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We need some kind of overlay language for M0 that we can use to reimplement
Parrot. Writing poke_caller was hard and writing a factorial program will be
harder. Anything less trivial will require an actual compiler or code
generator. We have the following options:
1 (PIR/M0) emit M0 from PIR
2 (nqp/M0) emit M0 from nqp
3 (winxed/M0) emit M0 from winxed
4 (new-nqp/M0) write a new compiler targeting M0 using nqp
5 (new-custom/M0) write a new compiler targeting M0 using a separate toolchain
6 (steal/retarget) take someone's existing compiler, retarget it to M0
PIR/M0 has the advantage that we'll need to do something similar later anyway.
Being able to translate from PIR to M0 will be necessary if we want to continue
to support PIR, and we do. I'm not sure if we'll want to replace Parrot's
current C code with PIR, though. This approach is worth considering.
I don't like the idea of nqp/M0. nqp is already quite slow and I don't see it
being feasible to get a speed improvement by using it more internally. It
might be the case that we don't need to generate inefficient code to deal with
lvalue semantics if the translation is well-designed. There's also the concern
that using nqp almost universally means using a bunch of pir:: garbage, which
would make M0 translation less efficient. Overall it's a fairly nice language,
but I'm not certain that nqp/M0 is the best way forward.
winxed/M0 is a nice option. The compiler already exists and has an alternate
version (winxedxx) that targets C++. Unfortunately winxed isn't designed to
support multiple codegen backends, so we'd have to either refactor codegen into
a separate step (probably slowing down performance) or just fork it and write a
new backend. The language itself is quite pleasant, but the compiler needs
work. I'm still not convinced that this is a bad way forward.
new-nqp/M0 brings with it the speed issues of nqp. nqp is very much designed
to support a highly flexible compilation workflow, so using it to generate M0
is a reasonable approach. I'm not a fan of the langauge's speed and quirks,
though. This approach could be made to work but it doesn't sound like the best
new-custom/M0 is almost included for the sake of completeness. I'm tired of
writing meta-things and want to get some real work done. Writing a new
compiler from scratch is decidedly non-lazy.
steal/retarget is a generalization of the winxed/M0 approach. Instead of
retargeting winxed, we'd take an unrelated compiler for some language (js comes
to mind) and target that at M0 using CPS for control flow. This has the
advantage that we're not writing (and debugging) a whole new compiler from
scratch, but it depends on us finding an appropriately-licensed compiler for a
suitable language and constructed in a modular fashion.
Probably the biggest question is what language we want to use. We'll be
spending a huge amount of time writing whatever it is. We need some criteria:
* how easy is the language to learn for people accustomed to C
* does the language has an object system? We'll be implementing cmop (6model)
in it, so the object system needs to be either self-hosting or non-existent
* how efficiently does the language map to M0? We want to generate efficient
M0 and to have a clear idea of what the M0 for a given snippet looks like
* it shouldn't allow things that don't make sense in M0 (not sure what this means)
* the language should allow CPS stuff, either directly or indirectly.
Indirectly is probably easier.
* need distinction between compile-time and runtime constructs
* need typed variables
* something struct-like
* a way to define M1 ops composed from M0 ops
* light syntax, easy to implement, no optimizations
* if we want macros, they need to be way smaller than C's
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