= Investigation of Boehm's GC as GCC GC =

LaurynasBiveinis

== Current status ==
Conservative marking works. Work in progress on exact marking using auto-generated custom marking routines by gengtype. This might be reused for other collectors too.

== Submitted patches ==
 1. http://gcc.gnu.org/ml/gcc-patches/2006-06/msg01267.html
 1. http://gcc.gnu.org/ml/gcc-patches/2006-07/msg01071.html
 1. http://gcc.gnu.org/ml/gcc-patches/2006-07/msg00695.html
== Latest performance results ==
http://gcc.gnu.org/ml/gcc/2006-07/msg00520.html

== Old performance results ==
[[http://gcc.gnu.org/ml/gcc-patches/2006-06/msg01267.htmlFirst|http://gcc.gnu.org/ml/gcc-patches/2006-06/msg01267.html]]

== Tech notes ==
The results of previous investigation performed few years ago can be found at http://gcc.gnu.org/ml/gcc/2002-08/msg00837.html. Note that it has different design.

With a simple implementation of {{{ggc_alloc_stat()}}} by means of {{{GC_MALLOC()}}} and {{{ggc_collect()}}} by {{{GC_collect()}}}, GCC builds and compiles small programs. With anything larger it fails with

{{{
../../gcc-boehm-test/gcc/libgcc2.c:32:2: error: invalid preprocessing directive #include
[...]
}}}
This is caused by collecting allocated nodes from {{{ident_hash}}} prematurely.  This issue can be resolved by registering GC roots for the identifier table. Since table might be relocated and expanded during compilation, currently it is done before every collection.  See {{{ggc_stringpool_roots}}}, {{{ggc_register_stringpool_roots()}}}, {{{ggc_unregister_stringpool_roots()}}} and their usage in {{{ggc_collect}}}.

The other {{{malloc()}}}'ed memory areas that contain pointers do not move. It is OK to register them once per lifetime, see {{{register_gty_roots()}}} in {{{ggc-boehm.c}}}.

With these changes GCC bootstraps. The code can be found in {{{boehms-gc}}} branch.

Does Boehm's GC support reallocation of non-atomic data structures?

=== Things to do if this code is going to be in production ===
 * Top-level {{{configure}}} should build the collector on host only if {{{--with-gc=boehm}}} is given.
 * Top-level {{{configure}}} should be taught to pass on appropriate {{{--enable-threads}}} value.  Right now the collector must be configured with {{{--disable-threads}}} by hand.
 * Solve the weak pointer support patch issues, see below.
 * The built host library should be renamed to libgc from libgcjgc. Target library can be left alone.
 * Boehm's GC {{{make check}}} target should be integrated into main {{{make check}}}.
=== Weak pointer support in Boehm's GC ===
The collector in branch contains my own patch for weak pointer support.

 * For production, it probably should be changed to http://www.eideticdew.org/culibs/bdwgc-rn.html or something else that would be accepted upstream.
 * In my patch, is memory size increase of {{{struct disappearing_link}}} something to be concerned about?
 * Is there a need for a external function for unregistering all the weak pointers?
 * Could the weak pointer callback be implemented with http://comments.gmane.org/gmane.comp.programming.garbage-collection.boehmgc/759 ?
 * How should the case of registering the same link twice be handled?
 * Boehm's GC appears to support pre-standard C dialects. Should this part be tested for the patches submitted upstream?
== Custom marking tech notes ==
gt_types_enum_free_list_obj = 0 is required because Boehm's GC may invoke marker routine on an object residing on free list. Such object contents are cleared except the first word, which is a linked list pointer to the next object. Since type id is after the allocated memory, free list objects will be recognized as such.