On Tue, 2007-08-07 at 15:38 -0600, Chris Friesen wrote:Chris Snook wrote:
That's why we define atomic_read like so:Can you guarantee that the pointer dereference cannot be optimised away on any architecture? Without other restrictions, a suficiently intelligent optimiser could notice that the address of v doesn't change in the loop and the destination is never written within the loop, so the read could be hoisted out of the loop.
#define atomic_read(v) ((v)->counter)
This avoids the aliasing problem, because the compiler must de-reference the pointer every time, which requires a memory fetch.
Even now, powerpc (as an example) defines atomic_t as:
typedef struct { volatile int counter; } atomic_t
That volatile is there precisely to force the compiler to dereference it every single time.
I just tried this with GCC 4.2 on x86_64 because I was curious.
struct counter_t { volatile int counter; } test;
struct counter_t *tptr = &test;
int main() {
int i;
tptr->counter = 0;
i = 0;
while(tptr->counter < 100) {
i++;
}
return 0;
}
$ gcc -O3 -S t.c
a snippet of t.s:
main:
.LFB2:
movq tptr(%rip), %rdx
movl $0, (%rdx)
.p2align 4,,7
.L2:
movl (%rdx), %eax
cmpl $99, %eax
jle .L2
Now with the volatile removed:
main:
.LFB2:
movq tptr(%rip), %rax
movl $0, (%rax)
.L2:
jmp .L2
If the compiler can see it clearly, it will optimize out the load
without the volatile.