[Vm-dev] goto instruction with Cog VM

Eliot Miranda eliot.miranda at gmail.com
Sun Nov 2 06:12:00 UTC 2014

Hi Ralph,

On Sat, Nov 1, 2014 at 1:37 PM, Ralph Boland <rpboland at gmail.com> wrote:

> I am working on a parser generator tool (a replacement for SmaCC) and
> one of the things I a m interested in is the direct translation of
> language specifications into the virtual machine code (SmaCC and my
> current version of it use Squeak as the target language).

First, a different approach than compiling to Smalltalk is to compile to a
parse tree.  We do this in the pseudo JavaScript compiler we've implemented
at Cadence, translating an AST into a Squeak compiler parse tree for code
generation.  Targeting a parse tree gives you much more freedom; you can
express things that aren't expressible in Smalltalk.  And if you target
bytecodes you can do even more.

> One of the problems I have is that, for some languages, the natural
> translation
> into VM code uses computed gotos.
> There are two scenarios here:
>      1) goto X  where X is a variable.
>      2) goto  (coll at: y)  where coll is a Collection.

There are several ways of implementing this without computed bytecodes in
the instruction set, but there is also the possibility of implementing it
directly in the instruction set.

Off the top of my head one could

- map to perform: using some mangled selector.  Yes this is problematic
because one has to split the scope between the two methods, so in general
it's not a solution

- map to a case statement, which is what Squeak does. Just map it to a
sequence of compare and branches.  Or better still, to a binary tree.
Coincidentally this is used by the JIT to implement block dispatch in
methods that contain more than one block.  I know of other VM
implementations using it for PIC dispatch with really good performance.

- use thisContext pc: value.  This /should/ be fine in the stack VM, but
slooooow in the JIT because internally mapping bytecode pcs to machine code
pcs is slow, and currently slower still because the frame will be converted
to a pure context and then converted back into a frame on the return from
pc:.  But this solution isn't to be rejected out-of-hand.  It can be
optimized to avoid the frame conversion and the JIT might be able to
optimize it.  The main problem is the compiler has no support for labels so
there would be work here.

> For example, one such language is that of regular expressions, which I
> wish to translate into finite state machines implemented in VM code.
> In this case I need case 2) gotos where coll is a collection of
> associations, possibly a
> Dictionary. I also plan to write a debugger for this (and other languages)
> but that is another story.
> I realize that the Cog VM is being built for Smalltalk (Squeak? Pharo?)
> for which the goto instructions are not needed and thus I assume
> unavailable. But there is something to
> viewing a virtual machine as general purpose and thus the target of
> multiple languages as is
> the case for the Java virtual machine.
> If the Cog VM is viewed this way then I argue there is a need for my goto
> instructions
> because some languages have need for them.
> For example, many languages have case statements.  (I am all for object
> oriented
> but I would be willing to accept a case statement in Smalltalk too;  the
> Squeak code
> implemented one in Squeak doesn't cut it).

I've occasionally thought about this for many years.  A computed jump might
be nice.  Eg index an Array literal of pcs with the integer on top of
stack, falling through on bad type or out of range.

> Anyway, I am not arguing to Change Squeak or Smalltalk but I am arguing
> to have my goto instructions in Cog VM. Is there any chance of this?????

There's no chance of me spending time implementing this any time soon.  I
have too much high-priority tasks to tackle this.  But I want to encourage
you or others to have a go implementing it.  It's fun!

I don't know the Squeak VM or the Cog VM either but I assume these
> instructions don't exist because I see no need of them when the source
> language is
> Squeak or any version of Smalltalk for that matter. I also assume that
> there is already
> a full list of 256 instructions in the Cog VM and thus no room for my goto
> instructions
> unless some instructions are removed.
> Are there Cog VM instructions that are so rarely used that they could be
> removed without
> unreasonably slowing down the Cog VM interpretation of byte codes
> generated from Squeak source code?????

The current set has 3 unused bytecodes, one of which Spur uses, so
effectively there are two unused bytecodes.

The Cog VMs support multiple bytecode sets.  If you look at the
BytecodeSets package on VMMaker you can read the class comments of the
BytecodeEncoder subclasses such as EncoderForSistaV1.  These bytecode sets
have a few more unused bytecodes.  This multiple bytecode set support is
better implemented in Spur where there is only one compiled method header
format and support for 64k literals.  So let me encourage you to move to
Spur and to look at the Sista set.  The class comment of each encoder class
specifies the instruction set it targets.

> I accept that it will always be that almost all byte codes to be
> interpreted by the
> Cog VM are generated from Smalltalk source.

Sure, but in Sista, the adaptive optimizer Clément Bera, Ronie Salgado and
I are working on, the optimizer generates special bytecodes based in
analysis of running code, /not/ from source.

> Good luck with the Cog VM.  I look forward to seeing it used in
> Squeak/Pharo.

Cog is already the standard VM in Squeak/Pharo.  Do you mean Spur?  Yes
it's going well and should be ready by the end if the year.  Sista will be
even better.  Spur is ~ 2x faster than Cog, and we hope Sista will get
another 3x :-).

> Ralph Boland

and welcome to vm-dev!

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