Re: Fix performance of generic atomics

Andres Freund <andres@anarazel.de> writes:
> On 2017-09-06 15:25:20 -0400, Tom Lane wrote:
>> 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.

> I think we might have been talking past each other - I thought you were
> talking about changing the pg_atomic_read_u64_impl implementation for
> external users.

Ah.  I was not thinking of touching pg_atomic_read_u32/u64_impl,
although now that you mention it, it's not clear to me why we
couldn't simplify

-	return *(&ptr->value);
+	return ptr->value;

AFAIK, the compiler is entitled to, and does, simplify away that
take-a-pointer-and-immediately-dereference-it dance.  If it did
not, a whole lot of standard array locutions would be much less
efficient than they should be.  What matters here is the volatile
qualifier, which we've already got.

			regards, tom lane