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Message-ID: <20170625130629.03dc67bc@inria.fr>
Date: Sun, 25 Jun 2017 13:06:29 +0200
From: Jens Gustedt <jens.gustedt@...ia.fr>
To: musl@...ts.openwall.com
Subject: Re: more on missing volatile qualifications
Hello Szabolcs,
On Sun, 25 Jun 2017 12:17:04 +0200 Szabolcs Nagy <nsz@...t70.net> wrote:
> pthread_once_t and pthread_spinlock_t qualifiers are
> visible in the c++ name mangling if a c++ function takes
> pointer to them as arguments so the change is an abi break.
too bad, so we can't change these two
There is a reading of the C standard that says that volatile only has
implications if an object itself is such qualified, having a volatile
qualified lvalue access isn't enough. I don't think that any current
compiler does such weird things, but who knows where optimisers will
go in the future.
> > Also, I can't think of any semantics for the three, where
> > opitimizing out loads or stores makes any sense, so this also
> > should never see any kind of performance regression.
>
> there was a case in glibc when volatile caused problems:
> some generic atomic macro tried to create a temporary using
>
> __typeof(*(p)) __tmp = *(p);
>
> but then __tmp become volatile and operations on it generated
> useless load/stores to the stack. it could be worked around as
>
> __typeof( (__typeof(*(p))) *(p) ) __tmp = *(p);
>
> is not volatile because the cast expression is unqualified.
> (musl does not have such __typeof hacks, but it is an
> example where volatile caused unexpeced regression)
AFAICS for the third finding in sigaction.c this would not be an
issue. Since in addition this is something dealing with signal stuff,
I still think that volatile would be in order, here.
Thanks
Jens
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