Re: Hot and Cold Partitioning (Was: GCC 4.1 Projects)

[Dale Johannesen <dalej at apple dot com>]

On Feb 28, 2005, at 4:43 AM, Joern RENNECKE wrote:

> Dale Johannesen wrote:
>
> >    Well, no, what is supposed to happen (I haven't tried it for a 
> > while, so I don't promise
> > this still works) is code like this:
> >
> > .hotsection:
> > loop:
> >   conditional branch (i?==1000) to L2
> > L1:
> >   /* do stuff */
> > end loop:
> >
> > /* still in hot section  */
> > L2:  jmp L3
> >
> > .coldsection:
> > L3:
> >   i = 0;
> >   jmp L1
>
> Well, even then, using of the cold section can increase the hot 
> section size, depending on target, and for some
> targets the maximum supported distance of the cold section.

Certainly.  In general it will make the total size bigger, as does 
inlining.  If you have good
information about what's hot and cold, it should reduce the number of 
pages that actually
get swapped in.  The information has to be good, though, as a branch 
from
hot<->cold section becomes more expensive.  I'd recommend it only if 
you have
profiling data (this is a known winner on Spec in that situation).

> Should I do custom basic block reordering in machine_dependent_reorg 
> to clean up
> the turds of hot and cold partitioning?

No, you should not turn on partitioning in situations where code size 
is important to you.