http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50569
--- Comment #7 from Mikael Pettersson <mikpe at it dot uu.se> 2011-11-04 15:31:25 UTC --- Created attachment 25716 --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=25716 reduced test case This reduced test case causes an alignment fault when compiled with gcc 4.7 or 4.6 on sparc64-linux and armv5tel-linux-gnueabi, but not when compiled with gcc 4.5 or older. On sparc64-linux the alignment fault results in a fatal SIGBUS, on armv5tel-linux-gnueabi the fault can be fixed up by the kernel but that incurs massive runtime overhead. The core of the test case is the following: struct event { struct { unsigned int sec; } sent __attribute__((packed)); }; void __attribute__((noinline,noclone)) frob_entry(char *buf) { struct event event; __builtin_memcpy(&event, buf, sizeof(event)); if (event.sent.sec < 64) { event.sent.sec = -1U; __builtin_memcpy(buf, &event, sizeof(event)); } } With gcc 4.6/4.7 the __builtin_memcpy() calls result in plain int-sized loads and stores on a misaligned address. Removing '__attribute__((packed))' from the 'sent' struct avoids the bug. Removing the 'struct {...} sent' wrapper around the 'unsigned int sec' but keeping '__attribute__((packed))' also avoids the bug.
