Success! (Precisely)

[dsimcha]

== Quote from Jacob Carlborg (doob at me.com)'s article
> On 10/30/09 06:08, dsimcha wrote:
> > After a few evenings of serious hacking, I've integrated precise heap scanning
> > into the GC.  Right now, I still need to test it better and debug it, but it
> > at least basically works.  I also still need to write the templates to
> > generate bit masks at compile time, but this is a simple matter of programming.
> >
> > A few things:
> >
> > 1.  Who knows how to write some good stress tests to make sure this works?
> >
> > 2.  I'm thinking about how to write the bitmask templates.  In the next
> > release of DMD, when static arrays are value types and returnable from
> > functions, will they be returnable from functions in CTFE?
> >
> > 3.  new only takes RTTI.  It is not a template.  Unless RTTI gets bitmasks in
> > the format I created (which I'll document once I clean things up and release,
> > but has only deviated slightly from what I had talked about here), stuff
> > allocated using it won't be able to take advantage of precise heap scanning.
> > The default bitmask, if none is provided, uses good (bad) old-fashioned
> > conservative scanning unless the entire block has no pointers, in which case
> > it isn't scanned.  This means that we have all the more incentive to replace
> > new with a template of some kind.
> >
> > 4.  I solved the static array problem, but the solution required using up one
> > of the high-order bits.  We have at least one more to play with in my bitmask
> > scheme, because I'm storing things by word offsets, not byte offsets, since
> > the GC isn't supposed to work with misaligned pointers anyhow.  This leaves
> > one more bit before we start limiting T.sizeof to less than full address space
> > (on 32-bit, where a word is 4 bytes).  I think it needs to be reserved for
> > pinning, in case a copying collector ever gets implemented.  If we're willing
> > to not let any precisely scanned object have a T.sizeof of more than half the
> > address space (a ridiculously minor limitation; this does not limit the size
> > of arrays, only the size of classes and the elements of an array), we could
> > throw in a third bit for weak references.
> Would this be possible to use with D1 ?

The precise heap scanning would probably work, if the bit masks were generated
manually, but I don't know if D1's templates are powerful enough to generate them.
 I had D2 in mind as a target, but I'll document the format once everything's
cleaned up, tested, etc. and if someone wants to try to make it work on D1, they can.

The only thing, though, is that D1 is supposed to be stable, and adding bit masks
as an argument o GC.malloc might not fly.  Then again, it would only be a lib
change, probably in Tango.  Furthermore, I made old-fashioned conservative
scanning the default (a bit mask called conservativeBitMask is stored in the
static data segment and is the default argument to GC.malloc) specifically to
avoid breaking any compatibility at the source level.