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Date: Mon, 18 May 2015 00:23:07 +0200
From: Jens Gustedt <jens.gustedt@...ia.fr>
To: musl@...ts.openwall.com
Subject: Re: Deduplicating atomics written in terms of CAS

Am Sonntag, den 17.05.2015, 13:59 -0400 schrieb Rich Felker:
> > Ah sorry, I probably went too fast. My last paragraph would be for all
> > atomic operations, so in particular 32 bit. A macro "a_load" would
> > make intentions clearer and would perhaps allow to implement an
> > optional compile time check to see if we use any object consistently
> > as atomic or not.
> 
> The reason I'm mildly against this is that all current reads of
> atomics, except via the return value of a_cas or a_fetch_add, are
> relaxed-order. We don't care if we see a stale value; if staleness
> could be a problem, the caller takes care of that in an efficient way.
> Having a_load that's relaxed-order whereas all the existing atomics
> are seq_cst order would be an inconsistent API design.

I still wasn't clear enough, sorry. My idea was not that such a
function or macro should change anything on the binary code that is
produced, at least for production builds. I just thought to
encapsulate all atomic accesses into a type and functions that allow
to have a compile check. With that we could ensure that actually all
accesses to such an object are through these functions.

The advantage of C11's model for atomic is that this a qualifier, and
then the compiler automatically checks (or ensures) that all accesses
are atomic. We don't have that luxury, here, but we could get a bit
closer to it.

Jens



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