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From "Mikhail Fursov" <mike.fur...@gmail.com>
Subject Re: [drlvm] interface call devirtualization
Date Fri, 21 Jul 2006 11:59:58 GMT
Egor,
I promised to start to investigate devirtualization techniques and here is
the first result:
I hacked the entry-backedge profiler in DRLVM and added the dump of virtual
methods call frequencies when VM is about to be shutted down.
The goal is to test that the last method in class hierarchy is the most
probable one.

The numbers I've got for different runtimes:

Eclipse startup (with a couple of opened projects):
Number of methods compiled: 18136
Number of virtual methods with multiple dispatches: 4676
Percent of the top methods frequency: 76%

DeCapo bloat:
Number of methods compiled: 1635
Number of virtual methods with multiple dispatches: 401
Percent of the top methods frequency: 88%

DeCapo hsqldb:
Number of methods compiled: 2322
Number of virtual methods with multiple dispatches: 512
Percent of the top methods frequency: 64%

DeCapo xalan:
Number of methods compiled: 2857
Number of virtual methods with multiple dispatches: 824
Percent of the top methods frequency: 59%

The results show that the heuristics to devirtualize the top  method in
hierarchy is practical, but the frequency of 'middle' methods is too high
and more advanced methods should be used.

Could I ask you or someone else to review my patch? It's a pity if I had an
errors in it :)

As the next task I will modify devirtualizer to select a method to
devirtualize by the hottest frequency and will compare the results.


On 20 Jul 2006 09:41:37 +0700, Egor Pasko <egor.pasko@gmail.com> wrote:
>
> On the 0x1AB day of Apache Harmony Mikhail Fursov wrote:
> > > Seems like the best choice is to start from a couple of easy
> heuristics:
> > > * if there is only one loaded class to implement the interface, choose
> it
> > > * if there are more, choose the one with it's method invoked earlier
> > > (compiled
> > >   by some JIT, possibly, some other JIT),
> > > * if we have many candidate methods that are compiled, choose the most
> > > frequent
> > >   one (need a method-entry profile, the feature is likely to stay
> > > untouched for
> > >   a while, I guess)
> > >
> > >
> > > 1. Does anybody have some additional elegant ideas?
> > >
> > >
> > Egor,  I'm interested in devirtualizer improvement too.
>
> Great!
>
> > IMO the profile based devirtualization will probably have the best
> results
> > and is easy to implement and to check: infrastructure in Jitrino is
> ready to
> > do it right now with a help of entry-backedge profile collected for OPT
> by
> > JET.
>
> OK, if it is not so difficult to use the profile. BTW, if I have a
> backedge in my HIR, how do I identify it with JET's backedge? Or is
> this done automatically in some way I should not care about?
>
> > + more ideas I have:
> > 1) to use edge profiler as value based one if initial compilation
> > devirtualize all possible dispatches and count their edge frequencies.
>
> Oh, that's a real code bloater!:) IMHO, inlining of these 'possible
> dispatches' should be disabled. Needs to make a hint for inliner. Not
> a difficult task though. Just an extra flag in call instruction.
>
> > 2) implement a real value profiler - this is not an easy task, but may
> be
> > reused in other optimizations
>
> Yes, sure. This is a big item for ongoing development.
>
> > 3) Add special annotations to classlib code about the most probable
> > dispatch. E.g. if the variable type does not depend on user's
> environment
> > and developer can prove that 90% of time the variable is of specific
> class -
> > JIT can read this annotation from method during the compilation and to
> > devirtualize it.
>
> Okay, waiting for 1.5 support here. I like the idea of writing
> annotations for devirtualization, parallelization, and other. They can
> provide hints without changing program semantics. I think, they should
> be available not only in classlib, but for user code too.
>
> > I will read more about other devirtualization techniques we can use and
> will
> > reply in a several days with results.
>
> That would be great! Thanks for ideas!
>
> --
> Egor Pasko, Intel Managed Runtime Division
>
>
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>


-- 
Mikhail Fursov

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