The -O0 result looks right. This behavior observed on x86 using r157445 and x64 using r157542.
reg...@john-home:~$ current-gcc -O0 small.c -o small -Wall reg...@john-home:~$ ./small 1 reg...@john-home:~$ current-gcc -O1 small.c -o small -Wall reg...@john-home:~$ ./small 0 reg...@john-home:~$ cat small.c extern int printf (__const char *__restrict __format, ...); static unsigned char g_2 = 1; static int g_9; static int *l_8 = &g_9; static void func_12(int p_13) { int * l_17 = &g_9; *l_17 &= 0 < p_13; } int main(void) { unsigned char l_11 = 254; *l_8 |= g_2; l_11 |= *l_8; func_12(l_11); printf("%d\n", g_9); return 0; } reg...@john-home:~$ current-gcc -v Using built-in specs. COLLECT_GCC=current-gcc COLLECT_LTO_WRAPPER=/home/regehr/z/compiler-install/gcc-r157445-install/libexec/gcc/i686-pc-linux-gnu/4.5.0/lto-wrapper Target: i686-pc-linux-gnu Configured with: ../configure --with-libelf=/usr/local --enable-lto --prefix=/home/regehr/z/compiler-install/gcc-r157445-install --program-prefix=r157445- --enable-languages=c,c++ Thread model: posix gcc version 4.5.0 20100314 (experimental) (GCC) -- Summary: possible wrong code bug Product: gcc Version: unknown Status: UNCONFIRMED Severity: normal Priority: P3 Component: c AssignedTo: unassigned at gcc dot gnu dot org ReportedBy: regehr at cs dot utah dot edu GCC build triplet: i686-pc-linux-gnu GCC host triplet: i686-pc-linux-gnu GCC target triplet: i686-pc-linux-gnu http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43438