== Quote from Andrei Alexandrescu (seewebsiteforem...@erdani.org)'s article
> dsimcha wrote:
> > == Quote from Andrei Alexandrescu (seewebsiteforem...@erdani.org)'s article
> >> Steven Schveighoffer wrote:
> >>> On Thu, 19 Nov 2009 12:01:25 -0500, dsimcha <dsim...@yahoo.com> wrote:
> >>>
> >>>> == Quote from Andrei Alexandrescu (seewebsiteforem...@erdani.org)'s
> >>>> article
> >>>>> dsimcha wrote:
> >>>>>> == Quote from Andrei Alexandrescu (seewebsiteforem...@erdani.org)'s
> >>>>> article
> >>>>>>> Yes, it will be because the book has a few failing unittests. In
> >>>>> fact, I
> >>>>>>> was hoping I could talk you or David into doing it :o).
> >>>>>>> Andrei
> >>>>>> Unfortunately, I've come to hate the MRU idea because it would fail
> >>>>> miserably for
> >>>>>> large arrays. I've explained this before, but not particularly
> >>>>> thoroughly, so
> >>>>>> I'll try to explain it more thoroughly here. Let's say you have an
> >>>>> array that
> >>>>>> takes up more than half of the total memory you are using. You try
> >>>>> to append to
> >>>>>> it and:
> >>>>>>
> >>>>>> 1. The GC runs. The MRU cache is therefore cleared.
> >>>>>>
> >>>>>> 2. Your append succeeds, but the array is reallocated.
> >>>>>>
> >>>>>> 3. You try to append again. Now, because you have a huge piece of
> >>>>> garbage that
> >>>>>> you just created by reallocating on the last append, the GC needs
> >>>>> to run again.
> >>>>>> The MRU cache is cleared again.
> >>>>>>
> >>>>>> 4. Goto 2.
> >>>>> This is not a matter of principles, but one of implementation. When you
> >>>>> GC, you can adjust the cache instead of clearing it.
> >>>> Technically true, but what is a matter of principles is whether the
> >>>> implementation
> >>>> of arrays should be very tightly coupled to the implementation of the
> >>>> GC. Fixing
> >>>> this issue would have massive ripple effects throughout the already
> >>>> spaghetti
> >>>> code-like GC, and might affect GC performance. For every single
> >>>> object the GC
> >>>> freed, it would have to look through the MRU cache and remove it from
> >>>> there if
> >>>> present, too.
> >>> You perform the lookup via MRU cache (after mark, before sweep). I see
> >>> it as a single function call at the right place in the GC.
> >>>
> >>>> The point is that this **can** be done, but we probably don't **want** to
> >>>> introduce this kind of coupling, especially if we want our GC model to
> >>>> be sane
> >>>> enough that people might actually come along and write us a better GC
> >>>> one day.
> >>> What about implementing it as a hook "do this between mark and sweep"?
> >>> Then it becomes decoupled from the GC.
> >>>
> >>> -Steve
> >> I think these are great ideas, but you'd need to transport certain
> >> information to the cache so it can adjust its pointers. Anyhow, I
> >> believe this is worth exploring because it can help with a great many
> >> other things such as weak pointers and similar checks and adjustments
> >> (there was a paper on GC assertions that I don't have time to dig right
> >> now. Aw what the heck, found it:
> >> http://www.eecs.tufts.edu/~eaftan/gcassertions-mspc-2008.pdf
> >> Andrei
> >
> > The hook doesn't sound like a bad idea, but it raises a lot of issues with
> > the
> > implementation details. These are things I could figure out given plenty
> > of time.
> > I'd like weak refs, too. However, I don't think this makes the short list
> > for D2
> > because:
> >
> > 1. Doing it at all properly requires a lot of thought about what a good
> > design
> > for such an API should be and how to implement it efficiently.
> >
> > 2. I think we still need an ArrayBuilder or something because, while the
> > MRU
> > would be reasonably efficient, it still wouldn't be as efficient as an
> > ArrayBuilder, and would do nothing to solve the uniqueness problem.
> > Therefore, I
> > think fleshing out ArrayBuilder is a higher priority. I was thinking of a
> > design
> > something like this:
> >
> > abstract class Array {
> > // A bunch of final methods for .length, opIndex, etc.
> > // No .ptr or opSlice.
> > }
> >
> > class UniqueArray : Array {
> > // Still no .ptr or opSlice. Has .toImmutable, which allows
> > // for conversion to immutability iff the elements are either
> > // pure value types or themselves immutable.
> > //
> > // Also, can deterministically delete old arrays on reallocation,
> > // since it owns a unique reference, leading to more GC-efficient
> > // appending.
> > }
> >
> > class ArrayBuilder : Array {
> > // Add opSlice and .ptr. Appending doesn't deterministically
> > // delete old arrays, even if the GC supports this. No guarantees
> > // about uniqueness.
> > }
> What does .toImmutable return? As far as I can tell making UniqueArray a
> class can't work because by definition you give up controlling how many
> references to the array there could be in a program.
> Andrei
Sorry, forgot to flesh out a few details.
1. .toImmutable() returns an immutable slice, but also sets UniqueArray's
pointer
member to null, so no instance of UniqueArray has a mutable reference any
longer.
After this is called, the UniqueArray object will be invalid unless
reinitialized.
2. After thinking about this some more, the big issue I see is ref opIndex. We
can either:
a. Disallow it for both UniqueArray and ArrayBuilder.
b. Allow it for both UniqueArray and ArrayBuilder and accept
that a sufficiently dumb programmer can invalidate the
guarantees of UniqueArray by taking the address of one of the
elements and saving it somewhere. Probably a bad idea, since
assumeUnique() already works for the careful programmer, and
UniqueArray is supposed to provide ironclad guarantees.
c. Don't define opIndex in the abstract base class at all, thus
making Array almost useless as an abstract base class.
