From owner-freebsd-hackers Thu Jun 4 04:49:15 1998 Return-Path: Received: (from majordom@localhost) by hub.freebsd.org (8.8.8/8.8.8) id EAA24380 for freebsd-hackers-outgoing; Thu, 4 Jun 1998 04:49:15 -0700 (PDT) (envelope-from owner-freebsd-hackers@FreeBSD.ORG) Received: from ns1.yes.no (ns1.yes.no [195.119.24.10]) by hub.freebsd.org (8.8.8/8.8.8) with ESMTP id EAA24374; Thu, 4 Jun 1998 04:49:10 -0700 (PDT) (envelope-from eivind@bitbox.follo.net) Received: from bitbox.follo.net (bitbox.follo.net [195.204.143.218]) by ns1.yes.no (8.8.7/8.8.7) with ESMTP id LAA19061; Thu, 4 Jun 1998 11:49:09 GMT Received: (from eivind@localhost) by bitbox.follo.net (8.8.8/8.8.6) id NAA08867; Thu, 4 Jun 1998 13:48:40 +0200 (MET DST) Message-ID: <19980604134839.09175@follo.net> Date: Thu, 4 Jun 1998 13:48:40 +0200 From: Eivind Eklund To: Mike Smith , Donn Miller Cc: "John S. Dyson" , hackers@FreeBSD.ORG Subject: Re: Recompiling sources with "-O2 -m486 -pipe" References: <199806040546.WAA00663@antipodes.cdrom.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii X-Mailer: Mutt 0.89.1i In-Reply-To: <199806040546.WAA00663@antipodes.cdrom.com>; from Mike Smith on Wed, Jun 03, 1998 at 10:46:28PM -0700 Sender: owner-freebsd-hackers@FreeBSD.ORG Precedence: bulk X-Loop: FreeBSD.ORG On Wed, Jun 03, 1998 at 10:46:28PM -0700, Mike Smith wrote: > > Some postings in DejaNews claim that the -mno-486 runs faster on a Pentium > > (than -m486). Other postings say just the opposite. > > -m486 optimises alignment for the '486 by padding. On the Pentium and > above alignment is not so significant, This is a very, very interesting statement. Unfortunately, it is also false. The _only_ thing I've found that affect PPro timing much is alignement, and alignment of inner loops can make a 50% speed difference. It caused me a lot of hassle until I found out about it ("Why the **** does changing that instruction, which is 20 instructions before the main loop, speed up things by 30%?") > and the padding wastes space in the cache and time for fetching and > discarding. Unless you intend to only run on a 486, it is generally > wrong to use it. What does it align to? PPro want alignment of the start of inner loops on a 16-byte boundary, IIRC... Eivind. To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message