http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49893
Summary: [4.7 Regression] -O3 generates wrong code for volatile
Product: gcc
Version: 4.7.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: hjl.to...@gmail.com
On Linux/x86-64, GCC 4.7 compiles this test into an infinite loop:
[hjl@gnu-mic-2 time]$ cat /tmp/x.c
extern unsigned int alarm (unsigned int __seconds) __attribute__
((__nothrow__));
typedef void (*__sighandler_t) (int);
extern __sighandler_t signal (int __sig, __sighandler_t __handler)
__attribute__ ((__nothrow__));
typedef long int clock_t;
extern clock_t clock (void) __attribute__ ((__nothrow__));
volatile int gotit = 0;
static void
alarm_handler (int signal)
{
gotit = 1;
}
int
main (int argc, char ** argv)
{
clock_t start, stop;
if (signal(14, alarm_handler) == ((__sighandler_t) -1))
{
return -1;
}
alarm(1);
start = clock ();
while (!gotit);
stop = clock ();
return start - stop;
}
[hjl@gnu-mic-2 time]$ /usr/gcc-4.7.0-x32/bin/gcc -O3 /tmp/x.c
[hjl@gnu-mic-2 time]$ ./a.out
^C
[hjl@gnu-mic-2 time]$
It forgot "gotit" was marked volatile and might change.
main:
.LFB1:
.cfi_startproc
pushq %rbx
.cfi_def_cfa_offset 16
.cfi_offset 3, -16
movl $alarm_handler, %esi
movl $14, %edi
call signal
cmpq $-1, %rax
je .L3
movl $1, %edi
call alarm
call clock
movq %rax, %rbx
movl gotit(%rip), %eax
testl %eax, %eax
je .L4
call clock
subl %eax, %ebx
movl %ebx, %eax
.L3:
popq %rbx
.cfi_remember_state
.cfi_def_cfa_offset 8
ret
.L4:
.cfi_restore_state
.p2align 4,,4
jmp .L4
.cfi_endproc
