Aug 28 09:33:01 2006 UTC, so IMHO
it shouldn't be the cause of the problem I'm reporting today.

However, if you have a suggestion to improve this code, I'm all ears

Guillaume


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29440

[Bug middle-end/29440] 4.2 20061007 experimental misscompiles libavcodec/h264.o

[poirierg at gmail dot com]

------- Comment #7 from poirierg at gmail dot com  2006-10-13 07:37 -------
(In reply to comment #3)
> Does it work when compiled with -O0 instead of -O4?  How about -O1?
> 
> Besies above, I noticed that the asm in get_cabac looks to be clobbering memory
> but is not marked as such.

Damn, that was the bug. I don't quite understand why 4.0 and 4.1 weren't
affected on Linux, whereas 3.4.6 was affected.

Sorry for the trouble, and thanks for the great work!


Guillaume


-- 

poirierg at gmail dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29440