On Tue, 30 Aug 2005, Knut Petersen wrote:

quoted

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.

On modern CPUs, a multiplication indeed takes 1 cycle, just like an addition.
But on older CPUs (still supported by Linux), this is not true.

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.

But you are right. On actual inspection of the generated assembly code for a
very simple test case, it turns out both (m68k-linux-)gcc 2.95.2 and 3.3.3 are
smart enough to convert the multiplication to an addition...

And interestingly, if I avoid the multiplication explicitly, gcc 2.95.2 still
generates the same code, but 3.3.3 adds a few extra instructions to
save/restore local vars. So this probably explains why it turned out to be
slower for you. Ugh...

Gr{oetje,eeting}s,

						Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
							    -- Linus Torvalds