Re: [Linux-fbdev-devel] [PATCH 1/1 2.6.13] framebuffer: bit_putcs() optimization for 8x* fonts

[Knut Petersen <Knut_Petersen@t-online.de>]

> Probably you can make it even faster by avoiding the multiplication, like
>
>    unsigned int offset = 0;
>    for (i = 0; i < image.height; i++) {
> 	dst[offset] = src[i];
> 	offset += pitch;
>    }
More than two decades ago I learned to avoid mul and imul. Use shifts, 
add and lea instead,
that was the credo those days. The name of the game was CP/M 80/86, a86, 
d86 and ddt ;-)
But let´s get serious again.

Your proposed change of the patch results in a 21 ms performance 
decrease on my system.
Yes, I do know that this is hard to believe. I tested a similar 
variation before, and the results
were even worse.

Avoiding mul is a good idea in assembly language today, but often it is 
better to write a
multiplication  with the loop counter in C and not to introduce an extra 
variable instead. The
compiler will optimize the code and it´s easier for gcc without that 
extra variable.
More interesting would be the question what should be done for idx==2 or 
idx==3. Probably
fb_pad_aligned_buffer() is also slower for those cases. But does anybody 
use such fonts?
cu,
knut
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