[Bug c/46620] New: 32-bit structures containing bitfields are not copied correctly on -O2 , x86 backend

bnitulescu at tremend dot ro gcc-bugzilla@gcc.gnu.org
Tue Nov 23 16:42:00 GMT 2010


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46620

           Summary: 32-bit structures containing bitfields are not copied
                    correctly on -O2 , x86 backend
           Product: gcc
           Version: 4.4.3
            Status: UNCONFIRMED
          Severity: major
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: bnitulescu@tremend.ro


Created attachment 22499
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=22499
Test case exhibiting the error

The attached code contains a copy between two structures containing bitfields.

The x86 code generated on -O2 fails to copy two fields in the structure. It is
also very complex, trying to break the structure in sets of bitfields and copy
them independently.

I have attached an archive containing
- file exhibiting the error (bitfield-error.c)
- main file driving the example
- generated assembly code (gcc -c -O2 bitfield-error.c -S)

To run the code in the attachment

gcc -c -O2 bitfield-error.c
gcc -c -O0 bitfield-error-main.c
gcc -o bitfield-error bitfield-error.o bitfield-error-main.o
./bitfield-error

Result: 0, expected 1

Running at -O0 gives a different answer

gcc -c -O0 bitfield-error.c
gcc -o bitfield-error bitfield-error.o bitfield-error-main.o
./bitfield-error

Result: 1, expected 1

The gcc used is the one from Ubuntu 10.04.1 LTS :

$ gcc --version
gcc (Ubuntu 4.4.3-4ubuntu5) 4.4.3
Copyright (C) 2009 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ gcc -v
Using built-in specs.
Target: i486-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 4.4.3-4ubuntu5'
--with-bugurl=file:///usr/share/doc/gcc-4.4/README.Bugs
--enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr --enable-shared
--enable-multiarch --enable-linker-build-id --with-system-zlib
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--with-gxx-include-dir=/usr/include/c++/4.4 --program-suffix=-4.4 --enable-nls
--enable-clocale=gnu --enable-libstdcxx-debug --enable-plugin --enable-objc-gc
--enable-targets=all --disable-werror --with-arch-32=i486 --with-tune=generic
--enable-checking=release --build=i486-linux-gnu --host=i486-linux-gnu
--target=i486-linux-gnu
Thread model: posix
gcc version 4.4.3 (Ubuntu 4.4.3-4ubuntu5)



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