gcc/boehm-gc/doc/README.changes

This is a rough history of garbage collector bugs and versions.

This has been maintained with varying diligence over the years.

I made an attempt to include recent contributors here.  I apologize for any
omissions.

-------------------------

  Version 1.3 and immediately preceding versions contained spurious
assembly language assignments to TMP_SP.  Only the assignment in the PC/RT
code is necessary.  On other machines, with certain compiler options,
the assignments can lead to an unsaved register being overwritten.
Known to cause problems under SunOS 3.5 WITHOUT the -O option.  (With
-O the compiler recognizes it as dead code.  It probably shouldn't,
but that's another story.)

  Version 1.4 and earlier versions used compile time determined values
for the stack base.  This no longer works on Sun 3s, since Sun 3/80s use
a different stack base.  We now use a straightforward heuristic on all
machines on which it is known to work (incl. Sun 3s) and compile-time
determined values for the rest.  There should really be library calls
to determine such values.

  Version 1.5 and earlier did not ensure 8 byte alignment for objects
allocated on a sparc based machine.

  Version 1.8 added ULTRIX support in gc_private.h.
  
  Version 1.9 fixed a major bug in gc_realloc.
  
  Version 2.0 introduced a consistent naming convention for collector
routines and added support for registering dynamic library data segments
in the standard mark_roots.c.  Most of the data structures were revamped.
The treatment of interior pointers was completely changed.  Finalization
was added.  Support for locking was added.  Object kinds were added.
We added a black listing facility to avoid allocating at addresses known
to occur as integers somewhere in the address space.  Much of this
was accomplished by adapting ideas and code from the PCR collector.
The test program was changed and expanded.

  Version 2.1 was the first stable version since 1.9, and added support
for PPCR.

  Version 2.2 added debugging allocation, and fixed various bugs.  Among them:
- GC_realloc could fail to extend the size of the object for certain large object sizes.
- A blatant subscript range error in GC_printf, which unfortunately
  wasn't exercised on machines with sufficient stack alignment constraints.
- GC_register_displacement did the wrong thing if it was called after
  any allocation had taken place.
- The leak finding code would eventually break after 2048 byte
  byte objects leaked.
- interface.c didn't compile.
- The heap size remained much too small for large stacks.
- The stack clearing code behaved badly for large stacks, and perhaps
  on HP/PA machines.

  Version 2.3 added ALL_INTERIOR_POINTERS and fixed the following bugs:
- Missing declaration of etext in the A/UX version.
- Some PCR root-finding problems.
- Blacklisting was not 100% effective, because the plausible future
  heap bounds were being miscalculated.
- GC_realloc didn't handle out-of-memory correctly.
- GC_base could return a nonzero value for addresses inside free blocks.
- test.c wasn't really thread safe, and could erroneously report failure
  in a multithreaded environment.  (The locking primitives need to be
  replaced for other threads packages.)
- GC_CONS was thoroughly broken.
- On a SPARC with dynamic linking, signals stayed diabled while the
  client code was running.
  (Thanks to Manuel Serrano at INRIA for reporting the last two.)
  
  Version 2.4 added GC_free_space_divisor as a tuning knob, added
  support for OS/2 and linux, and fixed the following bugs:
- On machines with unaligned pointers (e.g. Sun 3), every 128th word could
  fail to be considered for marking.
- Dynamic_load.c erroneously added 4 bytes to the length of the data and
  bss sections of the dynamic library.  This could result in a bad memory
  reference if the actual length was a multiple of a page.  (Observed on
  Sun 3.  Can probably also happen on a Sun 4.)
  (Thanks to Robert Brazile for pointing out that the Sun 3 version
  was broken.  Dynamic library handling is still broken on Sun 3s
  under 4.1.1U1, but apparently not 4.1.1.  If you have such a machine,
  use -Bstatic.)
  
  Version 2.5 fixed the following bugs:
- Removed an explicit call to exit(1)
- Fixed calls to GC_printf and GC_err_printf, so the correct number of
  arguments are always supplied.  The OS/2 C compiler gets confused if
  the number of actuals and the number of formals differ.  (ANSI C
  doesn't require this to work.  The ANSI sanctioned way of doing things
  causes too many compatibility problems.)
  
  Version 3.0  added generational/incremental collection and stubborn
  objects.

  Version 3.1 added the following features:
- A workaround for a SunOS 4.X SPARC C compiler
  misfeature that caused problems when the collector was turned into
  a dynamic library.  
- A fix for a bug in GC_base that could result in a memory fault.
- A fix for a performance bug (and several other misfeatures) pointed
  out by Dave Detlefs and Al Dosser.
- Use of dirty bit information for static data under Solaris 2.X.
- DEC Alpha/OSF1 support (thanks to Al Dosser).
- Incremental collection on more platforms.
- A more refined heap expansion policy.  Less space usage by default.
- Various minor enhancements to reduce space usage, and to reduce
  the amount of memory scanned by the collector.
- Uncollectable allocation without per object overhead.
- More conscientious handling of out-of-memory conditions.
- Fixed a bug in debugging stubborn allocation.
- Fixed a bug that resulted in occasional erroneous reporting of smashed
  objects with debugging allocation.
- Fixed bogus leak reports of size 4096 blocks with FIND_LEAK.

  Version 3.2 fixed a serious and not entirely repeatable bug in
  the incremental collector.  It appeared only when dirty bit info
  on the roots was available, which is normally only under Solaris.
  It also added GC_general_register_disappearing_link, and some
  testing code.  Interface.c disappeared.

  Version 3.3 fixes several bugs and adds new ports:
- PCR-specific bugs.
- Missing locking in GC_free, redundant FASTUNLOCK
  in GC_malloc_stubborn, and 2 bugs in
  GC_unregister_disappearing_link.
  All of the above were pointed out by Neil Sharman
  (neil@cs.mu.oz.au).
- Common symbols allocated by the SunOS4.X dynamic loader
  were not included in the root set.
- Bug in GC_finalize (reported by Brian Beuning and Al Dosser)
- Merged Amiga port from Jesper Peterson (untested)
- Merged NeXT port from Thomas Funke (significantly
  modified and untested)

  Version 3.4:
- Fixed a performance bug in GC_realloc.
- Updated the amiga port.
- Added NetBSD and 386BSD ports.
- Added cord library.
- Added trivial performance enhancement for
  ALL_INTERIOR_POINTERS.  (Don't scan last word.)
  
  Version 3.5
- Minor collections now mark from roots only once, if that
  doesn't cause an excessive pause.
- The stack clearing heuristic was refined to prevent anomalies
  with very heavily recursive programs and sparse stacks.
- Fixed a bug that prevented mark stack growth in some cases.
  GC_objects_are_marked should be set to TRUE after a call
  to GC_push_roots and as part of GC_push_marked, since
  both can now set mark bits.  I think this is only a performance
  bug, but I wouldn't bet on it.  It's certainly very hard to argue
  that the old version was correct.
- Fixed an incremental collection bug that prevented it from
  working at all when HBLKSIZE != getpagesize()
- Changed dynamic_loading.c to include gc_priv.h before testing
  DYNAMIC_LOADING.  SunOS dynamic library scanning
  must have been broken in 3.4.
- Object size rounding now adapts to program behavior.
- Added a workaround (provided by Manuel Serrano and
  colleagues) to a long-standing SunOS 4.X (and 3.X?) ld bug
  that I had incorrectly assumed to have been squished.
  The collector was broken if the text segment size was within
  32 bytes of a multiple of 8K bytes, and if the beginning of
  the data segment contained interesting roots.  The workaround
  assumes a demand-loadable executable.  The original may have
  have "worked" in some other cases.
- Added dynamic library support under IRIX5.
- Added support for EMX under OS/2 (thanks to Ari Huttunen).
  
Version 3.6:
- fixed a bug in the mark stack growth code that was introduced
  in 3.4.
- fixed Makefile to work around DEC AXP compiler tail recursion
  bug.

Version 3.7:
- Added a workaround for an HP/UX compiler bug.
- Fixed another stack clearing performance bug.  Reworked
  that code once more.
  
