On 5/1/14, 12:43 AM, Dmitry Olshansky wrote:
01-May-2014 08:05, Andrei Alexandrescu пишет:
So far allocators have been quite GC-agnostic; deallocation would be
needed for each allocation, and there was little structure to help
tracing.
The primitive resolveInternalPointer has made a step toward more precise
structuring of heaps, and now it's time to look at some real GC helpers.
These are markAllAsUnused, markAsUsed, and doneMarking. Find their
description here:
http://erdani.com/d/phobos-prerelease/std_allocator.html
There are a few proof of concept implementations here:
https://github.com/andralex/phobos/blob/allocator/std/allocator.d
The basic idea is that the GC would initiate a collection by calling
markAllAsUnused. That would cause the underlying untyped allocator to
softly mark all memory as free, but without actually messing it up.
(Many allocators are capable of implementing such a primitive cheaply.)
It would be interesting to define where GC sits and how it integrates
with allocators to begin with. Given that currently GCAllocator is
mentioned
somewhere at the bottom of heap layers I'm really at loss about how
these helpers fit into the picture.
Well currently GCAllocator is only included for completeness. The
purpose of std.allocator/std.typed_allocator is to be a complete
redesign, not an adaptation, of the current GC.
Then, the GC would trace objects starting from roots. Whenever it finds
a used pointer, it would mark the corresponding memory as allocated by
using markAsUsed, effectively undoing the soft freeing for it.
How GC determines which allocator an object belongs to ?
The idea is, for each root (whether conservative or typed), the GC first
calls resolveInternalPointer. That gives the memory chunk encompassing
that pointer. The GC uses that chunk to retrieve metadata about it and
then passes it to setAsUsed.
Andrei
