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Date: Mon, 18 May 2015 13:03:30 +0200
From: Jens Gustedt <jens.gustedt@...ia.fr>
To: musl@...ts.openwall.com
Subject: Re: Deduplicating atomics written in terms of CAS
Am Montag, den 18.05.2015, 12:19 +0200 schrieb Szabolcs Nagy:
> * Jens Gustedt <jens.gustedt@...ia.fr> [2015-05-18 00:23:07 +0200]:
> >
> > 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.
> >
>
> the qualifierness of atomic is a bit confusing
>
> differently qualified types had the same alignment and representation
> so far and in some cases pointers to differently qualified types are
> implicitly convertible (eg assigning char* to const char* is ok).
>
> atomic is special: when the standard says 'qualified or unqualified type'
> it does not include _Atomic even though it is called a qualifier.
> (atomic is always mentioned explicitly, it can have different representation
> and alignment to allow implementation with locks and thus the pointers
> cannot be convertible)
>
> annotating everything with _Atomic in musl is problematic because the
> atomic bits are publicly visible in pthread types (we could use _Atomic
> only when building musl, but i think its usefulness should be demonstrated
> with examples etc before doing something that ugly.. having a_load_relaxed
> without typesystem help does not sound very useful)
I didn't say that I wanted to use _Atomic, we can't do that for
obvious reasons. I just want to hide the atomic objects behind a
typedef, such that production compiles generate exactly the same
binary, but that we also can use the compiler to have a check for
consistency.
Jens
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