Version 4.0:
- Added support for Solaris threads (which was possible
  only by reimplementing some fraction of Solaris threads,
  since Sun doesn't currently make the thread debugging
  interface available).
- Added non-threads win32 and win32S support.
- (Grudgingly, with suitable muttering of obscenities) renamed
  files so that the collector distribution could live on a FAT
  file system.  Files that are guaranteed to be useless on
  a PC still have long names.  Gc_inline.h and gc_private.h
  still exist, but now just include  gc_inl.h and gc_priv.h.
- Fixed a really obscure bug in finalization that could cause
  undetected mark stack overflows.  (I would be surprised if
  any real code ever tickled this one.)
- Changed finalization code to dynamically resize the hash
  tables it maintains.  (This probably does not matter for well-
  -written code.  It no doubt does for C++ code that overuses
  destructors.)
- Added typed allocation primitives.  Rewrote the marker to
  accommodate them with more reasonable efficiency.  This
  change should also speed up marking for GC_malloc allocated
  objects a little.  See gc_typed.h for new primitives.
- Improved debugging facilities slightly.  Allocation time
  stack traces are now kept by default on SPARC/SUNOS4.
  (Thanks to Scott Schwartz.)
- Added better support for small heap applications.
- Significantly extended cord package.  Fixed a bug in the
  implementation of lazily read files.  Printf and friends now
  have cord variants.  Cord traversals are a bit faster.
- Made ALL_INTERIOR_POINTERS recognition the default.
- Fixed de so that it can run in constant space, independent
  of file size.  Added simple string searching to cords and de.
- Added the Hull-Ellis C++ interface.
- Added dynamic library support for OSF/1.
  (Thanks to Al Dosser and Tim Bingham at DEC.)
- Changed argument to GC_expand_hp to be expressed
  in units of bytes instead of heap blocks.  (Necessary
  since the heap block size now varies depending on
  configuration.  The old version was never very clean.)
- Added GC_get_heap_size().  The previous "equivalent"
  was broken.
- Restructured the Makefile a bit.  

Since version 4.0:
- Changed finalization implementation to guarantee that
  finalization procedures are called outside of the allocation
  lock, making direct use of the interface a little less dangerous.
  MAY BREAK EXISTING CLIENTS that assume finalizers
  are protected by a lock.  Since there seem to be few multithreaded
  clients that use finalization, this is hopefully not much of
  a problem.
- Fixed a gross bug in CORD_prev.
- Fixed a bug in blacklst.c that could result in unbounded
  heap growth during startup on machines that do not clear
  memory obtained from the OS (e.g. win32S).
- Ported de editor to win32/win32S.  (This is now the only
  version with a mouse-sensitive UI.)
- Added GC_malloc_ignore_off_page to allocate large arrays
  in the presence of ALL_INTERIOR_POINTERS.
- Changed GC_call_with_alloc_lock to not disable signals in
  the single-threaded case.
- Reduced retry count in GC_collect_or_expand for garbage
  collecting when out of memory.
- Made uncollectable allocations bypass black-listing, as they
  should.
- Fixed a bug in typed_test in test.c that could cause (legitimate)
  GC crashes.
- Fixed some potential synchronization problems in finalize.c
- Fixed a real locking problem in typd_mlc.c.
- Worked around an AIX 3.2 compiler feature that results in
  out of bounds memory references.
- Partially worked around an IRIX5.2 beta problem (which may
  or may not persist to the final release).
- Fixed a bug in the heap integrity checking code that could
  result in explicitly deallocated objects being identified as
  smashed.  Fixed a bug in the dbg_mlc stack saving code
  that caused old argument pointers to be considered live.
- Fixed a bug in CORD_ncmp (and hence CORD_str).
- Repaired the OS2 port, which had suffered from bit rot
  in 4.0.  Worked around what appears to be CSet/2 V1.0
  optimizer bug.
- Fixed a Makefile bug for target "c++".

Since version 4.1:
- Multiple bug fixes/workarounds in the Solaris threads version.
  (It occasionally failed to locate some register contents for
  marking.  It also turns out that thr_suspend and friends are
  unreliable in Solaris 2.3.  Dirty bit reads appear
  to be unreliable under some weird 
  circumstances.  My stack marking code
  contained a serious performance bug.  The new code is
  extremely defensive, and has not failed in several cpu
  hours of testing.  But  no guarantees ...)
- Added MacOS support (thanks to Patrick Beard.)
- Fixed several syntactic bugs in gc_c++.h and friends.  (These
  didn't bother g++, but did bother most other compilers.)
  Fixed gc_c++.h finalization interface.  (It didn't.)
- 64 bit alignment for allocated objects was not guaranteed in a
  few cases in which it should have been.
- Added GC_malloc_atomic_ignore_off_page.
- Added GC_collect_a_little.
- Added some prototypes to gc.h.
- Some other minor bug fixes (notably in Makefile).
- Fixed OS/2 / EMX port (thanks to Ari Huttunen).
- Fixed AmigaDOS port. (thanks to Michel Schinz).
- Fixed the DATASTART definition under Solaris.  There
  was a 1 in 16K chance of the collector missing the first
  64K of static data (and thus crashing).
- Fixed some blatant anachronisms in the README file.
- Fixed PCR-Makefile for upcoming PPCR release.

Since version 4.2:
- Fixed SPARC alignment problem with GC_DEBUG.
- Fixed Solaris threads /proc workaround.  The real
  problem was an interaction with mprotect.
- Incorporated fix from Patrick Beard for gc_c++.h (now gc_cpp.h).
- Slightly improved allocator space utilization by
  fixing the GC_size_map mechanism.
- Integrated some Sony News and MIPS RISCos 4.51
  patches.  (Thanks to Nobuyuki Hikichi of
  Software Research Associates, Inc. Japan)
- Fixed HP_PA alignment problem.  (Thanks to
  xjam@cork.cs.berkeley.edu.)
- Added GC_same_obj and friends.  Changed GC_base
  to return 0 for pointers past the end of large objects.
  Improved GC_base performance with ALL_INTERIOR_POINTERS
  on machines with a slow integer mod operation.
  Added GC_PTR_ADD, GC_PTR_STORE, etc. to prepare
  for preprocessor.
- changed the default on most UNIX machines to be that
  signals are not disabled during critical GC operations.
  This is still ANSI-conforming, though somewhat dangerous
  in the presence of signal handlers. But the performance
  cost of the alternative is sometimes problematic.
  Can be changed back with a minor Makefile edit.
- renamed IS_STRING in gc.h, to CORD_IS_STRING, thus
  following my own naming convention.  Added the function
  CORD_to_const_char_star.
- Fixed a gross bug in GC_finalize.  Symptom: occasional
  address faults in that function.  (Thanks to Anselm
  Baird-Smith (Anselm.BairdSmith@inria.fr)
- Added port to ICL DRS6000 running DRS/NX.  Restructured
  things a bit to factor out common code, and remove obsolete
  code.  Collector should now run under SUNOS5 with either
  mprotect or /proc dirty bits.  (Thanks to Douglas Steel
  (doug@wg.icl.co.uk)).
- More bug fixes and workarounds for Solaris 2.X.  (These were
  mostly related to putting the collector in a dynamic library,
  which didn't really work before.  Also SOLARIS_THREADS
  didn't interact well with dl_open.)  Thanks to btlewis@eng.sun.com.
- Fixed a serious performance bug on the DEC Alpha.  The text
  segment was getting registered as part of the root set.
  (Amazingly, the result was still fast enough that the bug
  was not conspicuous.) The fix works on OSF/1, version 1.3.
  Hopefully it also works on other versions of OSF/1 ...
- Fixed a bug in GC_clear_roots.
- Fixed a bug in GC_generic_malloc_words_small that broke
  gc_inl.h.  (Reported by Antoine de Maricourt.  I broke it
  in trying to tweak the Mac port.) 
- Fixed some problems with cord/de under Linux.
- Fixed some cord problems, notably with CORD_riter4.
- Added DG/UX port.
  Thanks to Ben A. Mesander (ben@piglet.cr.usgs.gov)
- Added finalization registration routines with weaker ordering
  constraints.  (This is necessary for C++ finalization with
  multiple inheritance, since the compiler often adds self-cycles.)
- Filled the holes in the SCO port. (Thanks to Michael Arnoldus
  <chime@proinf.dk>.)
- John Ellis' additions to the C++ support:  From John:

* I completely rewrote the documentation in the interface gc_c++.h
(later renamed gc_cpp.h).  I've tried to make it both clearer and more
precise.

* The definition of accessibility now ignores pointers from an
finalizable object (an object with a clean-up function) to itself.
This allows objects with virtual base classes to be finalizable by the
collector.  Compilers typically implement virtual base classes using
pointers from an object to itself, which under the old definition of
accessibility prevented objects with virtual base classes from ever
being collected or finalized.

* gc_cleanup now includes gc as a virtual base.  This was enabled by
the change in the definition of accessibility.

* I added support for operator new[].  Since most (all?) compilers
don't yet support operator new[], it is conditionalized on
-DOPERATOR_NEW_ARRAY.  The code is untested, but its trivial and looks
correct.

