Date: Fri, 15 Feb 2019 13:55:51 +0000 From: Alexey Dokuchaev <danfe@freebsd.org> To: Bruce Evans <brde@optusnet.com.au> Cc: Konstantin Belousov <kostikbel@gmail.com>, 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> In-Reply-To: <20190215233444.F2229@besplex.bde.org> References: <201902141353.x1EDrB0Z076223@repo.freebsd.org> <20190215071604.GA89653@FreeBSD.org> <20190215103644.GN24863@kib.kiev.ua> <20190215233444.F2229@besplex.bde.org>
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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
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