Date: Mon, 26 Feb 2018 13:45:26 +0100 From: Tijl Coosemans <tijl@FreeBSD.org> To: Bruce Evans <brde@optusnet.com.au> Cc: Mark Millard <marklmi26-fbsd@yahoo.com>, FreeBSD Hackers <freebsd-hackers@freebsd.org>, FreeBSD Standards <freebsd-standards@freebsd.org>, Kevin Lo <kevlo@freebsd.org> Subject: Re: Marking select(2) as restrict Message-ID: <20180226134526.425aa33c@kalimero.tijl.coosemans.org> In-Reply-To: <20180226200951.V2634@besplex.bde.org> References: <CAF6rxg=h_oMiUu7P=GAOQf_OySQM2w31hg6Kas%2B3jeEM3qq_Cg@mail.gmail.com> <CAF6rxgnt9c0n8i-nHQwoKGbZKF2hM5AZqEJnz0CLo26XOO4_sg@mail.gmail.com> <20180221032247.GA81670@ns.kevlo.org> <CAF6rxg=WwqeBnmJzfOZgtwrYesXPfvJFeaVmQwtTa_89_sxaJg@mail.gmail.com> <CANCZdfo46bhfaRpbqOmJjk4%2B=1R2c5kvmrJPENaxNgK==5M4kg@mail.gmail.com> <CAF6rxg=wNVgDUF9o744ngmzPNeHB3hqdrLufy=yS3D4osczxFQ@mail.gmail.com> <20180221104400.GU94212@kib.kiev.ua> <20180222112752.10da7e51@kalimero.tijl.coosemans.org> <20180222105608.GE94212@kib.kiev.ua> <20180225214813.776a9f58@kalimero.tijl.coosemans.org> <2909E983-953A-4463-959C-F3C386BC6C9A@yahoo.com> <20180226135457.B1203@besplex.bde.org> <1A2830F4-A00B-4C56-8D28-C46715DC7C9E@yahoo.com> <20180226200951.V2634@besplex.bde.org>
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On Mon, 26 Feb 2018 21:00:21 +1100 (EST) Bruce Evans <brde@optusnet.com.au> wrote: > For select() with restrict, I think the compiler cannot assert that > the args don't overlap since (even without the detailed specification > and Example 3), the compiler cannot know if select() modifies its args. > For all that the compiler knows, select() might be a stub that never > modifies or even reads anything. I think doing no accesses satifies > the constraints of restrict. It might be valid for the compiler to > assert that the (values pointed to by) the fdset args don't change for > select(nfd, &fdset, &fdset, &fdset, &tv) (because any write access > through an fdset type would give undefined behaviour on the other fdset > args; tv can still change since it has a different type). But this is > precisely what is wanted for the original example where we only care > about the return value -- then fdset is indeterminate after the call > so we shouldn't use it; the compiler is unlikely to optimize the non-use > of it and the worst that it can do is add a runtime assertion that the > arg didn't change. I don't think the compiler can assert that fdset is unmodified. It's valid for select to modify fdset through one pointer if it doesn't dereference the other two.
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