* The test program test_gc_c++ (later renamed test_cpp.cc)
tries to test for the C++-specific functionality not tested by the
other programs.
- Added <unistd.h> include to misc.c.  (Needed for ppcr.)
- Added PowerMac port. (Thanks to Patrick Beard again.)
- Fixed "srcdir"-related Makefile problems.  Changed things so
  that all externally visible include files always appear in the
  include subdirectory of the source.  Made gc.h directly
  includable from C++ code.  (These were at Per
  Bothner's suggestion.)
- Changed Intel code to also mark from ebp (Kevin Warne's
  suggestion).
- Renamed C++ related files so they could live in a FAT
  file system. (Charles Fiterman's suggestion.)
- Changed Windows NT Makefile to include C++ support in
  gc.lib.  Added C++ test as Makefile target.
  
Since version 4.3:
 - ASM_CLEAR_CODE was erroneously defined for HP
   PA machines, resulting in a compile error.
 - Fixed OS/2 Makefile to create a library.  (Thanks to
   Mark Boulter (mboulter@vnet.ibm.com)).
 - Gc_cleanup objects didn't work if they were created on
   the stack.  Fixed.
 - One copy of Gc_cpp.h in the distribution was out of 
   synch, and failed to document some known compiler
   problems with explicit destructor invocation.  Partially
   fixed.  There are probably other compilers on which
   gc_cleanup is miscompiled.
 - Fixed Makefile to pass C compiler flags to C++ compiler.
 - Added Mac fixes.
 - Fixed os_dep.c to work around what appears to be
   a new and different VirtualQuery bug under newer
   versions of win32S.
 - GC_non_gc_bytes was not correctly maintained by
   GC_free.  Fixed.  Thanks to James Clark (jjc@jclark.com).
 - Added GC_set_max_heap_size.
 - Changed allocation code to ignore blacklisting if it is preventing
   use of a very large block of memory.  This has the advantage
   that naive code allocating very large objects is much more
   likely to work.  The downside is you might no
   longer find out that such code should really use
   GC_malloc_ignore_off_page.
 - Changed GC_printf under win32 to close and reopen the file
   between calls.  FAT file systems otherwise make the log file
   useless for debugging.
 - Added GC_try_to_collect and GC_get_bytes_since_gc.  These
   allow starting an abortable collection during idle times. 
   This facility does not require special OS support.  (Thanks to
   Michael Spertus of Geodesic Systems for suggesting this.  It was
   actually an easy addition.  Kumar Srikantan previously added a similar
   facility to a now ancient version of the collector.  At the time
   this was much harder, and the result was less convincing.)
 - Added some support for the Borland development environment.  (Thanks
   to John Ellis and Michael Spertus.)
 - Removed a misfeature from checksums.c that caused unexpected 
   heap growth.  (Thanks to Scott Schwartz.)
 - Changed finalize.c to call WARN if it encounters a finalization cycle.
   WARN is defined in gc_priv.h to write a message, usually to stdout.
   In many environments, this may be inappropriate.
 - Renamed NO_PARAMS in gc.h to GC_NO_PARAMS, thus adhering to my own
   naming convention.
 - Added GC_set_warn_proc to intercept warnings.
 - Fixed Amiga port. (Thanks to Michel Schinz (schinz@alphanet.ch).)
 - Fixed a bug in mark.c that could result in an access to unmapped
   memory from GC_mark_from_mark_stack on machines with unaligned
   pointers.
 - Fixed a win32 specific performance bug that could result in scanning of
   objects allocated with the system malloc.
 - Added REDIRECT_MALLOC.

Since version 4.4:
 - Fixed many minor and one major README bugs. (Thanks to Franklin Chen
   (chen@adi.com) for pointing out many of them.)
 - Fixed ALPHA/OSF/1 dynamic library support. (Thanks to Jonathan Bachrach
   (jonathan@harlequin.com)).
 - Added incremental GC support (MPROTECT_VDB) for Linux (with some
   help from Bruno Haible).
 - Altered SPARC recognition tests in gc.h and config.h (mostly as
   suggested by Fergus Henderson).
 - Added basic incremental GC support for win32, as implemented by
   Windows NT and Windows 95.  GC_enable_incremental is a noop
   under win32s, which doesn't implement enough of the VM interface.
 - Added -DLARGE_CONFIG.
 - Fixed GC_..._ignore_off_page to also function without
   -DALL_INTERIOR_POINTERS.
 - (Hopefully) fixed RS/6000 port.  (Only the test was broken.)
 - Fixed a performance bug in the nonincremental collector running
   on machines supporting incremental collection with MPROTECT_VDB
   (e.g. SunOS 4, DEC AXP).  This turned into a correctness bug under
   win32s with win32 incremental collection.  (Not all memory protection
   was disabled.)
 - Fixed some ppcr related bit rot.
 - Caused dynamic libraries to be unregistered before reregistering.
   The old way turned out to be a performance bug on some machines.
 - GC_root_size was not properly maintained under MSWIN32.
 - Added -DNO_DEBUGGING and GC_dump.
 - Fixed a couple of bugs arising with SOLARIS_THREADS +
   REDIRECT_MALLOC.
 - Added NetBSD/M68K port.  (Thanks to Peter Seebach
   <seebs@taniemarie.solon.com>.)
 - Fixed a serious realloc bug.  For certain object sizes, the collector
   wouldn't scan the expanded part of the object.  (Thanks to Clay Spence
   (cds@peanut.sarnoff.com) for noticing the problem, and helping me to
   track it down.)
   
Since version 4.5:
 - Added Linux ELF support.  (Thanks to Arrigo Triulzi <arrigo@ic.ac.uk>.)
 - GC_base crashed if it was called before any other GC_ routines.
   This could happen if a gc_cleanup object was allocated outside the heap
   before any heap allocation.
 - The heap expansion heuristic was not stable if all objects had finalization
   enabled.  Fixed finalize.c to count memory in finalization queue and
   avoid explicit deallocation.  Changed alloc.c to also consider this count.
   (This is still not recommended.  It's expensive if nothing else.)  Thanks
   to John Ellis for pointing this out.
 - GC_malloc_uncollectable(0) was broken.  Thanks to Phong Vo for pointing
   this out.
 - The collector didn't compile under Linux 1.3.X.  (Thanks to Fred Gilham for
   pointing this out.)  The current workaround is ugly, but expected to be
   temporary.
 - Fixed a formatting problem for SPARC stack traces.
 - Fixed some '=='s in os_dep.c that should have been assignments.
   Fortunately these were in code that should never be executed anyway.
   (Thanks to Fergus Henderson.)
 - Fixed the heap block allocator to only drop blacklisted blocks in small
   chunks.  Made BL_LIMIT self adjusting.  (Both of these were in response
   to heap growth observed by Paul Graham.)
 - Fixed the Metrowerks/68K Mac code to also mark from a6.  (Thanks
   to Patrick Beard.)
 - Significantly updated README.debugging.
 - Fixed some problems with longjmps out of signal handlers, especially under
   Solaris.  Added a workaround for the fact that siglongjmp doesn't appear to
   do the right thing with -lthread under Solaris.
 - Added MSDOS/djgpp port.  (Thanks to Mitch Harris  (maharri@uiuc.edu).)
 - Added "make reserved_namespace" and "make user_namespace".  The
   first renames ALL "GC_xxx" identifiers as "_GC_xxx".  The second is the
   inverse transformation.  Note that doing this is guaranteed to break all
   clients written for the other names.
 - descriptor field for kind NORMAL in GC_obj_kinds with ADD_BYTE_AT_END
   defined should be -ALIGNMENT not WORDS_TO_BYTES(-1).  This is
   a serious bug on machines with pointer alignment of less than a word.
 - GC_ignore_self_finalize_mark_proc didn't handle pointers to very near the
   end of the object correctly.  Caused failures of the C++ test on a DEC Alpha
   with g++.
 - gc_inl.h still had problems.  Partially fixed.  Added warnings at the
   beginning to hopefully specify the remaining dangers.
 - Added DATAEND definition to config.h.
 - Fixed some of the .h file organization.  Fixed "make floppy".
 
Since version 4.6:
 - Fixed some compilation problems with -DCHECKSUMS (thanks to Ian Searle)
 - Updated some Mac specific files to synchronize with Patrick Beard.
 - Fixed a serious bug for machines with non-word-aligned pointers.
   (Thanks to Patrick Beard for pointing out the problem.  The collector
   should fail almost any conceivable test immediately on such machines.)

