From owner-freebsd-hardware  Sat Sep 20 05:05:07 1997
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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 
In-reply-to: Your message of "Sat, 20 Sep 1997 01:37:27 PDT."
             <199709200837.BAA22167@MindBender.serv.net> 
From: David Greenman <dg@root.com>
Reply-To: dg@root.com
Date: Sat, 20 Sep 1997 05:05:48 -0700
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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