[Vm-dev] [VM-dev] Can we be a JavaScript engine?

Denis Kudriashov dionisiydk at gmail.com
Tue Feb 20 21:20:26 UTC 2018

Thank's Eliot.
It is very interesting.

2018-02-20 21:51 GMT+01:00 Eliot Miranda <eliot.miranda at gmail.com>:

> Hi Denis,
> On Tue, Feb 20, 2018 at 6:56 AM, Denis Kudriashov <dionisiydk at gmail.com>
> wrote:
>> Hi.
>> It is just a question from the space :)
>> I wonder, can we be a JavaScript engine? Like replacing nodeJs with
>> Pharo/Squeak.
>> I imagine that JS prototypes can be easily supported by anonymous
>> classes. What is missing?
> Well, Vassili Bykov and I did a JavaScript implementation above
> VisualWorks in 2005 is.  One can do a very good job.  There are three
> strong ideas one can use.  The first two are Vassili's, the last is Claus
> Gittinger's.
> Vassili's ideas were to
> a) analyse constructors and map the accessors in constructors to inst var
> offsets.  If one does inst var access by property lookup (an object is a
> dictionary from field name to field) performance will be very poor.  If one
> can analyze a constructor and map each field to an inst var offset
> performance can be much better.
> b) the first field of any object is the prototype slot which points to the
> prototype or nil.  An inst var holding nil means "unbound".  Therefore a
> new JavaScript object will have all its fields unbound.  To access an
> object's field Vassili minted special accessors, one for each offset.  They
> can't be written directly in Smalltalk but can be expressed using Smalltalk
> bytecodes.  They look like this, where instVarAt: is actually a bytecode
> that fetches that inst var from the receiver
>     | current value |
>     current := self.
>     [current isNil ifTrue: [self error: 'field not found'].
>      value := current instVarAt: N. "N is the offset of the inst var this
> accessor should return"
>      value ~~ nil ifTrue: [^value].
>      current := current instVarAt: 0] repeat
> or in bytecode:
> pushReceiver
> popIntoTemp: 0 "set current to be the receiver"
> L1:
> pushTemp: N inVectorAt: 0.  "fetch Nth field of current"
> dup
> pushConstant: nil  "if Nth field is non-nil (is bound) answer it"
> send: ~~
> jumpFalseTo: L2
> returnTop
> L2:
> pop  "discard nil"
> pushTemp: 0 inVectorAt: 0.  "fetch prototype field of current"
> dup
> pushTemp: 0 "if prototype is nil, slot is not found"
> pushConstant: nil
> send: ==
> jumpFalseTo: L3
> self
> send: errorFieldNotFound
> pop
> L3:
> popIntoTemp: 0 "set current to be the current's prototype"
> jumpTo: L1
> returnSelf
> We used a class per constructor, and were able to get performance about 4x
> of Netscape's VM at the time.
> Claus' idea is to create a circular instance, an object whose class is the
> object itself.   The idea follows from the fact that if one has the
> changeClass/adoptInstance primitive one can create a prototype by creating
> an Array whose first three fields are laid out like a Behavior's fields,
> superclass/prototype, methodDictionsry, format, and whose subsequent fields
> are the inst vars of the object.  One then uses adoptInstance/change class
> primitive to change the class of the Array to itself, so that it is an
> instance of itself; it has its own methods in its methodDictionary, its
> superclass field is now the prototype.  The format field must specify a
> pointer object with N inst vars.  If using this scheme to implement
> JavaScript objects, then slot 4 (immediately after format) is probably the
> sot for dynamically added properties.  And of course one would implement
> prototype creation in a special primitive rather than using the standard
> adoptInstance: primitive.
> Using tricks like these one can get reasonably high performance.  In fact,
> at Cadence we have a JavaScript implementation which is has evolved from
> Vassili's two ideas.  The implementation is in Newspeak running above the
> 64-bit Spur VM.
> _,,,^..^,,,_
> best, Eliot
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