Re: Fix performance of generic atomics

Andres Freund <andres(at)anarazel(dot)de> writes:
> On 2017-09-06 15:12:13 -0400, Tom Lane wrote:
>> It looks to me like two of the three implementations promise no such
>> thing.

> They're volatile vars, so why not?

Yeah, but so are the caller's variables. That is, in

pg_atomic_exchange_u64_impl(volatile pg_atomic_uint64 *ptr, uint64 xchg_)
{
uint64 old;
old = ptr->value;

ISTM that the compiler is required to actually fetch ptr->value, not
rely on some previous read of it. I do not think that (the first
version of) pg_atomic_read_u64_impl is adding any guarantee that wasn't
there already.

>> Even if they somehow do, it hardly matters given that the cmpxchg loop
>> would be self-correcting.

> Well, in this one instance maybe, hardly in others.

All the functions involved use nigh-identical cmpxchg loops.

> What are you suggesting as an alternative?

I think we can just use "old = ptr->value" to set up for the cmpxchg
loop in every generic.h function that uses such a loop.

regards, tom lane