From mboxrd@z Thu Jan 1 00:00:00 1970 From: Nick Piggin Subject: Re: Big git diff speedup by avoiding x86 "fast string" memcmp Date: Wed, 15 Dec 2010 16:54:50 +1100 Message-ID: <20101215055450.GC3398@amd> References: <20101209070938.GA3949@amd> <19324.1291990997@jrobl> <20101213014553.GA6522@amd> <9580.1292225351@jrobl> <20101215043840.GA7692@cr0.nay.redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: QUOTED-PRINTABLE Cc: Nick Piggin , "J. R. Okajima" , Nick Piggin , Linus Torvalds , linux-arch@vger.kernel.org, x86@kernel.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org To: =?iso-8859-1?Q?Am=E9rico?= Wang Return-path: Content-Disposition: inline In-Reply-To: <20101215043840.GA7692@cr0.nay.redhat.com> Sender: linux-arch-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org On Wed, Dec 15, 2010 at 12:38:40PM +0800, Am=E9rico Wang wrote: > On Mon, Dec 13, 2010 at 07:25:05PM +1100, Nick Piggin wrote: > >On Mon, Dec 13, 2010 at 6:29 PM, J. R. Okajima wrote: > >> But is the "rep cmp has quite a long latency" issue generic for al= l x86 > >> architecture, or Westmere system specific? > > > >I don't believe it is Westmere specific. Intel and AMD have > >been improving these instructions in the past few years, so > >Westmere is probably as good or better than any. > > > >That said, rep cmp may not be as heavily optimized as the > >set and copy string instructions. > > > >In short, I think the change should be suitable for all x86 CPUs, > >but I would like to hear more opinions or see numbers for other > >cores. > > >=20 > How about other arch? If this is only for x86, shouldn't it be > protected by CONFIG_X86? That's what I would like to know, but I suspect that for very short strings we are dealing with, the custom loop will be fine for everybody.