GC Precision

[Leandro Lucarella]

Andrei Alexandrescu, el 26 de octubre a las 11:01 me escribiste:
> Sean Kelly wrote:
> >dsimcha Wrote:
> >
> >>== Quote from Sean Kelly (sean at invisibleduck.org)'s article
> >>>dsimcha Wrote:
> >>>>I just realized last night that D's templates are probably powerful enough now
> >>>>to generate bit masks that can be used for precise GC heap scanning.  I'm
> >>>>halfway (emphasis on halfway) thinking of using this to try to hack the GC and
> >>>>make heap scanning fully precise except for the corner case of unions.
> >>>>However, this ties into several things that others in the D community are
> >>>>doing, so I want to gauge people's responses and make sure I'm not wasting
> >>>>effort on something that will be useless in 6 months.
> >>>>
> >>>>1.  Sean, Leonardo, whoever else may be working on GC implementations, have
> >>>>you by any chance broken ground on precise heap scanning already?
> >>>I've thought about it, but not done anything about it.  The compiler doesn't
> >>provide this information, so precise scanning would require a user-level call.
> >>You'll also have to deal with arrays of structs, by the way.
> >>
> >>Arrays of structs are easy:  Generate a bitmask for one element, and keep reusing
> >>that bitmask until the end of the block.  Am I missing something?
> >
> >Nope.
> 
> One question is, is there enough information for stack variables? My
> understanding from a while ago was that heap data could be
> reasonably analyzed, but stack data has no info associated with it.

There is some discussion about precise stack in the thread I cited too.
The stack can be also precise adding a shadow stack (like a TypeInfo for
the stack) but it's costly and it needs compiler support (LLVM has some
mechanisms to generate this stack information AFAIK but I never played
with it).

The problem with the stack is that you still have to interact with C,
which has no stack information, so it's a little more controversial about
how much the extra complexity will pay off.

I agree with David that it's much more reasonable to make the heap precise
first and then see how thing are going from that.

-- 
Leandro Lucarella (AKA luca)                     http://llucax.com.ar/
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