#include <signal.h> #include <unistd.h> int test;
void handler (int sig) { test = 1; } int main() { signal (SIGALRM, handler); alarm (1); while (test == 0) asm(""); return 0; } Results in an infinite loop with -O2. The load of test in main should not be hoisted out of the loop. A more x86-specific example (but without relying on unix syscalls might be int test; int main() { int x = test; asm("movl $1,test"); if (x + test != 1) __builtin_trap (); return 0; } It also wouldn't surprise me if the two tests have different culprits... -- Summary: Old-style asms don't clobber memory Product: gcc Version: 4.0.1 Status: UNCONFIRMED Severity: normal Priority: P2 Component: rtl-optimization AssignedTo: unassigned at gcc dot gnu dot org ReportedBy: rth at gcc dot gnu dot org http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24414