Re: [PATCH] Add stdatomic.h for clang>=3.1 and gcc>=4.1

[Jens Gustedt <jens.gustedt@inria.fr>]

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Am Sonntag, den 23.11.2014, 11:38 -0500 schrieb Rich Felker:
> On Sun, Nov 23, 2014 at 05:29:03PM +0100, Jens Gustedt wrote:
> > > > For the discussion about the second case for the type, this is the
> > > > question if there are archs that implement TAS operations with other
> > > > values than 0 for "unset" and 1 for "set". There seem to be archs out
> > > > there that implement TAS with other values, I vaguely remember having
> > > > heard about some risk arch (??). Actually this also is the reason why
> > > > the standard defines this type in such a weird manner, and why per the
> > > > standard it needs a dedicated initialization macro, default
> > > > initialization with 0 doesn't do in all cases.
> > > 
> > > These are not archs we can support with musl, so they wouldn't be
> > > relevant. And they're not archs that could support POSIX without a
> > > kernel stop-the-world approach for implementing CAS, or syscalls for
> > > every synchronization action.
> > 
> > Could you be more specific? Is it that you know that all the arch in
> > question and conclude about their behavior from you knowledge about
> > them?
> > 
> > Without more specific information I don't see any reason, that an arch
> > that has such specialized super-fast TAS with weird values couldn't
> > have a CAS that behaves "normal". But then I also don't see any reason
> > for such a TAS design in any case ...
> 
> If you have a CAS, you don't use the broken TAS to implement TAS. You
> just use the CAS.

We wouldn't do that, if we'd have a choice, sure. But, if gcc decides
to use the super-fast TAS that has other values, you'd give up binary
compatibility?

Jens


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