Zan Lynx wrote:
On Tue, 2007-08-07 at 15:38 -0600, Chris Friesen wrote:
Chris Snook wrote:
That's why we define atomic_read like so:
#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.
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.
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.
This is not a problem, since indirect references will cause the CPU to fetch the
data from memory/cache anyway.
-- Chris
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