Re: gcc 2.95 vs 3.21 performance

P
2003-02-04T13:58:52Z

Helge Hafting wrote:
> Padraig@Linux.ie wrote:
> [...]
>
>>Interesting. I just noticed that I get 50% decrease in
>>the speed of my program if I just insert a printf(). I.E.
>>my program is like:
>>
>>printf()
>>for(;;) {
>> do_sorting_loop_test();
>>}
>>
>>If I remove the initial printf it doubles in speed?
>>I assume this is some weird caching thing?
>
>
> Looks like a cacheline alignment issue to me.
> This loop of yours occupy x cachelines on your cpu,
> moving it in memory by adding the printf
> might cause it to ocupy x+1 cachelines.
> That might be noticeable if x is a really small number,
> such as 1.

OK it is (as I suspected and as you explained nicely)
related to the cachelines on my CPU (866 celery).

===============================
GCC options loops/s
===============================
gcc 2283
gcc -O3 -falign-loops=2 3451
gcc -O3 -falign-loops=4 3443
gcc -O3 -falign-loops=8 7045
gcc -march=i686 -O3 9101
===============================

cheers,
Pádraig.