Re: better atomics

On Wed, Oct 16, 2013 at 12:52 PM, Andres Freund <andres(at)2ndquadrant(dot)com> wrote:
>> I have a related problem, which is that some code I'm currently
>> working on vis-a-vis parallelism can run lock-free on platforms with
>> atomic 8 bit assignment but needs a spinlock or two elsewhere. So I'd
>> want to use pg_atomic_store_u64(), but I'd also need a clean way to
>> test, at compile time, whether it's available.
>
> Yes, definitely. There should be a couple of #defines that declare
> whether non-prerequisite options are supported. I'd guess we want at least:
> * 8byte math
> * 16byte compare_and_swap

I'm not terribly excited about relying on 16-byte CAS, but I agree
that 8-byte math, at least, is important. I've not been successful in
finding any evidence that gcc has preprocessor symbols to tell us
about the properties of 8-byte loads and stores. The closest thing
that I found is:

http://gcc.gnu.org/onlinedocs/gcc/_005f_005fatomic-Builtins.html

That page provides intrinsics for 8-byte atomic loads and stores,
among other things. But it seems that the only method for determining
whether they work on a particular target is to compile a test program
and see whether it complains about __atomic_load_n and/or
__atomic_store_n being unresolved symbols. I suppose we could add a
configure test for that. Yuck.

Anyone have a better idea?

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
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