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Text File | 1994-12-22 | 50KB | 992 lines

Copyright 1988, 1989 Hans-J. Boehm, Alan J. Demers
Copyright (c) 1991-1994 by Xerox Corporation.  All rights reserved.

THIS MATERIAL IS PROVIDED AS IS, WITH ABSOLUTELY NO WARRANTY EXPRESSED
OR IMPLIED.  ANY USE IS AT YOUR OWN RISK.

Permission is hereby granted to use or copy this program
for any purpose,  provided the above notices are retained on all copies.
Permission to modify the code and to distribute modified code is granted,
provided the above notices are retained, and a notice that the code was
modified is included with the above copyright notice.

This is version 4.3 of a conservative garbage collector for C and C++.

HISTORY -

  Early versions of this collector were developed as a part of research
projects supported in part by the National Science Foundation
and the Defense Advance Research Projects Agency.
Much of the code was rewritten by Hans-J. Boehm at Xerox PARC.
The SPARC specific code was contributed by Mark Weiser
([e-mail]).  The Encore Multimax modifications were supplied by
Kevin Kenny ([e-mail]).  The adaptation to the RT is largely due
to Vernon Lee ([e-mail]), on machines made available by IBM.
Much of the HP specific code and a number of good suggestions for improving the
generic code are due to Walter Underwood ([e-mail]).
Robert Brazile ([e-mail]) originally supplied the ULTRIX code.
Al Dosser ([e-mail]) and Regis Cridlig ([e-mail])
subsequently provided updates and information on variation between ULTRIX
systems.  Parag Patel ([e-mail]) supplied the A/UX code.
Jesper Peterson([e-mail]) and
Michel Schinz supplied the Amiga port.
Thomas Funke ([e-mail](?)) and
Brian D.Carlstrom ([e-mail]) supplied the NeXT ports.
Douglas Steel ([e-mail]) provided ICL DRS6000 code.
Bill Janssen ([e-mail]) supplied the SunOS dynamic loader
specific code. Manuel Serrano ([e-mail]) supplied linux and
Sony News specific code.  Al Dosser provided Alpha/OSF/1 code.  He and
Dave Detlefs([e-mail]) also provided several generic bug fixes.
Alistair G. Crooks([e-mail]) supplied the NetBSD and 386BSD ports.
Jeffrey Hsu ([e-mail]) provided the FreeBSD port.
Brent Benson ([e-mail]) ported the collector to
a Motorola 88K processor running CX/UX (Harris NightHawk).
Ari Huttunen ([e-mail]) generalized the OS/2 port to
nonIBM development environments (a nontrivial task).
Patrick Beard ([e-mail]) provided the initial MacOS port.
David Chase, then at Olivetti Research, suggested several improvements.
Scott Schwartz ([e-mail]) supplied some of the
code to save and print call stacks for leak detection on a SPARC.
Jesse Hull and John Ellis supplied the C++ interface code.
Zhong Shao performed much of the experimentation that led to the
current typed allocation facility.  (His dynamic type inference code hasn't
made it into the released version of the collector, yet.)
(Blame for misinstallation of these modifications goes to the first author,
however.)

    This is intended to be a general purpose, garbage collecting storage
allocator.  The algorithms used are described in:

Boehm, H., and M. Weiser, "Garbage Collection in an Uncooperative Environment",
Software Practice & Experience, September 1988, pp. 807-820.

Boehm, H., A. Demers, and S. Shenker, "Mostly Parallel Garbage Collection",
Proceedings of the ACM SIGPLAN '91 Conference on Programming Language Design
and Implementation, SIGPLAN Notices 26, 6 (June 1991), pp. 157-164.

Boehm, H., "Space Efficient Conservative Garbage Collection", Proceedings
of the ACM SIGPLAN '91 Conference on Programming Language Design and
Implementation, SIGPLAN Notices 28, 6 (June 1993), pp. 197-206.

  Possible interactions between the collector and optimizing compilers are
discussed in

Boehm, H., and D. Chase, "A Proposal for GC-safe C Compilation",
The Journal of C Language Translation 4, 2 (December 1992).
(Also available from parcftp.xerox.com:pub/gc, among other places.)