Since version 4.7:
 - Changed a "comment" in a MacOS specific part of mach-dep.c that caused
   gcc to fail on other platforms.

Since version 4.8
 - More README.debugging fixes.
 - Objects ready for finalization, but not finalized in the same GC
   cycle, could be prematurely collected.  This occasionally happened
   in test_cpp.
 - Too little memory was obtained from the system for very large
   objects.  That could cause a heap explosion if these objects were
   not contiguous (e.g. under PCR), and too much of them was blacklisted.
 - Due to an improper initialization, the collector was too hesitant to
   allocate blacklisted objects immediately after system startup.
 - Moved GC_arrays from the data into the bss segment by not explicitly
   initializing it to zero.  This significantly
   reduces the size of executables, and probably avoids some disk accesses
   on program startup.  It's conceivable that it might break a port that I
   didn't test.
 - Fixed EMX_MAKEFILE to reflect the gc_c++.h to gc_cpp.h renaming which
   occurred a while ago.

Since 4.9:
 - Fixed a typo around a call to GC_collect_or_expand in alloc.c.  It broke
   handling of out of memory.  (Thanks to Patrick Beard for noticing.)

Since 4.10:
 - Rationalized (hopefully) GC_try_to_collect in an incremental collection
   environment.  It appeared to not handle a call while a collection was in
   progress, and was otherwise too conservative.
 - Merged GC_reclaim_or_delete_all into GC_reclaim_all to get rid of some
   code.
 - Added Patrick Beard's Mac fixes, with substantial completely untested
   modifications.
 - Fixed the MPROTECT_VDB code to deal with large pages and imprecise
   fault addresses (as on an UltraSPARC running Solaris 2.5).  Note that this
   was not a problem in the default configuration, which uses PROC_VDB.
 - The DEC Alpha assembly code needed to restore $gp between calls.
   Thanks to Fergus Henderson for tracking this down and supplying a
   patch.
 - The write command for "de" was completely broken for large files.
   I used the easiest portable fix, which involved changing the semantics
   so that f.new is written instead of overwriting f.  That's safer anyway.
 - Added README.solaris2 with a discussion of the possible problems of
   mixing the collector's sbrk allocation with malloc/realloc.
 - Changed the data segment starting address for SGI machines.  The
   old code failed under IRIX6.
 - Required double word alignment for MIPS.
 - Various minor fixes to remove warnings.
 - Attempted to fix some Solaris threads problems reported by Zhiying Chen.
   In particular, the collector could try to fork a thread with the
   world stopped as part of GC_thr_init.  It also failed to deal with
   the case in which the original thread terminated before the whole
   process did.
 - Added -DNO_EXECUTE_PERMISSION.  This has a major performance impact
   on the incremental collector under Irix, and perhaps under other
   operating systems.
 - Added some code to support allocating the heap with mmap.  This may
   be preferable under some circumstances.
 - Integrated dynamic library support for HP.
   (Thanks to Knut Tvedten <knuttv@ifi.uio.no>.)
 - Integrated James Clark's win32 threads support, and made a number
   of changes to it, many of which were suggested by Pontus Rydin.
   This is still not 100% solid.
 - Integrated Alistair Crooks' support for UTS4 running on an Amdahl
   370-class machine.
 - Fixed a serious bug in explicitly typed allocation.  Objects requiring
   large descriptors where handled in a way that usually resulted in
   a segmentation fault in the marker.  (Thanks to Jeremy Fitzhardinge
   for helping to track this down.)
 - Added partial support for GNU win32 development.  (Thanks to Fergus
   Henderson.)
 - Added optional support for Java-style finalization semantics.  (Thanks
   to Patrick Bridges.)  This is recommended only for Java implementations.
 - GC_malloc_uncollectable faulted instead of returning 0 when out of
   memory.  (Thanks to dan@math.uiuc.edu for noticing.)
 - Calls to GC_base before the collector was initialized failed on a
   DEC Alpha.  (Thanks to Matthew Flatt.)
 - Added base pointer checking to GC_REGISTER_FINALIZER in debugging
   mode, at the suggestion of Jeremy Fitzhardinge.
 - GC_debug_realloc failed for uncollectable objects.  (Thanks to
   Jeremy Fitzhardinge.)
 - Explicitly typed allocation could crash if it ran out of memory.
   (Thanks to Jeremy Fitzhardinge.)
 - Added minimal support for a DEC Alpha running Linux.
 - Fixed a problem with allocation of objects whose size overflowed
   ptrdiff_t.  (This now fails unconditionally, as it should.)
 - Added the beginning of Irix pthread support.
 - Integrated Xiaokun Zhu's fixes for djgpp 2.01.
 - Added SGI-style STL allocator support (gc_alloc.h).
 - Fixed a serious bug in README.solaris2.  Multithreaded programs must include
   gc.h with SOLARIS_THREADS defined.
 - Changed GC_free so it actually deallocates uncollectable objects.
   (Thanks to Peter Chubb for pointing out the problem.)
 - Added Linux ELF support for dynamic libararies.  (Thanks again to
   Patrick Bridges.)
 - Changed the Borland cc configuration so that the assembler is not
   required.
 - Fixed a bug in the C++ test that caused it to fail in 64-bit
   environments.

Since 4.11:
 - Fixed ElfW definition in dyn_load.c. (Thanks to Fergus Henderson.)
   This prevented the dynamic library support from compiling on some
   older ELF Linux systems.
 - Fixed UTS4 port (which I apparently mangled during the integration)
   (Thanks to again to Alistair Crooks.)
 - "Make C++" failed on Suns with SC4.0, due to a problem with "bool".
   Fixed in gc_priv.h.
 - Added more pieces for GNU win32.  (Thanks to Timothy N. Newsham.)
   The current state of things should suffice for at least some
   applications.
 - Changed the out of memory retry count handling as suggested by
   Kenjiro Taura.  (This matters only if GC_max_retries > 0, which
   is no longer the default.)
 - If a /proc read failed repeatedly, GC_written_pages was not updated
   correctly.  (Thanks to Peter Chubb for diagnosing this.)
 - Under unlikely circumstances, the allocator could infinite loop in
   an out of memory situation.  (Thanks again to Kenjiro Taura for
   identifying the problem and supplying a fix.)
 - Fixed a syntactic error in the DJGPP code.  (Thanks to Fergus
   Henderson for finding this by inspection.)  Also fixed a test program
   problem with DJGPP (Thanks to Peter Monks.)
 - Atomic uncollectable objects were not treated correctly by the
   incremental collector.  This resulted in weird log statistics and
   occasional performance problems.  (Thanks to Peter Chubb for pointing
   this out.)
 - Fixed some problems resulting from compilers that dont define
   __STDC__.  In this case void * and char * were used inconsistently
   in some cases.  (Void * should not have been used at all.  If
   you have an ANSI superset compiler that does not define __STDC__,
   please compile with -D__STDC__=0. Thanks to Manuel Serrano and others
   for pointing out the problem.)
 - Fixed a compilation problem on Irix with -n32 and -DIRIX_THREADS.
   Also fixed some other IRIX_THREADS problems which may or may not have
   had observable symptoms.
 - Fixed an HP PA compilation problem in dyn_load.c.  (Thanks to
   Philippe Queinnec.)
 - SEGV fault handlers sometimes did not get reset correctly.  (Thanks
   to David Pickens.)
 - Added a fix for SOLARIS_THREADS on Intel.  (Thanks again to David
   Pickens.)  This probably needs more work to become functional.
 - Fixed struct sigcontext_struct in os_dep.c for compilation under
   Linux 2.1.X.	(Thanks to Fergus Henderson.)
 - Changed the DJGPP STACKBOTTOM and DATASTART values to those suggested
   by Kristian Kristensen.  These may still not be right, but it is
   it is likely to work more often than what was there before.  They may
   even be exactly right.
 - Added a #include <string.h> to test_cpp.cc.  This appears to help
   with HP/UX and gcc.  (Thanks to assar@sics.se.)
 - Version 4.11 failed to run in incremental mode on recent 64-bit Irix
   kernels.  This was a problem related to page unaligned heap segments.
   Changed the code to page align heap sections on all platforms.
   (I had mistakenly identified this as a kernel problem earlier.
   It was not.)
 - Version 4.11 did not make allocated storage executable, except on
   one or two platforms, due to a bug in a #if test.  (Thanks to Dave
   Grove for pointing this out.)
 - Added sparc_sunos4_mach_dep.s to support Sun's compilers under SunOS4.
 - Added GC_exclude_static_roots.
 - Fixed the object size mapping algorithm.  This shouldn't matter,
   but the old code was ugly.
 - Heap checking code could die if one of the allocated objects was
   larger than its base address.  (Unsigned underflow problem.  Thanks
   to Clay Spence for isolating the problem.)
 - Added RS6000 (AIX) dynamic library support and fixed STACK_BOTTOM.
   (Thanks to Fred Stearns.)
 - Added Fergus Henderson's patches for improved robustness with large
   heaps and lots of blacklisting.
 - Added Peter Chubb's changes to support Solaris Pthreads, to support
   MMAP allocation in Solaris, to allow Solaris to find dynamic libraries
   through /proc, to add malloc_typed_ignore_off_page, and a few other
   minor features and bug fixes.
 - The Solaris 2 port should not use sbrk.  I received confirmation from
   Sun that the use of sbrk and malloc in the same program is not
   supported.  The collector now defines USE_MMAP by default on Solaris.
 - Replaced the djgpp makefile with Gary Leavens' version.
 - Fixed MSWIN32 detection test.
 - Added Fergus Henderson's patches to allow putting the collector into
   a DLL under GNU win32.
 - Added Ivan V. Demakov's port to Watcom C on X86.
 - Added Ian Piumarta's Linux/PowerPC port.
 - On Brian Burton's suggestion added PointerFreeGC to the placement
   options in gc_cpp.h.  This is of course unsafe, and may be controversial.
   On the other hand, it seems to be needed often enough that it's worth
   adding as a standard facility.

