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Re: [Qemu-devel] [PATCH] vga: optimize ppm_save() divisions
From: |
Alon Levy |
Subject: |
Re: [Qemu-devel] [PATCH] vga: optimize ppm_save() divisions |
Date: |
Thu, 12 Jan 2012 17:16:03 +0200 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
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
>
>