[Vm-dev] Re: [squeak-dev] Compiler

Eliot Miranda eliot.miranda at gmail.com
Wed Jun 20 01:36:16 UTC 2012

On Tue, Jun 19, 2012 at 12:19 AM, Colin Putney <colin at wiresong.com> wrote:

> So I was poking around in Compiler today, and noticed that it's a
> bit... messy. I had a few ideas for improvement, but before I go
> monkeying with such an important part of the system, I thought I'd
> bring it up here. Do we have a long- or medium-term plan for how the
> compiler should evolve?
> As I see it there are a few paths available to us:
> 1. Incremental improvement. The compiler we have now is tried and
> true. Now that we have proper block closures and a high performance
> VM, there's no real need to improve bytecode generation, so we
> shouldn't put much effort into this part of the system. We'll just
> make small improvements and minor refactorings as needed.

I wouldn't say that the bytecode set is satisfactory.  Some things that are
wrong are
- limited to 256 literals
- limited to 32 temporaries, 16 arguments (IIRC)
- limited room for expansion (and in Newspeak there is no room for

I'd like to see a set with
- a primitive bytecode to move the primitive number field out of the header
and make more room for num args, num literals (64k would be a fine limit)
- a design based around prefixes for extended indices a la VW.  lifts the
limits on addressability, arity etc while keeping the bytecode compact
- some mild experiments such as a nop which can be used e.g. to express
metadata that guides the decompiler (this is a to:do:, etc).

Even more interesting would be metadata that allowed the discovery of
inlined blocks so that e.g. mustBeBoolean is instead handled by dynamically
creating closures and the relevant ifTrue:ifFalse: message so that these
can be inlined for true/false but reimplemented for other objects.

> 2. Adopt an existing project. There have been a few "new compiler"
> projects over the years, and one or another of them might present an
> opportunity for signifiant improvement over the status quo. I'm
> thinking of ByteSurgeon, Opal, AOStA etc. It's not something we'll
> rush into, but eventually, when the code is mature, we'll want to
> replace the current compiler.
> 3. Something completely new. Now that we have closures and a fast VM,
> existing projects aren't relevant anymore, but we have new
> opportunities for improvement. VM-level changes, such as a new object
> format or new bytecodes could drive this option, if they're big enough
> that significant work on the compiler is required anyway. Maybe we can
> only see the broad outlines of what the project might look like at the
> moment, but we can see it on the horizon.

Well, my refactoring of the compiler to move instruction encoding out of
ParseNode general instances and into BytecodeEncoder takes the pressure off
as far as changing the bytecode set.  There's still a need for refactoring
in InstructionStream and CompiledMethod to handle bytecode set change.  It
is really a BytecodeEncoder or InstructionStream that understands how a
bytecode set works, and not CompiledMethod (in e.g. readsField etc).

So, are there any pain points right now that we should think about
> addressing? Is anybody planning or considering working on something
> compiler-related?

For me at least has to take second place to the new object representation
because there's much more benefit to be derived from the object

Eliot, is there anything in the new object format that will have an
> impact on image-side compilation?

I don't think so.  It should be entirely orthogonal.

> I seem to remember you mentioning
> something about efficiently supporting alternate bytecode sets. Is
> that meant for Newspeak, or do you have something in mind for
> Smalltalk?

It is a convenient way of migrating the bytecode set.  Better than my
EncoderForLongFormV3 approach.

I don't think we have to come up with a definitive plan just now, I
> just want to get a sense of what people are thinking.
> Colin

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