Date: Thu, 21 Apr 2016 13:25:55 -0500 From: Pedro Giffuni <pfg@FreeBSD.org> To: Hans Petter Selasky <hps@selasky.org>, Juli Mallett <juli@clockworksquid.com> Cc: Ngie Cooper <yaneurabeya@gmail.com>, "src-committers@freebsd.org" <src-committers@freebsd.org>, "svn-src-all@freebsd.org" <svn-src-all@freebsd.org>, "svn-src-head@freebsd.org" <svn-src-head@freebsd.org> Subject: Re: svn commit: r298412 - head/sys/ofed/drivers/infiniband/core Message-ID: <57191B33.7070406@FreeBSD.org> In-Reply-To: <5719180F.90509@selasky.org> References: <201604211604.u3LG4wmT035874@repo.freebsd.org> <CAGHfRMC3UW6dAy6cW%2B8EJ1f=RxQy%2BTJV1dvHozGhaSmUWqAdNA@mail.gmail.com> <57190C47.8070106@selasky.org> <CACVs6=-iKVChHOiJjbn%2BOft-uUE603rwnfnk=fhZFtRVW0mfHg@mail.gmail.com> <5719150F.7030200@FreeBSD.org> <5719180F.90509@selasky.org>
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On 21/04/2016 13:12, Hans Petter Selasky wrote: > On 04/21/16 19:59, Pedro Giffuni wrote: >> >> >> On 21/04/2016 12:52, Juli Mallett wrote: >>> On Thu, Apr 21, 2016 at 10:22 AM, Hans Petter Selasky >>> <hps@selasky.org> wrote: >>>> On 04/21/16 19:12, Ngie Cooper wrote: > > Hi, > >>> Then there should be an assertion or something else of that sort (I >>> forget if we have a __builtin_unreachable()-alike macro in the kernel; >>> the lint(1)y NOTREACHED isn't as nice as actual code) so that it is >>> apparent to a human that this case cannot be reached. The presence of >>> a do-nothing default case is not typically indicative of unreachable >>> code. >> >> We do have __unreachable() in cdefs.h >> >> It should work with both GCC and clang. >> >> Pedro. >> > > I don't see anyone using __unreachable() yet in the kernel. Do you > recommend it over a KASSERT() ? > It's a rather recent addition (I added it after checking NetBSD's cdefs). It is meant mostly for the compiler/static analyzers. From the clang documentation: "The __builtin_unreachable() builtin has completely undefined behavior. Since it has undefined behavior, it is a statement that it is never reached and the optimizer can take advantage of this to produce better code. This builtin takes no arguments and produces a void result." There was some discussion before it was introduced about turning it into an assertion, but I recall it was not possible. hth, Pedro.
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