Re: [Qemu-devel] [PATCH] vga: optimize ppm_save() divisions

[Anthony Liguori <anthony@codemonkey.ws>]

On 01/03/2012 07:32 AM, Avi Kivity wrote:
> ppm_save() spends upwards of 50% of its time doing divisions. Replace them
> with shifts.
>
> Signed-off-by: Avi Kivity<avi@redhat.com>

Applied.  Thanks.

Regards,

Anthony Liguori

> ---
>   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;