Date: Thu, 22 Feb 2018 12:56:08 +0200 From: Konstantin Belousov <kostikbel@gmail.com> To: Tijl Coosemans <tijl@FreeBSD.org> Cc: Eitan Adler <lists@eitanadler.com>, Kevin Lo <kevlo@freebsd.org>, FreeBSD Standards <freebsd-standards@freebsd.org>, FreeBSD Hackers <freebsd-hackers@freebsd.org> Subject: Re: Marking select(2) as restrict Message-ID: <20180222105608.GE94212@kib.kiev.ua> In-Reply-To: <20180222112752.10da7e51@kalimero.tijl.coosemans.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>
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On Thu, Feb 22, 2018 at 11:27:52AM +0100, Tijl Coosemans wrote: > On Wed, 21 Feb 2018 12:44:00 +0200 Konstantin Belousov <kostikbel@gmail.com> wrote: > > On Tue, Feb 20, 2018 at 10:14:05PM -0800, Eitan Adler wrote: > >> On 20 February 2018 at 21:19, Warner Losh <imp@bsdimp.com> wrote: > >>> Once upon a time, this would break a lot of code. Perhaps times have > >>> changed. > >> > >> I've seen very little code that this would break though some of it > >> certainly exists. > > You certainly seen very little code, but the question was about the > > existed code. > > FWIW, it seems that glibc uses restrict since 2000 so there's unlikely to > be much fallout: > https://sourceware.org/git/?p=glibc.git;a=blob;f=misc/sys/select.h > https://sourceware.org/git/?p=glibc.git;a=commitdiff;h=98cbe360d947b59e7a5eda068581f4cfeb4b99b3 Clearly, nobody knowns. At least, glibc is used with gcc compilation, not with clang. Consider the recently changed devd code: select(n + 1, &fd, &fd, &fd); There, compiler can see that restrict is applied to arguments which are given same values. Since this leads to the self-contradicting statement fd != fd which cannot be true, compliler in its optimizing wisdom can assume that the code is never executing and remove it. I do not know whether clang actually makes such transformation, but it does not sound unfeasible looking at its other advances.
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