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Update Conservative GC section of manual

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* doc/ref/data-rep.texi (Conservative GC): Update notes with more
  details.
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Andy Wingo 2018-09-27 09:50:37 +02:00
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@ -1,6 +1,6 @@
@c -*-texinfo-*-
@c This is part of the GNU Guile Reference Manual.
@c Copyright (C) 1996, 1997, 2000, 2001, 2002, 2003, 2004, 2010, 2015
@c Copyright (C) 1996, 1997, 2000, 2001, 2002, 2003, 2004, 2010, 2015, 2018
@c Free Software Foundation, Inc.
@c See the file guile.texi for copying conditions.
@ -324,24 +324,28 @@ is a nightmare to maintain. Thus, the BDW-GC uses a technique called
@dfn{conservative garbage collection}, to make the local variable list
unnecessary.
The trick to conservative collection is to treat the stack as an
ordinary range of memory, and assume that @emph{every} word on the stack
is a pointer into the heap. Thus, the collector marks all objects whose
addresses appear anywhere in the stack, without knowing for sure how
that word is meant to be interpreted.
The trick to conservative collection is to treat the C stack as an
ordinary range of memory, and assume that @emph{every} word on the C
stack is a pointer into the heap. Thus, the collector marks all objects
whose addresses appear anywhere in the C stack, without knowing for sure
how that word is meant to be interpreted.
In addition to the stack, the BDW-GC will also scan static data
sections. This means that global variables are also scanned when looking
for live Scheme objects.
Obviously, such a system will occasionally retain objects that are
actually garbage, and should be freed. In practice, this is not a
problem. The alternative, an explicitly maintained list of local
variable addresses, is effectively much less reliable, due to programmer
error. Interested readers should see the BDW-GC web page at
@uref{http://www.hboehm.info/gc/}, for more
information.
actually garbage, and should be freed. In practice, this is not a
problem, as the set of conservatively-scanned locations is fixed; the
Scheme stack is maintained apart from the C stack, and is scanned
precisely (as opposed to conservatively). The GC-managed heap is also
partitioned into parts that can contain pointers (such as vectors) and
parts that can't (such as bytevectors), limiting the potential for
confusing a raw integer with a pointer to a live object.
Interested readers should see the BDW-GC web page at
@uref{http://www.hboehm.info/gc/}, for more information on conservative
GC in general and the BDW-GC implementation in particular.
@node The SCM Type in Guile
@subsection The SCM Type in Guile