From owner-freebsd-current@FreeBSD.ORG Tue Apr 22 04:43:44 2003 Return-Path: Delivered-To: freebsd-current@freebsd.org Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125]) by hub.freebsd.org (Postfix) with ESMTP id 11DDD37B401; Tue, 22 Apr 2003 04:43:44 -0700 (PDT) Received: from mailman.zeta.org.au (mailman.zeta.org.au [203.26.10.16]) by mx1.FreeBSD.org (Postfix) with ESMTP id 0140B43F93; Tue, 22 Apr 2003 04:43:42 -0700 (PDT) (envelope-from bde@zeta.org.au) Received: from katana.zip.com.au (katana.zip.com.au [61.8.7.246]) by mailman.zeta.org.au (8.9.3p2/8.8.7) with ESMTP id VAA31013; Tue, 22 Apr 2003 21:43:39 +1000 Date: Tue, 22 Apr 2003 21:43:38 +1000 (EST) From: Bruce Evans X-X-Sender: bde@gamplex.bde.org To: David Schultz In-Reply-To: <20030421211440.GA5507@HAL9000.homeunix.com> Message-ID: <20030422210632.U16843@gamplex.bde.org> References: <20030420192319.GB4963@HAL9000.homeunix.com> <20030421231756.H11214@gamplex.bde.org> <20030421211440.GA5507@HAL9000.homeunix.com> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII cc: Robert Watson cc: current@freebsd.org Subject: Re: HEADS UP: UFS2 now the default creation type on 5.0-CURRENT X-BeenThere: freebsd-current@freebsd.org X-Mailman-Version: 2.1.1 Precedence: list List-Id: Discussions about the use of FreeBSD-current List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Tue, 22 Apr 2003 11:43:44 -0000 On Mon, 21 Apr 2003, David Schultz wrote: > On Mon, Apr 21, 2003, Bruce Evans wrote: > > On Mon, 21 Apr 2003, David Schultz wrote: > > > Index: ufsread.c > > > =================================================================== > > > RCS file: /cvs/src/sys/boot/common/ufsread.c,v > > > retrieving revision 1.11 > > > diff -u -r1.11 ufsread.c > > > --- ufsread.c 25 Feb 2003 00:10:20 -0000 1.11 > > > +++ ufsread.c 21 Apr 2003 10:10:01 -0000 > > > ... > > > @@ -47,11 +59,11 @@ > > > ... > > > -#define FS_TO_VBA(fs, fsb, off) (fsbtodb(fs, fsb) + \ > > > - ((off) / VBLKSIZE) * DBPERVBLK) > > > +#define FS_TO_VBA(fs, fsb, off) ma((off) / VBLKSIZE, DBPERVBLK, \ > > > + fsbtodb((fs), (fsb))) > > > > The division by VBLKSIZE should probably be a shift. ufsread.c has > > VBLKSHIFT and uses it for all multiplications and divisions by VBLKSIZE > > except this one. gcc can't optimize to just a shift since all the > > types are signed and C99 specifies that division of negative integers > > by positive ones has the usual hardware brokenness. > > As I recall, signed division gets optimized into a sign test, some ^ by a power of 2 > bit fiddling for negative numbers, and a division. The additional shift > cost is nominal if you only care about speed, but I'm sure using a > shift directly would save a few more bytes. I tried this, but it had no effect since FS_TO_VBA() is never actually used. So there is a much better optimization for it :-). I think this makes ma() unused too. I thought that the savings for unsigned division were more for long longs than for longs, but they are actually relatively smaller. On i386's, signed division (when optimized to shifts) of %edx:%eax by 2^12 takes 19 bytes and right shift takes 7 bytes; the corresponding numbers for %eax are 12 bytes and 3 bytes. Optimization for space should not use shifts for the signed case. Bruce