Date: Tue, 26 Oct 2021 21:49:58 -0700 From: Gleb Smirnoff <glebius@freebsd.org> To: Konstantin Belousov <kostikbel@gmail.com> Cc: mjg@freebsd.org, jhb@freebsd.org, jeff@freebsd.org, arch@freebsd.org Subject: Re: rwlock(9) and mutex(9) definitions Message-ID: <YXjadvxmJRoXwWUj@FreeBSD.org> In-Reply-To: <YXjYCj2zh0cjL/%2Bq@kib.kiev.ua> References: <YXiw1afVlQyEhQyc@FreeBSD.org> <YXizhiRnByvyisXe@kib.kiev.ua> <YXi0PM6babQKFulv@kib.kiev.ua> <YXjS16MQZiKm4E/r@FreeBSD.org> <YXjYCj2zh0cjL/%2Bq@kib.kiev.ua>
next in thread | previous in thread | raw e-mail | index | archive | help
On Wed, Oct 27, 2021 at 07:39:38AM +0300, Konstantin Belousov wrote:
K> > K> Both rw_rlock and rw_wlock are in tail context. You cannot _return_ void.
K> >
K> > You actually can return void to hint compiler for a tail call optimization.
K> > It is not a wrong syntax.
K> >
K> > Other code that is working with true void functions (e.g. with WITNESS) and
K> > doesn't work with "do {} while" is:
K> >
K> > void
K> > something(bool clue)
K> > {
K> > return (clue ? rw_rlock(lock) : rw_wlock(lock));
K> > }
K> >
K> > This is explicitly allowed in 6.5.15 of the C11 standard.
K> >
K> > Of course all this code can be written in some other way, so constraint
K> > of KPI not being true functions can be worked around, but I believe better
K> > it be fixed.
K>
K> Hum. I know about ternary operator allowing void-typed expressions on both
K> sides of ':'. What I replied to is the following, from C17
K>
K> 6.8.6.4 The return statement
K> Constraints
K> 1 A return statement with an expression shall not appear in a function
K> whose return type is void. A return statement without an expression
K> shall only appear in a function whose return type is void.
K>
K> So syntax above is explicitly prohibited by the standard. I was quite
K> surprised that both gcc and clang silently accept this, unless -pedantic
K> is specified. There is no mention of this extension in gcc manual.
K> Compiler explorer demonstrates that compilers like msvc do warn about
K> the construct, and some even error out (tendra):
K> https://godbolt.org/z/xqcPssTcY
K>
K> Another part of the confusion, perhaps, is that
K> return <void expression>;
K> is explicitly allowed by the C++ standard (I looked at 2020 version),
K> unlike C.
Hmm, so I mistakenly transferred my knowledge about the tail call
optimization hint from C++ to C.
Okay, let's put return aside. This would compile with true
functions (e.g. WITNESS), otherwise not:
void
something(bool clue)
{
clue ? rw_rlock(lock) : rw_wlock(lock);
}
And this is correct code per 6.5.15.
--
Gleb Smirnoff
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?YXjadvxmJRoXwWUj>