Re: GCC mini-summit - compiling for a particular architecture

[Kenneth dot Hoste at elis dot UGent dot be]

Citeren Diego Novillo <dnovillo@redhat.com>:

> Mark Mitchell wrote on 04/23/07 13:56:
>
> > So, I think there's a middle ground between "exactly the same passes on
> > all targets" and "use Acovea for every CPU to pick what -O2 means".
> > Using Acovea to reveal some of the suprising, but beneficial results,
> > seems like a fine idea, though.
>
> I'm hoping to hear something along those lines at the next GCC Summit. I
> have heard of a bunch of work in academia doing extensive optimization
> space searches looking for combinations of pass sequencing and
> repetition to achieve optimal results.
>
> My naive idea is for someone to test all these different combinations
> and give us a set of -Ox recipes that we can use by default in the compiler.


Sorry to be blunt, but that's indeed quite naive :-)

Currently, the -On flags set/unset 60 flags, which yields 2^60 conbinations.
If you also kind the passes not controlled by a flag, but decided upon  
depending on the optimization level, that adds another, virtual flag  
(i.e. using -O1, -O2, -O3 or -Os as base setting).

A serious set of programs can easily take tens of minutes, so I think  
you can easily see trying all possible combinations is totally  
unfeasible. The nice thing is you don't have to: you can learn from  
the combinations you've evaluated, you can start with only a subset of  
programs and add others gradually, ...

My work is actually concentrating on building a framework to do  
exactly that: give a set of recipes for -On flags which allow a  
choice, and which are determined by trading off compilation time,  
execution time and code size.

I won't be at the GCC summit in Canada (I'm in San Diego then  
presenting some other work), but I'll make sure to announce our work  
when it's finished...

greetings,

Kenneth

--

Statistics are like a bikini. What they reveal is suggestive, but what  
they conceal is vital (Aaron Levenstein)

Kenneth Hoste
ELIS - Ghent University
kenneth.hoste@elis.ugent.be
http://www.elis.ugent.be/~kehoste

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