Re: Hotspot. Dynamic compilers "better" than static one?

[Lieven Marchand <mal@bewoner.dma.be>]

"Dr Richard A. O'Keefe" <ok@atlas.otago.ac.nz> writes:

> Robert Dewar wrote:
> <<Many modern compilers support profiling feedback.>>
> 
> > Actually fewer production compilers than you might imagine actually use
> > this approach, though of course it has appeared in research compilers for
> > a long time.
> 
> Sun's C, Pascal, and Fortran compilers for SPARC do this.
> Digital's compilers for the Alpha do this.

The K&R C compiler for Ultrix already had profiling feedback about 10 years 
ago.

> What may be of more interest is that I have seen profile-driven
> feedback make a difference of 0-20%, with 0% actually being quite
> common, and I've had better performance using gcc -O6 (without
> feedback) on an Alpha than I've had from the Dec compiler _with_
> feedback.
> 

I've never seen it made much difference however. 

> So profile-driven feedback HAS BEEN SHIPPING FOR SEVERAL YEARS
> in production compilers from MIPS, Digital, and Sun, and that's
> just the ones I've used.  But it's only one technique, and not
> always the most important.

Has anybody done any studies on how to choose input for the trial runs 
you're going to feed back to the optimizer? There seems to be a silent
assumption that "typical" input values are adequate and that the choice
of input values won't make the performance worse on some other type of
values.

-- 
Lieven Marchand <mal@bewoner.dma.be> 
------------------------------------------------------------------------------
Few people have a talent for constructive laziness. -- Lazarus Long




















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