Re: Counting bits

[gtoal@an-teallach.com (Graham Toal)]

: Both Sun C and GCC on a Sun SPARC system running 4.1.3 produced this code
: for each bit-count line (-O4 optimization used):

: L77042:
:         andcc   %o0,2,%g0: : ; AND the bit
:         bne,a   L77044: : : ; branch/anull if zero
:         inc     %o5: : : ; increment bitcount
: L77044:

: This, I believe, is as optimized as it is possible to get on a uniprocessor
: machine.

Using branches is seriously bad news on some machines, especially
risk machines which are using a prefetched instruction pipeline.
Then of course you get machines with an on-chip cache, in which
case the looping variant becomes the best choice again.

And you have to figure architectures where every instruction is
conditional on the CC so you can have branches over (some) short
instruction sequences for free.

Serious optimization isn't a child's game.  When we did the 1's-counting
code for the Acorn RISC machine, every programmer in the office worked
on it for a week.  I think the best version in the end was a variation
of the trick shown earlier and some sneaky use of ARM conditionals and
address-loading instructions that could do arbitrary shifts on the fly
while adding.

I wish I'd kept it.  If anyone bumps into Paul Bond, I think he was
the guy who wrote the best one.  I'd like to see that one again for
nostalgia's sake :-)

G