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

[Chris Snook <csnook@redhat.com>]

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