Re: Scripting

[Steve Baker <sjbaker1@airmail.net>]

Jorrit Tyberghein wrote:
> 
> Steve Baker wrote:
> 
> > ...yes - and after all, a non-realtime "traditional" compiler can just optimise
> > *ALL* of the code that well - regardless of whether it's used frequently or not.
> 
> >
> > The reason HotSpot is a good idea for an interpreter is that for infrequently
> > executed code, you can't optimise the code as well as you'd like to because
> > it takes longer than the time you'd save.  Hence, recognising the hot spots
> > allows you to come *closer* to the performance of a conventional compiler.
> >
> > I think this is a ridiculous claim - and if it were remotely likely to be
> > true then that's how C++ compilers would work.  You'd compile the program
> > offline - optimise it the best you possibly could with static knowledge of
> > the nature of the program - then keep a copy of the code and re-optimise
> > it based on this nebulous "additional information" that somehow becomes
> > available.
> 
> You're focusing too much on loop unrolling for which it is true that a 'static'
> compiler can often do a better optimizing job easier. But HotSpot can in
> theory (don't think it already does that at this moment) do much more than that.
> It can do statistical analyzes on parameters to functions and base optimizations
> on that. For example my switch example where statistical analyze showed
> that the parameter given to switch is between 1 and 10 for 90% of the cases
> (just an example). If a run time environment can detect that then it can
> optimize the switch code to be most optimal for values between 1 and 10.
> A static compiler can never do that because it doesn't have that statistical
> information.

...and you really think it can do all of that analysis, recompilation and
optimisation fast enough to actually save more than it costs?

I *really* doubt that.  You'd have to have a pretty amazing amount of
speedup to make it faster than a simple compiler which can do some of
that at compile time and imposes zero overhead at runtime.

It's not a matter of whether using dynamic information enables better
optimisation than static analysis could provide - it's a matter of whether
you can do that analysis (and recompilation) fast enough to beat a really
*deep* look at the code.

The compiler can do very global optimisations because it can take it's
time to look through all of it.  Your hotspot analyser isn't going to
have the time to optimise more than a small peephole at a time.

----------------------------- Steve Baker -------------------------------
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