2010/3/9 Marcin Baczyński marb...@gmail.com:
Hi,
the following piece of code produces different output on svn trunk and
gcc-4_4-branch:
#include stdio.h
int main()
{
struct { unsigned bar:1; } foo;
foo.bar = 0x1;
printf(%08x\n, (unsigned char)(foo.bar * 0xfe));
printf(%08x\n, (unsigned char)(foo.bar * 0xff));
return 0;
}
monst...@yggdrasil /data/tmp $ gcc -v
Using built-in specs.
Target: x86_64-unknown-linux-gnu
Configured with: ../gcc-svn/configure --enable-stage1-languages-c
--enable-languages=c,c++
Thread model: posix
gcc version 4.4.4 20100309 (prerelease) (GCC)
monst...@yggdrasil /data/tmp $ ./a.out
00fe
0001
monst...@yggdrasil /data/tmp $ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/local/libexec/gcc/x86_64-unknown-linux-gnu/4.5.0/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with: ../gcc-svn/configure --enable-stage1-languages-c
--enable-languages=c,c++
Thread model: posix
gcc version 4.5.0 20100309 (experimental) (GCC)
monst...@yggdrasil /data/tmp $ ./a.out
00fe
00ff
Is there something illegal in this code or is it a bug somewhere?
Probably something that was fixed for 4.5 but not backported to 4.4.
The 4.4 C frontend produces for the 2nd printf:
printf ((const char * restrict) %08x\n, (int) -foo.bar);
while 4.5 does
printf ((const char * restrict) %08x\n, (int) -(unsigned char) foo.bar);
I'm not sure where this difference comes from (and I can't convince
myself that this is a real fix). Please file a bugreport.
Btw, 4.3 produced
printf ((const char * restrict) (char *) %08x\n, (int) (unsigned
char) ((int) foo.bar * 255));
and the same runtime result as 4.5.
Thanks,
Richard.
Thanks,
Marcin