Since 4.12:
 - Fixed a crucial bug in the Watcom port.  There was a redundant decl
   of GC_push_one in gc_priv.h.
 - Added FINALIZE_ON_DEMAND.
 - Fixed some pre-ANSI cc problems in test.c.
 - Removed getpagesize() use for Solaris.  It seems to be missing in one
   or two versions.
 - Fixed bool handling for SPARCCompiler version 4.2.
 - Fixed some files in include that had gotten unlinked from the main
   copy.
 - Some RS/6000 fixes (missing casts).  Thanks to Toralf Foerster.
 - Fixed several problems in GC_debug_realloc, affecting mostly the
   FIND_LEAK case.
 - GC_exclude_static_roots contained a buggy unsigned comparison to
   terminate a loop.  (Thanks to Wilson Ho.)
 - CORD_str failed if the substring occurred at the last possible position.
   (Only affects cord users.)
 - Fixed Linux code to deal with RedHat 5.0 and integrated Peter Bigot's
   os_dep.c code for dealing with various Linux versions.
 - Added workaround for Irix pthreads sigaction bug and possible signal
   misdirection problems.
Since alpha1:
 - Changed RS6000 STACKBOTTOM.
 - Integrated Patrick Beard's Mac changes.
 - Alpha1 didn't compile on Irix m.n, m < 6.
 - Replaced Makefile.dj with a new one from Gary Leavens.
 - Added Andrew Stitcher's changes to support SCO OpenServer.
 - Added PRINT_BLACK_LIST, to allow debugging of high densities of false
   pointers.
 - Added code to debug allocator to keep track of return address
   in GC_malloc caller, thus giving a bit more context.
 - Changed default behavior of large block allocator to more
   aggressively avoid fragmentation.  This is likely to slow down the
   collector when it succeeds at reducing space cost.
 - Integrated Fergus Henderson's CYGWIN32 changes.  They are untested,
   but needed for newer versions.
 - USE_MMAP had some serious bugs.  This caused the collector to fail
   consistently on Solaris with -DSMALL_CONFIG.
 - Added Linux threads support, thanks largely to Fergus Henderson.
Since alpha2:
 - Fixed more Linux threads problems.
 - Changed default GC_free_space_divisor to 3 with new large block allocation.
   (Thanks to Matthew Flatt for some measurements that suggest the old
   value sometimes favors space too much over time.)
 - More CYGWIN32 fixes.
 - Integrated Tyson-Dowd's Linux-M68K port.
 - Minor HP PA and DEC UNIX fixes from Fergus Henderson.
 - Integrated Christoffe Raffali's Linux-SPARC changes.
 - Allowed for one more GC fixup iteration after a full GC in incremental
   mode.  Some quick measurements suggested that this significantly
   reduces pause times even with smaller GC_RATE values.
 - Moved some more GC data structures into GC_arrays.  This decreases
   pause times and GC overhead, but makes debugging slightly less convenient.
 - Fixed namespace pollution problem ("excl_table").
 - Made GC_incremental a constant for -DSMALL_CONFIG, hopefully shrinking
   that slightly.
 - Added some win32 threads fixes.
 - Integrated Ivan Demakov and David Stes' Watcom fixes.
 - Various other minor fixes contributed by many people.
 - Renamed config.h to gcconfig.h, since config.h tends to be used for
   many other things.
 - Integrated Matthew Flatt's support for 68K MacOS "far globals".
 - Fixed up some of the dynamic library Makefile targets for consistency
   across platforms.
 - Fixed a USE_MMAP typo that caused out-of-memory handling to fail
   on Solaris.
 - Added code to test.c to test thread creation a bit more.
 - Integrated GC_win32_free_heap, as suggested by Ivan Demakov.
 - Fixed Solaris 2.7 stack base finding problem.  (This may actually
   have been done in an earlier alpha release.)
Since alpha3:
 - Fixed MSWIN32 recognition test, which interfered with cygwin.
 - Removed unnecessary gc_watcom.asm from distribution.  Removed
   some obsolete README.win32 text.
 - Added Alpha Linux incremental GC support.  (Thanks to Philipp Tomsich
   for code for retrieving the fault address in a signal handler.)
   Changed Linux signal handler context argument to be a pointer.
 - Took care of some new warnings generated by the 7.3 SGI compiler.
 - Integrated Phillip Musumeci's FreeBSD/ELF fixes.
 - -DIRIX_THREADS was broken with the -o32 ABI (typo in gc_priv.h>

Since 4.13:
 - Fixed GC_print_source_ptr to not use a prototype.
 - generalized CYGWIN test.
 - gc::new did the wrong thing with PointerFreeGC placement.
   (Thanks to Rauli Ruohonen.)
 - In the ALL_INTERIOR_POINTERS (default) case, some callee-save register
   values could fail to be scanned if the register was saved and
   reused in a GC frame.  This showed up in verbose mode with gctest
   compiled with an unreleased SGI compiler.  I vaguely recall an old
   bug report that may have been related.  The bug was probably quite old.
   (The problem was that the stack scanning could be deferred until
   after the relevant frame was overwritten, and the new save location
   might be outside the scanned area.  Fixed by more eager stack scanning.)
 - PRINT_BLACK_LIST had some problems.  A few source addresses were garbage.
 - Replaced Makefile.dj and added -I flags to cord make targets.
   (Thanks to Gary Leavens.)
 - GC_try_to_collect was broken with the nonincremental collector.
 - gc_cleanup destructors could pass the wrong address to
   GC_register_finalizer_ignore_self in the presence of multiple
   inheritance.  (Thanks to Darrell Schiebel.)
 - Changed PowerPC Linux stack finding code.

Since 4.14alpha1
 - -DSMALL_CONFIG did not work reliably with large (> 4K) pages.
   Recycling the mark stack during expansion could result in a size
   zero heap segment, which confused things.  (This was probably also an
   issue with the normal config and huge pages.)
 - Did more work to make sure that callee-save registers were scanned
   completely, even with the setjmp-based code.  Added USE_GENERIC_PUSH_REGS
   macro to facilitate testing on machines I have access to.
 - Added code to explicitly push register contents for win32 threads.
   This seems to be necessary.  (Thanks to Pierre de Rop.)

Since 4.14alpha2
 - changed STACKBOTTOM for DJGPP (Thanks to Salvador Eduardo Tropea).
 
