From owner-svn-src-head@freebsd.org Fri Feb 15 14:04:19 2019 Return-Path: Delivered-To: svn-src-head@mailman.ysv.freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2610:1c1:1:606c::19:1]) by mailman.ysv.freebsd.org (Postfix) with ESMTP id 3E9F514DC4DA; Fri, 15 Feb 2019 14:04:19 +0000 (UTC) (envelope-from kostikbel@gmail.com) Received: from kib.kiev.ua (kib.kiev.ua [IPv6:2001:470:d5e7:1::1]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by mx1.freebsd.org (Postfix) with ESMTPS id 553F46BA7F; Fri, 15 Feb 2019 14:04:18 +0000 (UTC) (envelope-from kostikbel@gmail.com) Received: from tom.home (kib@localhost [127.0.0.1]) by kib.kiev.ua (8.15.2/8.15.2) with ESMTPS id x1FE49SL083824 (version=TLSv1.2 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=NO); Fri, 15 Feb 2019 16:04:13 +0200 (EET) (envelope-from kostikbel@gmail.com) DKIM-Filter: OpenDKIM Filter v2.10.3 kib.kiev.ua x1FE49SL083824 Received: (from kostik@localhost) by tom.home (8.15.2/8.15.2/Submit) id x1FE49ZY083823; Fri, 15 Feb 2019 16:04:09 +0200 (EET) (envelope-from kostikbel@gmail.com) X-Authentication-Warning: tom.home: kostik set sender to kostikbel@gmail.com using -f Date: Fri, 15 Feb 2019 16:04:09 +0200 From: Konstantin Belousov To: Bruce Evans Cc: Alexey Dokuchaev , src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: Re: svn commit: r344118 - head/sys/i386/include Message-ID: <20190215140409.GQ24863@kib.kiev.ua> References: <201902141353.x1EDrB0Z076223@repo.freebsd.org> <20190215071604.GA89653@FreeBSD.org> <20190215103644.GN24863@kib.kiev.ua> <20190215233444.F2229@besplex.bde.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190215233444.F2229@besplex.bde.org> User-Agent: Mutt/1.11.2 (2019-01-07) X-Spam-Status: No, score=-1.0 required=5.0 tests=ALL_TRUSTED,BAYES_00, DKIM_ADSP_CUSTOM_MED,FORGED_GMAIL_RCVD,FREEMAIL_FROM, NML_ADSP_CUSTOM_MED autolearn=no autolearn_force=no version=3.4.2 X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on tom.home 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: Fri, 15 Feb 2019 14:04:19 -0000 On Sat, Feb 16, 2019 at 12:27:16AM +1100, Bruce Evans wrote: > On Fri, 15 Feb 2019, Konstantin Belousov wrote: > > > On Fri, Feb 15, 2019 at 07:16:04AM +0000, Alexey Dokuchaev wrote: > >> On Thu, Feb 14, 2019 at 01:53:11PM +0000, Konstantin Belousov wrote: > >>> New Revision: 344118 > >>> URL: https://svnweb.freebsd.org/changeset/base/344118 > >>> > >>> Log: > >>> Provide userspace versions of do_cpuid() and cpuid_count() on i386. > >>> > >>> Some older compilers, when generating PIC code, cannot handle inline > >>> asm that clobbers %ebx (because %ebx is used as the GOT offset > >>> register). Userspace versions avoid clobbering %ebx by saving it to > >>> stack before executing the CPUID instruction. > >>> > >>> ... > >>> +static __inline void > >>> +do_cpuid(u_int ax, u_int *p) > >>> +{ > >>> + __asm __volatile( > >>> + "pushl\t%%ebx\n\t" > >>> + "cpuid\n\t" > >>> + "movl\t%%ebx,%1\n\t" > >>> + "popl\t%%ebx" > >> > >> Is there a reason to prefer pushl+movl+popl instead of movl+xchgl? > >> > >> "movl %%ebx, %1\n\t" > >> "cpuid\n\t" > >> "xchgl %%ebx, %1" > > > > xchgl seems to be slower even in registers format (where no implicit > > lock is used). If you can demonstrate that your fragment is better in > > some microbenchmark, I can change it. But also note that its use is not > > on the critical path. > > The should have the same speed on modern x86. xchgl %reg1,%reg2 is > not slow, but it changes 2 visible registers and a needs somwhere to > hold one of the registers while changing it, so on 14 year old AthlonXP > where I know the times in cycles better, register xchgl was twice as slow > as register move (2 cycles latency instead of 1, and throughput == > latency (?)). On 2015 Haswell, register movl in a loop is in parallel > with the loop overhead (1 cycle), while xchgl and pushl/popl take 0.5 > cycles longer on average. Latency might be a problem for pushl/popl > in critical paths. There aren't many of those. I think on modern Intels xchgl is implemented by renaming. Still it is slower than typically highly optimized push/pops. That said, what is your preference ? My version or xchgl ? My own preference is to leave it as is, since it is slightly slower, and I do not want to spend several hours again, re-testing libc changes.