Date: Sat, 20 Sep 1997 05:05:48 -0700 From: David Greenman <dg@root.com> To: "Michael L. VanLoon -- HeadCandy.com" <michaelv@MindBender.serv.net> Cc: Mark Mayo <mark@quickweb.com>, Robert Schien <robsch@robkaos.ruhr.de>, hardware@freebsd.org Subject: Re: 'make world' on P6 system takes 3 h Message-ID: <199709201205.FAA19765@implode.root.com> In-Reply-To: Your message of "Sat, 20 Sep 1997 01:37:27 PDT." <199709200837.BAA22167@MindBender.serv.net>
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>>Just curious. Did the -m486 ever really do anything?? I've always used it >>for 'make world' and kernel compiles in my 486's, but now that I'm using a >>PPro I suppose it's useless, no? > >No, it's not useless. Think about it: -m486 assumes that you want >code to be optimized for processors _better_ than a 386. Why would >then removing it for a Pentium Pro be good? That means you're going >back to code that works better on a 386. -m486 means to specifically optimize for the 486. It does not mean to optimize for the greater than 386. >>From what I understand, most of the "benefit" from -m486 is that it >aligns data on 16-byte boundaries instead of 4 or 8-byte boundaries. >This supposedly helps cache loads happen more efficiently. Back in >"the old days", when 486s ruled, the XFree86 guys said that -m486 gave >them 10-20% improvement in speed, if I remember right. They said >speed gains were similar on Pentiums (and why shouldn't they be? -- >optimizations for 486 shouldn't hurt Pentiums). > >There's no reason to assume it should hurt a Pentium Pro -- in fact >going back to 4 or 8-byte alignment is almost certainly going to be >slower on the PPro, which is highly-optimized for 32-bit code, with >64-bit and 128-bit internal busses. Yes, but the cache works very different on the P6 and it's been found that the alignment just results in a larger, slower program. -DG David Greenman Core-team/Principal Architect, The FreeBSD Project
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