Since 4.14
 - Reworked large block allocator.  Now uses multiple doubly linked free
   lists to approximate best fit.
 - Changed heap expansion heuristic.  Entirely free blocks are no longer
   counted towards the heap size.  This seems to have a major impact on
   heap size stability; the old version could expand the heap way too
   much in the presence of large block fragmentation.
 - added -DGC_ASSERTIONS and some simple assertions inside the collector.
   This is mainlyt for collector debugging.
 - added -DUSE_MUNMAP to allow the heap to shrink.  Suupported on only
   a few UNIX-like platforms for now.
 - added GC_dump_regions() for debugging of fragmentation issues.
 - Changed PowerPC pointer alignment under Linux to 4.  (This needs
   checking by someone who has one.  The suggestions came to me via a
   rather circuitous path.)
 - Changed the Linux/Alpha port to walk the data segment backwards until
   it encounters a SIGSEGV.  The old way to find the start of the data
   segment broke with a recent release.
 - cordxtra.c needed to call GC_REGISTER_FINALIZER instead of
   GC_register_finalizer, so that it would continue to work with GC_DEBUG.
 - allochblk sometimes cleared the wrong block for debugging purposes
   when it dropped blacklisted blocks.  This could result in spurious
   error reports with GC_DEBUG.
 - added MACOS X Server support.  (Thanks to Andrew Stone.)
 - Changed the Solaris threads code to ignore stack limits > 8 MB with
   a warning.  Empirically, it is not safe to access arbitrary pages
   in such large stacks.  And the dirty bit implementation does not
   guarantee that none of them will be accessed.
 - Integrated Martin Tauchmann's Amiga changes.
 - Integrated James Dominy's OpenBSD/SPARC port.

Since 5.0alpha1
 - Fixed bugs introduced in alpha1 (OpenBSD & large block initialization).
 - Added -DKEEP_BACK_PTRS and backptr.h interface.  (The implementation
   idea came from Al Demers.)

Since 5.0alpha2
 - Added some highly incomplete code to support a copied young generation.
   Comments on nursery.h are appreciated.
 - Changed -DFIND_LEAK, -DJAVA_FINALIZATION, and -DFINALIZE_ON_DEMAND,
   so the same effect could be obtained with a runtime switch.   This is
   a step towards standardizing on a single dynamic GC library.
 - Significantly changed the way leak detection is handled, as a consequence
   of the above.

Since 5.0 alpha3
 - Added protection fault handling patch for Linux/M68K from Fergus
   Henderson and Roman Hodek.
 - Removed the tests for SGI_SOURCE in new_gc_alloc.h.  This was causing that
   interface to fail on nonSGI platforms.
 - Changed the Linux stack finding code to use /proc, after changing it
   to use HEURISTIC1.  (Thanks to David Mossberger for pointing out the
   /proc hook.)
 - Added HP/UX incremental GC support and HP/UX 11 thread support.
   Thread support is currently still flakey.
 - Added basic Linux/IA64 support.
 - Integrated Anthony Green's PicoJava support.
 - Integrated Scott Ananian's StrongARM/NetBSD support.
 - Fixed some fairly serious performance bugs in the incremental
   collector.  These have probably been there essentially forever.
   (Mark bits were sometimes set before scanning dirty pages.
   The reclaim phase unnecessarily dirtied full small object pages.)
 - Changed the reclaim phase to ignore nearly full pages to avoid
   touching them.
 - Limited GC_black_list_spacing to roughly the heap growth increment.
 - Changed full collection triggering heuristic to decrease full GC
   frequency by default, but to explicitly trigger full GCs during
   heap growth.  This doesn't always improve things, but on average it's
   probably a win.
 - GC_debug_free(0, ...) failed.  Thanks to Fergus Henderson for the
   bug report and fix.

Since 5.0 alpha4
 - GC_malloc_explicitly_typed and friends sometimes failed to
   initialize first word.
 - Added allocation routines and support in the marker for mark descriptors
   in a type structure referenced by the first word of an object.  This was
   introduced to support gcj, but hopefully in a way that makes it
   generically useful.
 - Added GC_requested_heapsize, and inhibited collections in nonincremental
   mode if the actual used heap size is less than what was explicitly
   requested.
 - The Solaris pthreads version of GC_pthread_create didn't handle a NULL
   attribute pointer.  Solaris thread support used the wrong default thread
   stack size.  (Thanks to Melissa O'Neill for the patch.)
 - Changed PUSH_CONTENTS macro to no longer modify first parameter.
   This usually doesn't matter, but it was certainly an accident waiting
   to happen ...
 - Added GC_register_finalizer_no_order and friends to gc.h.  They're
   needed by Java implementations.
 - Integrated a fix for a win32 deadlock resulting from clock() calling
   malloc.  (Thanks to Chris Dodd.)
 - Integrated Hiroshi Kawashima's port to Linux/MIPS.  This was designed
   for a handheld platform, and may or may not be sufficient for other
   machines.
 - Fixed a va_arg problem with the %c specifier in cordprnt.c.  It appears
   that this was always broken, but recent versions of gcc are the first to
   report the (statically detectable) bug.
 - Added an attempt at a more general solution to dlopen races/deadlocks.
   GC_dlopen now temporarily disables collection.  Still not ideal, but ...
 - Added -DUSE_I686_PREFETCH, -DUSE_3DNOW_PREFETCH, and support for IA64
   prefetch instructions.  May improve performance measurably, but I'm not
   sure the code will run correctly on processors that don't support the
   instruction.  Won't build except with very recent gcc.
 - Added caching for header lookups in the marker.  This seems to result
   in a barely measurable performance gain.  Added support for interleaved
   lookups of two pointers, but unconfigured that since the performance
   gain is currently near zero, and it adds to code size.
 - Changed Linux DATA_START definition to check both data_start and
   __data_start, since nothing else seems to be portable.
 - Added -DUSE_LD_WRAP to optionally take advantage of the GNU ld function
   wrapping mechanism.  Probably currently useful only on Linux.
 - Moved some variables for the scratch allocator into GC_arrays, on
   Martin Hirzel's suggestion.
 - Fixed a win32 threads bug that caused the collector to not look for
   interior pointers from one of the thread stacks without
   ALL_INTERIOR_POINTERS.  (Thanks to Jeff Sturm.)
 - Added Mingw32 support.  (Thanks again to Jeff Sturm for the patch.)
 - Changed the alpha port to use the generic register scanning code instead
   of alpha_mach_dep.s.  Alpha_mach_dep.s doesn't look for pointers in fp
   registers, but gcc sometimes spills pointers there.  (Thanks to Manuel
   Serrano for helping me debug this by email.)  Changed the IA64 code to
   do something similar for similar reasons.

[5.0alpha5 doesn't really exist, but it may have escaped.]

Since 5.0alpha6:
 - -DREDIRECT_MALLOC was broken in alpha6. Fixed.
 - Cleaned up gc_ccp.h slightly, thus also causing the HP C++ compiler to
   accept it.
 - Removed accidental reference to dbg_mlc.c, which caused dbg_mlc.o to be
   linked into every executable.
 - Added PREFETCH to bitmap marker.  Changed it to use the header cache.
 - GC_push_marked sometimes pushed one object too many, resulting in a
   segmentation fault in GC_mark_from_mark_stack.  This was probably an old
   bug.  It finally showed up in gctest on win32.
 - Gc_priv.h erroneously #defined GC_incremental to be TRUE instead of FALSE
   when SMALL_CONFIG was defined.  This was no doubt a major performance bug for
   the default win32 configuration.
 - Removed -DSMALL_CONFIG from NT_MAKEFILE.  It seemed like an anchronism now
   that the average PC has 64MB or so.
 - Integrated Bryce McKinley's patches for linux threads and dynamic loading
   from the libgcj tree.  Turned on dynamic loading support for Linux/PPC.
 - Changed the stack finding code to use environ on HP/UX.  (Thanks
   to Gustavo Rodriguez-Rivera for the suggestion.)  This should probably
   be done on other platforms, too.  Since I can't test those, that'll
   wait until after 5.0.

Since 5.0alpha7:
 - Fixed threadlibs.c for linux threads.  -DUSE_LD_WRAP was broken and
   -ldl was omitted.  Fixed Linux stack finding code to handle
   -DUSE_LD_WRAP correctly.
 - Added MSWIN32 exception handler around marker, so that the collector
   can recover from root segments that are unmapped during the collection.
   This caused occasional failures under Windows 98, and may also be
   an issue under Windows NT/2000.

Since 5.0
 - Fixed a gc.h header bug which showed up under Irix.  (Thanks to
   Dan Sullivan.)
 - Fixed a typo in GC_double_descr in typd_mlc.c.
   This probably could result in objects described by array descriptors not
   getting traced correctly.  (Thanks to Ben Hutchings for pointing this out.)
 - The block nearly full tests in reclaim.c were not correct for 64 bit
   environments.  This could result in unnecessary heap growth under unlikely
   conditions.

Since 5.1
 - dyn_load.c declared GC_scratch_last_end_ptr as an extern even if it
   was defined as a macro.  This prevented the collector from building on
   Irix.
 - We quietly assumed that indirect mark descriptors were never 0.
   Our own typed allocation interface violated that.  This could result
   in segmentation faults in the marker with typed allocation.
 - Fixed a _DUSE_MUNMAP bug in the heap block allocation code.
   (Thanks to Ben Hutchings for the patch.)
 - Taught the collector about VC++ handling array operator new.
   (Thanks again to Ben Hutchings for the patch.)
 - The two copies of gc_hdrs.h had diverged.  Made one a link to the other
   again.

