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Date: Thu, 12 Jan 2012 17:19:44 +0200
From: Alon Levy <alevy@redhat.com>
Message-ID: <20120112151944.GZ5762@garlic.tlv.redhat.com>
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	<20120112151603.GY5762@garlic.tlv.redhat.com>
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Subject: Re: [Qemu-devel] [PATCH] vga: optimize ppm_save() divisions
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To: Avi Kivity <avi@redhat.com>, qemu-devel@nongnu.org

On Thu, Jan 12, 2012 at 05:16:03PM +0200, Alon Levy wrote:
> On Tue, Jan 03, 2012 at 03:32:57PM +0200, Avi Kivity wrote:
> > ppm_save() spends upwards of 50% of its time doing divisions. Replace them
> > with shifts.
> > 
> 
> Reviewed-by: Alon Levy <alevy@redhat.com>
> 
> rmax/bmax/gmax are all uint8_t atm, could add a compilation error if
> sizeof(bmax)!=1 ever.

meant {r,g,b}bits.

> 
> > Signed-off-by: Avi Kivity <avi@redhat.com>
> > ---
> >  hw/vga.c |   10 ++++------
> >  1 files changed, 4 insertions(+), 6 deletions(-)
> > 
> > diff --git a/hw/vga.c b/hw/vga.c
> > index ca79aa1..a228cde 100644
> > --- a/hw/vga.c
> > +++ b/hw/vga.c
> > @@ -2370,12 +2370,10 @@ int ppm_save(const char *filename, struct DisplaySurface *ds)
> >                  v = *(uint32_t *)d;
> >              else
> >                  v = (uint32_t) (*(uint16_t *)d);
> > -            r = ((v >> ds->pf.rshift) & ds->pf.rmax) * 256 /
> > -                (ds->pf.rmax + 1);
> > -            g = ((v >> ds->pf.gshift) & ds->pf.gmax) * 256 /
> > -                (ds->pf.gmax + 1);
> > -            b = ((v >> ds->pf.bshift) & ds->pf.bmax) * 256 /
> > -                (ds->pf.bmax + 1);
> > +            /* Limited to 8 or fewer bits per channel: */
> > +            r = ((v >> ds->pf.rshift) & ds->pf.rmax) << (8 - ds->pf.rbits);
> > +            g = ((v >> ds->pf.gshift) & ds->pf.gmax) << (8 - ds->pf.gbits);
> > +            b = ((v >> ds->pf.bshift) & ds->pf.bmax) << (8 - ds->pf.bbits);
> >              *pbuf++ = r;
> >              *pbuf++ = g;
> >              *pbuf++ = b;
> > -- 
> > 1.7.7.1
> > 
> > 
>