http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52979
Bug #: 52979 Summary: likely wrong code bug w/packed bitfields Classification: Unclassified Product: gcc Version: 4.8.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c AssignedTo: unassig...@gcc.gnu.org ReportedBy: reg...@cs.utah.edu CC: cheny...@cs.utah.edu [regehr@dyson r30]$ current-gcc -O2 small.c ; ./a.out 0 [regehr@dyson r30]$ current-gcc -O3 small.c ; ./a.out 1 [regehr@dyson r30]$ cat small.c int printf (const char *, ...); int c, d, e; void fn1 () { } #pragma pack(1) struct S1 { int f0:31; int f1:6; } a = { 1 }; static struct S1 b = { 1 }; void fn2 () { for (;;) { a.f1 = 1; struct S1 f = { }; b = f; e = 0; if (d) c = a.f0; break; } } void fn3 () { fn2 (); a = b; } int main (void) { fn3 (); printf ("%d\n", a.f0); return 0; } [regehr@dyson r30]$ current-gcc -v Using built-in specs. COLLECT_GCC=current-gcc COLLECT_LTO_WRAPPER=/uusoc/exports/scratch/regehr/z/compiler-install/gcc-r186403-install/bin/../libexec/gcc/x86_64-unknown-linux-gnu/4.8.0/lto-wrapper Target: x86_64-unknown-linux-gnu Configured with: ../configure --with-libelf=/usr/local --enable-lto --prefix=/home/regehr/z/compiler-install/gcc-r186403-install --program-prefix=r186403- --enable-languages=c,c++ Thread model: posix gcc version 4.8.0 20120413 (experimental) (GCC)