Since 5.2  (A few 5.2 patches are not in 6.0alpha1)
 - Fixed _end declaration for OSF1.
 - There were lots of spurious leak reports in leak detection mode, caused
   by the fact that some pages were not being swept, and hence unmarked
   objects weren't making it onto free lists.  (This bug dated back to 5.0.)
 - Fixed a typo in the liblinuxgc.so Makefile rule.
 - Added the GetExitCodeThread to Win32 GC_stop_world to (mostly) work
   around a Windows 95 GetOpenFileName problem.  (Thanks to Jacob Navia.)

Since 5.3
 - Fixed a typo that prevented compilation with -DUSE_3DNOW_PREFETCH.
   (Thanks to Shawn Wagner for actually testing this.)
 - Fixed GC_is_thread_stack in solaris_threads.c.  It forgot to return a value
   in the common case.  I wonder why nobody noticed?
 - Fixed another silly syntax problem in GC_double_descr.  (Thanks to
   Fergus Henderson for finding it.)
 - Fixed a GC_gcj_malloc bug: It tended to release the allocator lock twice.

Since 5.4  (A few 5.3 patches are not in 6.0alpha2)
 - Added HP/PA prefetch support.
 - Added -DDBG_HDRS_ALL and -DSHORT_DBG_HDRS to reduce the cost and improve
   the reliability of generating pointer backtrace information, e.g. in
   the Bigloo environment.
 - Added parallel marking support (-DPARALLEL_MARK).  This currently
   works only under IA32 and IA64 Linux, but it shouldn't be hard to adapt
   to other platforms.  This is intended to be a lighter-weight (less
   new code, probably not as scalable) solution than the work by Toshio Endo
   et al, at the University of Tokyo.  A number of their ideas were
   reused, though the code wasn't, and the underlying data structure
   is significantly different.  In particular, we keep the global mark
   stack as a single shared data structure, but most of the work is done
   on smaller thread-local mark stacks.
 - Changed GC_malloc_many to be cheaper, and to require less mutual exclusion
   with -DPARALLEL_MARK.
 - Added full support for thread local allocation under Linux
   (-DTHREAD_LOCAL_ALLOC).  This is a thin veneer on GC_malloc_many, and
   should be easily portable to other platforms, especially those that
   support pthreads.
 - CLEAR_DOUBLE was not always getting invoked when it should have been.
 - GC_gcj_malloc and friends used different out of memory handling than
   everything else, probably because I forgot about one when I implemented
   the other.  They now both call GC_oom_fn(), not GC_oom_action().
 - Integrated Jakub Jelinek's fixes for Linux/SPARC.
 - Moved GC_objfreelist, GC_aobjfreelist, and GC_words_allocd out of
   GC_arrays, and separately registered the first two as excluded roots.
   This makes code compiled with gc_inl.h less dependent on the
   collector version.  (It would be nice to remove the inclusion of
   gc_priv.h by gc_inl.h completely, but we're not there yet.  The
   locking definitions in gc_priv.h are still referenced.)
   This change was later coniditoned on SEPARATE_GLOBALS, which
   is not defined by default, since it involves a performance hit.
 - Register GC_obj_kinds separately as an excluded root region.  The
   attempt to register it with GC_arrays was usually failing.  (This wasn't
   serious, but seemed to generate some confusion.) 
 - Moved backptr.h to gc_backptr.h.

Since 6.0alpha1
 - Added USE_MARK_BYTES to reduce the need for compare-and-swap on platforms
   for which that's expensive.
 - Fixed a locking bug ib GC_gcj_malloc and some locking assertion problems.
 - Added a missing volatile to OR_WORD and renamed the parameter to
   GC_compare_and_swap so it's not a C++ reserved word.  (Thanks to
   Toshio Endo for pointing out both of those.)
 - Changed Linux dynamic library registration code to look at /proc/self/maps
   instead of the rld data structures when REDIRECT_MALLOC is defined.
   Otherwise some of the rld data data structures may be prematurely garbage
   collected.  (Thanks to Eric Benson for helping to track this down.)
 - Fixed USE_LD_WRAP a bit more, so it should now work without threads.
 - Renamed XXX_THREADS macros to GC_XXX_THREADS for namespace correctness.
   Tomporarily added some backward compatibility definitions.  Renamed
   USE_LD_WRAP to GC_USE_LD_WRAP.
 - Many MACOSX POWERPC changes, some additions to the gctest output, and
   a few minor generic bug fixes.  (Thanks to Dietmar Planitzer.)

Since 6.0 alpha2
 - Fixed the /proc/self/maps code to not seek, since that apparently is not
   reliable across all interesting kernels.
 - Fixed some compilation problems in the absence of PARALLEL_MARK
   (introduced in alpha2).
 - Fixed an algorithmic problem with PARALLEL_MARK.  If work needs to
   be given back to the main mark "stack", the BOTTOM entries of the local
   stack should be given away, not the top ones.  This has substantial
   performance impact, especially for > 2 processors, from what I can tell.
 - Extracted gc_lock.h from gc_priv.h.  This should eventually make it a
   bit easier to avoid including gc_priv.h in clients.
 - Moved all include files to include/ and removed duplicate links to the
   same file.  The old scheme was a bad idea because it was too easy to get the
   copies out of sync, and many systems don't support hard links.
   Unfortunately, it's likely that I broke some of the non-Unix Makefiles in
   the process, although I tried to update them appropriately.
 - Removed the partial support for a copied nursery.  It's not clear that
   this would be a tremendous win, since we don't consistently lose to
   generational copying collectors.  And it would significantly complicate
   many things.  May be reintroduced if/when it really turns out to win.
 - Removed references to IRIX_JDK_THREADS, since I believe there never
   were and never will be any clients.
 - Added some code to linux_threads.c to possibly support HPUX threads
   using the Linux code.  Unfortunately, it doesn't work yet, and is
   currently disabled.
 - Added support under Linux/X86 for saving the call chain, both in (debug)
   objects for client debugging, and in GC_arrays._last_stack for GC
   debugging.  This was previously supported only under Solaris.  It is
   not enabled by default under X86, since it requires that code be compiled
   to explicitly dave frame pointers on the call stack.  (With gcc this
   currently happens by default, but is often turned off explicitly.)
   To turn it on, define SAVE_CALL_CHAIN.
 
Since 6.0 alpha3
 - Moved up the detection of mostly full blocks to the initiatiation of the
   sweep phase.  This eliminates some lock conention in the PARALLEL_MARK case,
   as multiple threads try to look at mostly full blocks concurrently.
 - Restored the code in GC_malloc_many that grabs a prefix of the global
   free list.  This avoids the case in which every GC_malloc_many call
   tries and fails to allocate a new heap block, and the returns a single
   object from the global free list.
 - Some minor fixes in new_hblk.c.  (Attempted to build free lists in order
   of increasing addresses instead of decreasing addresses for cache performance
   reasons.  But this seems to be only a very minor gain with -DEAGER_SWEEP,
   and a loss in other cases.  So the change was backed out.)
 - Fixed some of the documentation.  (Thanks in large part to Fergus
   Henderson.)
 - Fixed the Linux USE_PROC_FOR_LIBRARIES code to deal with apps that perform
   large numbers of mmaps.  (Thanks to Eric Benson.)  Also fixed that code to
   deal with short reads.
 - Added GC_get_total_bytes(). 
 - Fixed leak detection mode to avoid spurious messages under linuxthreads.
   (This should also now be easy for the other supported threads packages.
   But the code is tricky enough that I'm hesitant to do it without being able
   to test.  Everything allocated in the GC thread support itself should be
   explicitly deallocated.)
 - Made it possible (with luck) to redirect malloc to GC_local_malloc.

