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

[Alon Levy]

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 <address@hidden>

rmax/bmax/gmax are all uint8_t atm, could add a compilation error if
sizeof(bmax)!=1 ever.

> Signed-off-by: Avi Kivity <address@hidden>
> ---
>  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
> 
>