From owner-svn-src-head@freebsd.org Fri Feb 15 13:55:52 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 2244414DBFD6; Fri, 15 Feb 2019 13:55:52 +0000 (UTC) (envelope-from danfe@freebsd.org) Received: from freefall.freebsd.org (freefall.freebsd.org [96.47.72.132]) (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 "freefall.freebsd.org", Issuer "Let's Encrypt Authority X3" (verified OK)) by mx1.freebsd.org (Postfix) with ESMTPS id B1AE26B208; Fri, 15 Feb 2019 13:55:51 +0000 (UTC) (envelope-from danfe@freebsd.org) Received: by freefall.freebsd.org (Postfix, from userid 1033) id 8CB38FA0; Fri, 15 Feb 2019 13:55:51 +0000 (UTC) Date: Fri, 15 Feb 2019 13:55:51 +0000 From: Alexey Dokuchaev To: Bruce Evans Cc: Konstantin Belousov , 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: <20190215135551.GA99583@FreeBSD.org> 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/1.10.1 (2018-07-13) X-Rspamd-Queue-Id: B1AE26B208 X-Spamd-Bar: ------ Authentication-Results: mx1.freebsd.org X-Spamd-Result: default: False [-6.96 / 15.00]; NEURAL_HAM_MEDIUM(-1.00)[-0.998,0]; NEURAL_HAM_SHORT(-0.96)[-0.962,0]; REPLY(-4.00)[]; NEURAL_HAM_LONG(-1.00)[-1.000,0] 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 13:55:52 -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. > >>> ... > >>> +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. Thanks Bruce, I guess we can leave it as is then. ./danfe