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Postings from March 2008
On Tue, Mar 18, 2008 at 2:16 PM, Nicholas Clark <nick@ccl4.org> wrote:
> On Tue, Mar 18, 2008 at 06:22:17PM +0100, Rafael Garcia-Suarez wrote:
> > On 18/03/2008, Jim Cromie <jim.cromie@gmail.com> wrote:
> > > Ok, Ill bite.
> > >
> > > these are ordered relative to each other only, Ive made no attempt to
> > > grade them vs the other entries.
> > >
> > > feel free to discuss/dispute ..
> >
> > All those todoes are nice, although the "squeeze the optree" sounds a
> > bit like science fiction to me.
>
> I continue to be extremely doubtful that squeezing the optree by the method
> Jim suggests will work successfully. It's adding a lot of complexity for
> negative total memory saving (one has to store the map from ops to sibling
> pointers somewhere), with only local positive memory saving. The necessary
> extra complexity strikes me as something that bugs will love to breed in.
>
>
> > > +=head2 optimize tail-calls
> > > +
> > > +Tail-calls present an opportunity for broadly applicable optimization;
> > > +anywhere that C<< return foo(...) >> is called, the outer return can
> > > +be replaced by a goto, and foo will return directly to the outer
> > > +caller, saving (conservatively) 25% of perl's call&return cost, which
> > > +is relatively higher than in C. The scheme language is known to do
> > > +this heavily. B::Concise provides good insight into where this
> > > +optimization is possible, ie anywhere entersub,leavesub op-sequence
> > > +occurs.
> > > +
> > > + perl -MO=Concise,-exec,a,b,-main -e 'sub a{ 1 }; sub b {a()}; b(2)'
> > > +
> > > +Bottom line on this is probably a new pp_tailcall function which
> > > +combines the code in pp_entersub, pp_leavesub. This should probably
> > > +be done 1st in XS, and using B::Generate to patch the new OP into the
> > > +optrees.
>
> Will modules using caller for introspection choke on this?
I think I played around with optimizing tail calls to "goto &crraack".
I remember there were several outstanding issues with the debugger
that I hadn't the heart to tackle at the time.
>
> What would Devel::Cover think? (being a useful piece of code that it would be
> bad to break)
There are some things (ternary operator and for/foreach) that cause
Devel::Cover some mistranslation. Whether it would cause other
problems, I can't really say.
Steve Peters
steve@fisharerojo.org
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