From: Knut Petersen <Knut_Petersen@t-online.de>
Subject: Re: [PATCH 1/1 2.6.13] framebuffer: bit_putcs()
 optimization for 8x* fonts
Date: Tue, 30 Aug 2005 19:58:51 +0200
Message-ID: <43149E5B.7040006@t-online.de>
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Cc: Andrew Morton <akpm@osdl.org>, "Antonino A. Daplas" <adaplas@gmail.com>, Linux Kernel Development <linux-kernel@vger.kernel.org>, Jochen Hein <jochen@jochen.org>


>Probably you can make it even faster by avoiding the multiplication, lik=
e
>
>    unsigned int offset =3D 0;
>    for (i =3D 0; i < image.height; i++) {
>	dst[offset] =3D src[i];
>	offset +=3D pitch;
>    }
>

More than two decades ago I learned to avoid mul and imul. Use shifts,=20
add and lea instead,
that was the credo those days. The name of the game was CP/M 80/86, a86,=20
d86 and ddt ;-)

But let=B4s get serious again.

Your proposed change of the patch results in a 21 ms performance=20
decrease on my system.
Yes, I do know that this is hard to believe. I tested a similar=20
variation before, and the results
were even worse.

Avoiding mul is a good idea in assembly language today, but often it is=20
better to write a
multiplication  with the loop counter in C and not to introduce an extra=20
variable instead. The
compiler will optimize the code and it=B4s easier for gcc without that=20
extra variable.

More interesting would be the question what should be done for idx=3D=3D2=
 or=20
idx=3D=3D3. Probably
fb_pad_aligned_buffer() is also slower for those cases. But does anybody=20
use such fonts?

cu,
 knut


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