[Pharo-dev] DebugSession>>activePC:

Eliot Miranda eliot.miranda at gmail.com
Sun Jan 20 20:26:19 EST 2019

Hi Marcus,

On Fri, Jan 18, 2019 at 5:42 AM Marcus Denker <marcus.denker at inria.fr>

> > On 18 Jan 2019, at 14:26, ducasse <stepharo at netcourrier.com> wrote:
> >
> > I simply love the dynamic rewriting this is just too cool. We should
> systematically use it.
> > I will continue to use it in any deprecation.
> >
> On my TODO is to make it stand-alone and provide is as a “compatibility
> transform”, too.
> So we can add it to methods that we want to keep for compatibility, but
> they will nevertheless transform the code automatically.
> (this then might be disabled in production to not transform)
> > Now I have a simple question (You can explain it to me over lunch one of
> these days).
> >
> > I do not get why RBAST would not be a good representation for the
> compiler?
> > I would like to know what is the difference.
> >
> I think it is a good one. I have not yet seen a reason why not. But
> remember, Roel left Squeak because his visitor pattern for the compiler was
> rejected as a dumb idea… so there are definitely different views on core
> questions.
> E.g. the RB AST is annotated and the whole things for sure uses a bit more
> memory than the compiler designed for a machine from 1978.
> > You mean that before going from BC to AST was difficult?
> You need to do the mapping somehow, the compiler needs to remember the BC
> offset in the code generation phase and the AST (somehow) needs to store
> that information (either in every node or some table).
> > How opal performs it? It does not use the source of the method to
> recreate the AST but he can do it from the BC?
> >
> It uses the IR (which I still am not 100% sure about, it came from the old
> “ClosureCompiler” Design and it turned out to be quite useful, for example
> for the mapping: every IR node retains the offset of the BC it creates,
> then the IR Nodes
> retain the AST node that created them.
> -> so we just do a query: “IRMethod, give me the IRInstruction that
> created BC offset X. then “IR, which AST node did create you? then the AST
> Node: what is your highlight interval in the source?
> The devil is in the detail as one IR can produce multiple byte code
> offsets (and byte codes) and one byte code might be created by two IR
> nodes, but it does seem to work with some tricks.
> Which I want to remove by improving the mapping and even the IR more…
> there is even the question: do we need the IR? could we not do it simpler?
> The IR was quite nice back when we tried to do things with byte code
> manipulation (Bytesurgeon), now it feels a bit of an overkill. But it
> simplifies e.g. the bc mapping.

I find Bytesurgeon functionality, specifically a bytecode dis/assembler
very useful, but wouldn't use it for the back end of the bytecode
compiler.  It adds overhead that has no benefit.  But I use my version,
MethodMassage, for lots of things:

- transporting compiled methods from one dialect to another, e.g. to do
in-image JIT compilation omg a method from Pharo in Squeak.
- generating JIT test cases
- generating methods that can't be generated from Smalltalk source, e.g. an
accessor for a JavaScript implementation above Smalltalk where inst var 0
is the prototype slot and nil is unbound, and so in a loop one wants to
fetch the Nth inst var from a temporary initialized to self, and ion the
value is non-nil return it, otherwise setting the temporary to
the prototype slot, hence walking up the prototype chain until an
initialized inst var is found.

I based mine around the messages that InstructionStream sends to the client
in the interpretFooInstructionFor: methods; a client that catches
doesNotUnderstand: then forms the basis of the disassembler.  Simple and


best, Eliot
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