Date: Thu, 17 Aug 2006 00:34:31 +1000 (EST) From: Bruce Evans <bde@zeta.org.au> To: John Baldwin <jhb@FreeBSD.org> Cc: Kostik Belousov <kostikbel@gmail.com>, cvs-src@FreeBSD.org, src-committers@FreeBSD.org, David Xu <davidxu@FreeBSD.org>, cvs-all@FreeBSD.org Subject: Re: cvs commit: src/sys/amd64/amd64 support.S Message-ID: <20060817003125.I38538@delplex.bde.org> In-Reply-To: <200608151052.49421.jhb@freebsd.org> References: <200608151245.k7FCjpJo077372@repoman.freebsd.org> <200608152134.46359.davidxu@freebsd.org> <20060815135517.GB41562@deviant.kiev.zoral.com.ua> <200608151052.49421.jhb@freebsd.org>
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On Tue, 15 Aug 2006, John Baldwin wrote: > On Tuesday 15 August 2006 09:55, Kostik Belousov wrote: >> So, it seems that xorq %rax, %rax and xorl %eax, %eax will make the >> same results, but in the different ways. And xorq requires REX prefix, >> that shall make the decoding longer. > > Ok, thanks! David, can you revert your change? I had almost gone through > earlier and fixed all the places that did this a while back until I realized > that it must have been intentional. It is logged as intentional in the commit that went through and optimized all places that used xorq: %%% RCS file: /home/ncvs/src/sys/amd64/amd64/support.S,v Working file: support.S head: 1.118 ... ---------------------------- revision 1.116 date: 2005/09/27 18:32:46; author: peter; state: Exp; lines: +11 -11 Fix a minor nit that has been bugging me for a while. Fix the obvious cases of using a 64 bit operation to zero a register. 32 bit opcodes are smaller and supposedly faster, and clear the upper 32 bits for free. ---------------------------- %%% Bruce
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