Since 6.0 alpha4
 - Changed the definition of GC_pause in linux_threads.c to use a volatile
   asm.  Some versions of gcc apparently optimize away writes to local volatile
   variables.  This caused poor locking behaviour starting at about
   4 processors.
 - Added GC_start_blocking(), GC_end_blocking() calls and wrapper for sleep
   to linux_threads.c.
   The first two calls could be used to generally avoid sending GC signals to
   blocked threads, avoiding both premature wakeups and unnecessary overhead.
 - Fixed a serious bug in thread-local allocation.  At thread termination,
   GC_free could get called on small integers.  Changed the code for thread
   termination to more efficiently return left-over free-lists.
 - Integrated Kjetil Matheussen's BeOS support.
 - Rearranged the directory structure to create the doc and tests
   subdirectories.
 - Sort of integrated Eric Benson's patch for OSF1.  This provided basic
   OSF1 thread support by suitably extending hpux_irix_threads.c.  Based
   on earlier email conversations with David Butenhof, I suspect that it
   will be more reliable in the long run to base this on linux_threads.c
   instead.  Thus I attempted to patch up linux_threads.c based on Eric's code.
   The result is almost certainly broken, but hopefully close enough that
   someone with access to a machine can pick it up.
 - Integrated lots of minor changes from the NetBSD distribution.  (These
   were supplied by David Brownlee.  I'm not sure about the original
   authors.)
 - Hacked a bit more on the HP/UX thread-support in linux_threads.c.  It
   now appears to work in the absence of incremental collection.  Renamed
   hpux_irix_threads.c back to irix_threads.c, and removed the attempt to
   support HPUX there.
 - Changed gc.h to define _REENTRANT in cases in which it should already
   have been defined. It is still safer to also define it on the command
   line. 

Since 6.0alpha5:
 - Changed the definition of DATASTART on ALPHA and IA64, where data_start
   and __data_start are not defined by earlier versions of glibc.  This might
   need to be fixed on other platforms as well.
 - Changed the way the stack base and backing store base are found on IA64.
   This should now remain reliable on future kernels.  But since it relies
   on /proc, it will no longer work in the simulated NUE environment.
 - Made the call to random() in dbg_mlc.c with -DKEEP_BACK_PTRS dependent
   on the OS.  On non-Unix systems, rand() should be used instead.  Handled
   small RAND_MAX.  (Thanks to Peter Ross for pointing this out.)
 - Fixed the cord make rules to create the cord subdirectory, if necessary.
   (Thanks to Doug Moen.)
 - Changed fo_object_size calculation in finalize.c.  Turned finalization
   of nonheap object into a no-op.  Removed anachronism from GC_size()
   implementation.
 - Changed GC_push_dirty call in solaris_threads.c to GC_push_selected.
   It was missed in a previous renaming. (Thanks to Vladimir Tsichevski
   for pointing this out.)
 - Arranged to not not mask SIGABRT in linux_threads.c.  (Thanks to Bryce
   McKinlay.) 
 - Added GC_no_dls hook for applications that want to register their own
   roots.
 - Integrated Kjetil Matheussen's Amiga changes.
 - Added FREEBSD_STACKBOTTOM.  Changed the X86/FreeBSD port to use it.
   (Thanks to Matthew Flatt.)
 - Added pthread_detach interception for platforms supported by linux_threads.c
   and irix_threads.c.  Should also be added for Solaris?
 - Changed the USE_MMAP code to check for the case in which we got the
   high end of the address space, i.e. mem_ptr + mem_sz == 0.  It appears
   that this can happen under Solaris 7.  It seems to be allowed by what
   I would claim is an oversight in the mmap specification.  (Thanks to Toshio
   Endo for pointing out the problem.)
 - Cleanup of linux_threads.c.  Some code was originally cloned from
   irix_threads.c and now unnecessary.  Some comments were obviously wrong.
 - (Mostly) fixed a longstanding problem with setting of dirty bits from
   a signal handler.  In the presence of threads, dirty bits could get lost,
   since the etting of a bit in the bit vector was not atomic with respect
   to other updates.  The fix is 100% correct only for platforms for which
   GC_test_and_set is defined.  The goal is to make that all platforms with
   thread support.  Matters only if incremental GC and threads are both
   enabled.
 - made GC_all_interior_pointers (a.k.a. ALL_INTERIOR_POINTERS) an
   initialization time, instead of build-time option.  This is a 
   nontrivial, high risk change.  It should slow down the code measurably
   only if MERGE_SIZES is not defined, which is a very nonstandard
   configuration.   
 - Added doc/README.environment, and implemented what it describes.  This
   allows a number of additional configuration options to be set through
   the environment.  It documents a few previously undocumented options.
 - Integrated Eric Benson's leak testing improvements.
 - Removed the option to throw away the beginning of each page (DISCARD_WORDS).
   This became less and less useful as processors enforce stricter alignment.
   And it hadn't been tested in ages, and was thus probably broken anyway.

Since 6.0alpha6:
 - Added GC_finalizer_notifier.  Fixed GC_finalize_on_demand.  (The variable
   actually wasn't being tested at the right points.  The build-time flag
   was.)
 - Added Tom Tromey's S390 Linux patch.
 - Added code to push GC_finalize_now in GC_push_finalizer_structures.
   (Thanks to Matthew Flatt.)
 - Added GC_push_gc_structures() to push all GC internal roots.
 - Integrated some FreeBSD changes from Matthew Flatt.
 - It looks like USRSTACK is not always correctly defined under Solaris.
   Hacked gcconfig.h to attempt to work around the problem.  The result
   is not well tested.  (Thanks again to Matthew Flatt for pointing this
   out.  The gross hack is mine. - HB)
 - Added Ji-Yong Chung's win32 threads and C++ fixes.
 - Arranged for hpux_test_and_clear.s to no longer be needed or built.
   It was causing build problems with gas, and it's not clear this is
   better than the pthreads alternative on this platform.
 - Some MINGW32 fixes from Hubert Garavel.
 - Added Initial Hitachi SH4 port from Kaz Kojima.
 - Ported thread-local allocation and parallel mark code to HP/UX on PA_RISC.
 - Made include/gc_mark.h more public and separated out the really private
   pieces.  This is probably still not quite sufficient for clients that
   want to supply their own kind of type information.  But it's a start.
   This involved lots of identifier renaming to make it namespace clean.
 - Added GC_dont_precollect for clients that need complete control over
   the root set.
 - GC_is_visible didn't do the right thing with gcj objects.  (Not that
   many people are likely to care, but ...)
 - Don't redefine read with GC_USE_LD_WRAP.
 - Initial port to LINUX/HP_PA.  Incremental collection and threads are not
   yet supported.  (Incremental collection should work if you have the
   right kernel.  Threads may work with a sufficiently patched pthread
   library.)
 - Changed gcconfig.h to recognize __i386__ as an alternative to i386 in
   many places.  (Thanks to Benjamin Lerman.)
 - Made win32_threads.c more tolerant of detaching a thread that it didn't
   know about.  (Thanks to Paul Nash.)
 - Added Makefile.am and configure.in from gcc to the distribution, with
   minimal changes.  For the moment, those are just placeholders.  In the
   future, we're planning to switch to a GNU-style build environment for
   Un*x-like systems, though the old Makefile will remain as a backup.
 - Turned off STUBBORN_ALLOC by default, and added it back as a Makefile
   option.
 - Redistributed some functions between malloc.c and mallocx.c, so that
   simple statically linked apps no longer pull in mallocx.o.
 - Changed large object allocation to clear the first and last few words
   of each block before releassing the lock.  Otherwise the marker could see
   objects with nonsensical type descriptors.
 - Fixed a couple of subtle problems that could result in not recognizing
   interior pointers from the stack.  (I believe these were introduced
   in 6.0alpha6.)
 - GC_debug_free_inner called GC_free, which tried to reacquire the
   allocator lock, and hence deadlocked.  (DBG_HDRS_ALL probably never worked
   with threads?)
 - Fixed several problems with back traces.  Accidental references to a free
   list could cause the free list pointer to be overwritten by a back pointer.
   There seemed to be some problems with the encoding of root and finalizer
   references.
  
To do:
 - Very large root set sizes (> 16 MB or so) could cause the collector
   to abort with an unexpected mark stack overflow.  (Thanks again to
   Peter Chubb.)  NOT YET FIXED.  Workaround is to increase the initial
   size.
 - The SGI version of the collector marks from mmapped pages, even
   if they are not part of dynamic library static data areas.  This
   causes performance problems with some SGI libraries that use mmap
   as a bitmap allocator.  NOT YET FIXED.  It may be possible to turn
   off DYNAMIC_LOADING in the collector as a workaround.  It may also
   be possible to conditionally intercept mmap and use GC_exclude_static_roots.
   The real fix is to walk rld data structures, which looks possible.
 - Incremental collector should handle large objects better.  Currently,
   it looks like the whole object is treated as dirty if any part of it
   is.
 - Cord/cordprnt.c doesn't build on a few platforms (notably PowerPC), since
   we make some unwarranted assumptions about how varargs are handled.  This
   currently makes the cord-aware versions of printf unusable on some platforms.
   Fixing this is unfortunately not trivial.