From owner-svn-src-head@freebsd.org Sat Feb 22 16:29:29 2020 Return-Path: Delivered-To: svn-src-head@mailman.nyi.freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2610:1c1:1:606c::19:1]) by mailman.nyi.freebsd.org (Postfix) with ESMTP id 080E9243F3B; Sat, 22 Feb 2020 16:29:29 +0000 (UTC) (envelope-from kevans@freebsd.org) Received: from smtp.freebsd.org (smtp.freebsd.org [96.47.72.83]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) server-signature RSA-PSS (4096 bits) client-signature RSA-PSS (4096 bits) client-digest SHA256) (Client CN "smtp.freebsd.org", Issuer "Let's Encrypt Authority X3" (verified OK)) by mx1.freebsd.org (Postfix) with ESMTPS id 48Pv146VX5z3xtc; Sat, 22 Feb 2020 16:29:28 +0000 (UTC) (envelope-from kevans@freebsd.org) Received: from mail-qk1-f182.google.com (mail-qk1-f182.google.com [209.85.222.182]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (Client CN "smtp.gmail.com", Issuer "GTS CA 1O1" (verified OK)) (Authenticated sender: kevans) by smtp.freebsd.org (Postfix) with ESMTPSA id CDA7E5A65; Sat, 22 Feb 2020 16:29:28 +0000 (UTC) (envelope-from kevans@freebsd.org) Received: by mail-qk1-f182.google.com with SMTP id u124so4847832qkh.13; Sat, 22 Feb 2020 08:29:28 -0800 (PST) X-Gm-Message-State: APjAAAU55GjZBUDXuNsqAXv8Zc6NO6rS06JUL1UEK3PAi+hOs2aFVUG/ k/P6/GIa1EsBiwAB9sxp1J8EcSQhz0Jwr5tkjzg= X-Google-Smtp-Source: APXvYqysNew7p664ZpBk02eep1hY408dO8DhG3ixjgibR5U1n5q+jgffwlxJmh2SQlNr2i4i965vZMluB8EtCc1TNAQ= X-Received: by 2002:a05:620a:7f5:: with SMTP id k21mr14836293qkk.493.1582388968362; Sat, 22 Feb 2020 08:29:28 -0800 (PST) MIME-Version: 1.0 References: <202002221620.01MGK46E072303@repo.freebsd.org> In-Reply-To: From: Kyle Evans Date: Sat, 22 Feb 2020 10:29:17 -0600 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: svn commit: r358248 - head/sys/vm To: Ian Lepore Cc: src-committers , svn-src-all , svn-src-head Content-Type: text/plain; charset="UTF-8" X-BeenThere: svn-src-head@freebsd.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: SVN commit messages for the src tree for head/-current List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Sat, 22 Feb 2020 16:29:29 -0000 On Sat, Feb 22, 2020 at 10:25 AM Ian Lepore wrote: > > On Sat, 2020-02-22 at 16:20 +0000, Kyle Evans wrote: > > Author: kevans > > Date: Sat Feb 22 16:20:04 2020 > > New Revision: 358248 > > URL: https://svnweb.freebsd.org/changeset/base/358248 > > > > Log: > > vm_radix: prefer __builtin_unreachable() to an unreachable panic() > > > > This provides the needed hint to GCC and offers an annotation for readers to > > observe that it's in-fact impossible to hit this point. We'll get hit with a > > a -Wswitch error if the enum applicable to the switch above were to get > > expanded without the new value(s) being handled. > > > > Modified: > > head/sys/vm/vm_radix.c > > > > Modified: head/sys/vm/vm_radix.c > > ============================================================================== > > --- head/sys/vm/vm_radix.c Sat Feb 22 13:23:27 2020 (r358247) > > +++ head/sys/vm/vm_radix.c Sat Feb 22 16:20:04 2020 (r358248) > > @@ -208,8 +208,7 @@ vm_radix_node_load(smrnode_t *p, enum vm_radix_access > > case SMR: > > return (smr_entered_load(p, vm_radix_smr)); > > } > > - /* This is unreachable, silence gcc. */ > > - panic("vm_radix_node_get: Unknown access type"); > > + __unreachable(); > > } > > > > static __inline void > > What does __unreachable() do if the code ever becomes reachable? Like > if a new enum value is added and this switch doesn't get updated? > __unreachable doesn't help here, but the compiler will error out on the switch() if all enum values aren't addressed and there's no default: case. IMO, compilers could/should become smart enough to error if there's an explicit __builtin_unreachable() and they can trivially determine that all paths will terminate before this, independent of -Werror=switch*.