Re: HEADS UP: UFS2 now the default creation type on 5.0-CURRENT

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
> > > ...
> > > _at__at_ -47,11 +59,11 _at__at_
> > >  ...
> > > -#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