On Tue, 30 Aug 2005, Knut Petersen wrote:
> > 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 -- [email protected]
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
[Index of Archives]
[Kernel Newbies]
[Netfilter]
[Bugtraq]
[Photo]
[Gimp]
[Yosemite News]
[MIPS Linux]
[ARM Linux]
[Linux Security]
[Linux RAID]
[Video 4 Linux]
[Linux for the blind]
| |
 |