[Vm-dev] Re: goto instruction with Cog VM
btc at openInWorld.com
Sat Nov 8 23:35:37 UTC 2014
Eliot Miranda wrote:
> On Sat, Nov 8, 2014 at 11:21 AM, Ralph Boland <rpboland at gmail.com
> <mailto:rpboland at gmail.com>> wrote:
> > Hi Ralph,
> > >
> > > I was aware of caseOf: in Squeak. I always found it awkward to
> use and
> > > felt a true case statement would be simpler. Alas, it's
> impossible to
> > > have a true case statement added to Smalltalk now I think.
> > So what's a "true" case statement? For me, at least, the Squeak
> one *is*,
> > and is more general than one limited to purely integer keys, as
> for example
> > is C's switch statement. A number of languages provide case
> > that are like Squeak's. What do you consider a "true" case
> I mean that: caseOf: is not part of the language itself but rather
> part of the
> standard library or set of packages that one finds in the IDE. To
> be part of the
> language it would need to be something the compiler is aware of.
> Ah OK. I see what you mean. But you're wrong on a few counts. First,
> there are *no* control structures in the language beyond closures and
> polymorphism. ifTrue:, to:do:, and: whileTrue: et al are all defined in
> the library, not by the compiler. Second, tehse structures, /including/
> caseOf: are understood by the compiler and compiled to
> non-message-sending code. So none of the blocks in caseOf:, ifTrue:
> and: whileTrue: et al, the optimized selectors, are created and all are
> inlined by the compiler. So a) by your criterion of being in the
> compiler caseOf: is in the language, but b) it all control structures in
> Smalltalk are defined in the library, and some are optimized by the
Reviewing the code for the following is enlightening:
to see as the original implementation, but remembering that as an
optimization these are inlined, so that code is currently not executed.
Eliot, Would I be right to presume that the Interpreter does execute
those methods without optimisation?
> That is to
> day the Smalltalk language is not very much. Smalltalk (Squeak) the
> would not include Sets or Dictionaries but would include (some)
> Array classes
> because some aspects of Arrays are dealt with directly by the compiler.
> There is a syntactic form for creating Array, but really the notion that
> the Smalltalk compiler defines the language is a limited one. It's fair
> to say that language is defined by a small set of variables, return,
> blocks, an object representation (ability to create classes that define
> a sequence of named inst vars and inherit from other classes), and
> message lookup rules (normal sends and super sends), and a small number
> of literal forms (Array, Integer, Float, Fraction, ByteArray, String and
> Symbol literals), and a method syntax. The rest is in the library.
> What this really means is that Smalltalk can't be reduced to a language,
> becaue the anguage doesn't defne enough. Instead it is a small language
> and a large library.
> Selectors such as ifTrue: and to:do: are part of the language
> because they are inlined by the compiler.
> No. One can change the compiler to not inline them. This is merely an
> Put another way, if I could get my doBlockAt: method incorporated
> into the Squeak IDE
> it would nevertheless NOT be part of Squeak the language.
> The consequence of caseOf: not being part of the language is that
> the compiler/VM
> cannot perform optimizations when caseOf: is run into but must
> treat it as
> user written code.
> Squeak's caseOf: is more general than C's switch statement but it
> could be more
> general in that there is a hard coded message (=). I would like to
> be able to replace
> the '=' message by an arbitrary binary operator such as includes:
> or '>'.
> I have to backtrack here: I looked at the code and it looks like
> the compiler inlines
> caseOf: and caseOf:otherwise. If so then these selectors are part
> of the language
> by my definition.
> Well, live and learn :-)
> > > But I wouldn't want to be forced to implement my FSMs this way.
> > > It might be acceptable for small FSMs.
> > > I want to avoid sequential search and
> > > even binary search might be rather expensive.
> > > I look at computed gotos as the solution but,
> > > as you pointed out, computed gotos pose problems for JIT.
> > > Admittedly, for large FSM's, it might be best or necessary to
> > > use a FSM simulator anyway, as I do now.
> > Nah. One should always be able to map it down somehow. Tis will
> be easier
> > with the Spur instruction set which lifts number of literals and
> length of
> > branches limits.
> Good to hear.
> > > Again, for my FSM, case this would often be considered to be good.
> > > But if the state transition tables are sparse then Dictionaries
> > > might be preferable to Arrays.
> > Yes, but getting to the limit of what the VM can reasonably
> > Better would be an Array of value. pc pairs, where the keys are
> the values
> > the switch bytecode compares top of stack against, and the pcs
> are where to
> > jump to on a match. The JIT can therefore implement the table as
> it sees
> > fit, whereas the interpreter can just do a linear search through
> the Array.
> I am looking at this from the point of view of a compiler
> writer/generator and consider
> your proposal as inadequate for my needs. You, I think, are looking
> at this from
> the point of view of a VM writer and what can reasonably be
> delivered. I don't think
> what I want is overly difficult for the interpreter to deliver but
> as you pointed out,
> and you know much better than I, what I want causes serious problems
> for the VM.
> > > My expection is that at: be sent to the collection object
> > > to get the address to go to. Knowing that the collection
> > > is an array though makes it easier for the compiler/VM to
> > > ensure that the addresses stored in the collection are valid.
> > > Actually, the compiler will be generating the addresses.
> > > Does the VM have absolute trust in the compiler to generate valid
> > > addresses?
> > Yes. Generate bad bytecode and the VM crashes.
> This is what I expected to hear but wanted it to be clear for
> compilers generated
> by my parser generator tool as you did.
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