Rakudo Day: operator overloading, isa speed improvement, build improvements

July and August have been really busy months with conferences and vacations, so I've fallen a bit behind on Rakudo days for my grant. However, now my travels are largely finished and the kids are back in school again, so I'm hoping I can manage at least two Rakudo days per week until I'm caught up with the original schedule.

This week's major task did indeed need two days: I've now enabled operator overloading for many of Rakudo's builtin operators. Previously Rakudo has allowed some custom operators to be defined, but overloading the builtin operators (such as infix:<+>) would generally result in "Null PMC in find_method" errors.

The main approach I ended up taking for this was to re-implement most of the existing operators (previously written in PIR) as Perl 6 multisub definitions with inline PIR. This ended up being somewhat more difficult than it might sound or appear from simply reading the patch -- part of the challenge is that if any variant of an operator is written using Perl 6 multisubs, then all of them must be written that way. But since we had been generating PIR for the Whatever and Junction variants of some builtins, that meant rewriting those versions as well (and figuring out a way to get Rakudo to create WhateverCode objects from the setting).

Anyway, the hardest pieces are now done, so that many of the built-in operators can be overloaded with custom variants. I'm sure a lot of people will start taking advantage of those; for example, I'm hoping that SF will be able to update his Vector class to overload the builtin operators now.

There are still some operators that need to be moved to the setting; although I've been able to migrate prefix:<-> and prefix:<~>, attempting to do the same with prefix:<+> causes a failure in one of the spectests. We'll keep plugging away at it until we figure it out.

I"m also seeking clarification about the definitions of some of the relational ops; for example, infix:<==> and infix:<eq> are defined in terms of the infix:<===> operator, but I'm curious about the definitions of the other relational ops.

One item that greatly concerned me about moving the operators into the settings was the possibility that it would significantly slow down Rakudo, because subs compiled from Perl 6 result in a lot more code than hand-written PIR subs do. I even explored some ways to be able to automatically rebless PIR multisubs into Rakudo equivalents. However, I decided to just try the Perl 6 approach (we'll ultimately need to do that anyway), and I was pleasantly surprised that the changes didn't result in significant speed hit to the spectests. Indeed, my timings show that the use of the Perl 6 versions may in fact be slightly faster. I'm not sure how this can be possible, but my best guess at this point is that Rakudo's custom multisub dispatcher (which Jonathan wrote as part of his Hague Grant) is somehow significantly faster than Parrot's default multi-dispatch. But that's just a guess on my part...

Speaking of speed, yesterday I also finished and committed another significant change to Parrot's "isa" functions for testing if a PMC is an instance of a given type. For a long time Parrot has used type name (string) comparisons to check isa membership; this is not only a bit slow, but it can also result in unwanted type name collisions. Ideally the comparison should test the identities of the class objects; but getting this to work required cleaning a few other pieces of Parrot's object and class handling. When it was all finished we obtained a ~4.5% overall speed improvement in Rakudo's spectests.

Lastly, I've done some more work on improving Rakudo's build and install environment. We now have Rakudo building from an installed Parrot, and yesterday I improved Rakudo's so that it warns with a more useful error message if the files needed from a Parrot devel installation aren't present.

Next I'll need to review the ticket queue again to see how many tickets have been resolved by the above changes. I'll also want to see about adding the tighter/looser/equiv traits to user-defined operators. But mainly I'm glad that we can now start to do some more advanced operator handling and overloading in Rakudo.

Thanks as always to for sponsoring this work.


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