Re: why are some atomic_t's not volatile, while most are?

Chris Snook
2007-08-08T01:32:46Z

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