| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Andres Freund <andres(at)anarazel(dot)de> |
| Cc: | Sokolov Yura <funny(dot)falcon(at)postgrespro(dot)ru>, Robert Haas <robertmhaas(at)gmail(dot)com>, Simon Riggs <simon(at)2ndquadrant(dot)com>, Jeff Janes <jeff(dot)janes(at)gmail(dot)com>, Jesper Pedersen <jesper(dot)pedersen(at)redhat(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Fix performance of generic atomics |
| Date: | 2017-09-06 19:34:09 |
| Message-ID: | 1054.1504726449@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
Andres Freund <andres(at)anarazel(dot)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
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