  Unlike the collector described in the second reference, this collector
operates either with the mutator stopped during the entire collection
(default) or incrementally during allocations.  (The latter is supported
on only a few machines.)  It does not rely on threads, but is intended
to be thread-safe.

  Some of the ideas underlying the collector have previously been explored
by others.  (Doug McIlroy wrote a vaguely similar collector that is part of
version 8 UNIX (tm).)  However none of this work appears to have been widely
disseminated.

  Rudimentary tools for use of the collector as a leak detector are included, as
is a fairly sophisticated string package "cord" that makes use of the collector.
(See cord/README.)


GENERAL DESCRIPTION

  This is a garbage colecting storage allocator that is intended to be
used as a plug-in replacement for C's malloc.

  Since the collector does not require pointers to be tagged, it does not
attempt to ensure that all inaccessible storage is reclaimed.  However,
in our experience, it is typically more successful at reclaiming unused
memory than most C programs using explicit deallocation.  Unlike manually
introduced leaks, the amount of unreclaimed memory typically stays
bounded.

  In the following, an "object" is defined to be a region of memory allocated
by the routines described below.

  Any objects not intended to be collected must be pointed to either
from other such accessible objects, or from the registers,
stack, data, or statically allocated bss segments.  Pointers from
the stack or registers may point to anywhere inside an object.
However, it is usually assumed that all pointers originating in the
heap point to the beginning of an object.  (This does
not disallow interior pointers; it simply requires that there must be a
pointer to the beginning of every accessible object, in addition to any
interior pointers.)  There are two facilities for altering this behavior.
The macro ALL_INTERIOR_POINTERS may be defined in gc_private.h to
cause any pointer into an object (or one past the end) to retain the
object.  A routine GC_register_displacement is provided to allow for
more controlled interior pointer use in the heap.  Defining
ALL_INTERIOR_POINTERS is somewhat dangerous, in that it can result
in unnecessary memroy retention.  However this is much less of a
problem than with older collector versions.  The routine
GC_register_displacement is described in gc.h.

  Note that pointers inside memory allocated by the standard "malloc" are not
seen by the garbage collector.  Thus objects pointed to only from such a
region may be prematurely deallocated.  It is thus suggested that the
standard "malloc" be used only for memory regions, such as I/O buffers, that
are guaranteed not to contain pointers.  Pointers in C language automatic,
static, or register variables, are correctly recognized.  (Note that
GC_malloc_uncollectable has semantics similar to standard malloc,
but allocates objects that are traced by the collector.)

  The collector does not generally know how to find pointers in data
areas that are associated with dynamic libraries.  This is easy to
remedy IF you know how to find those data areas on your operating
system (see GC_add_roots).  Code for doing this under SunOS and IRIX 5.X is
included (see dynamic_load.c).

  Note that the garbage collector does not need to be informed of shared
read-only data.  However if the shared library mechanism can introduce
discontiguous data areas that may contain pointers, then the collector does
need to be informed.

  Signal processing for most signals is normally deferred during collection,
and during uninterruptible parts of the allocation process.  Unlike
standard ANSI C mallocs, it is intended to be safe to invoke malloc
from a signal handler while another malloc is in progress, provided
the original malloc is not restarted.  (Empirically, many UNIX
applications already asssume this.)  Even this modest level of signal-
safety may be too expensive on some systems.  If so, ENABLE_SIGNALS
and DISABLE_SIGNALS may be redefined to the empty statement in gc_private.h.

  The allocator/collector can also be configured for thread-safe operation.
(Full signal safety can also be acheived, but only at the cost of two system
calls per malloc, which is usually unacceptable.)

INSTALLATION AND PORTABILITY

  As distributed, the macro SILENT is defined in Makefile.
In the event of problems, this can be removed to obtain a moderate
amount of descriptive output for each collection.
(The given statistics exhibit a few peculiarities.
Things don't appear to add up for a variety of reasons, most notably
fragmentation losses.  These are probably much more significant for the
contrived program "test.c" than for your application.)

  Note that typing "make test" will automatically build the collector
and then run setjmp_test and gctest. Setjmp_test will give you information
about configuring the collector, which is useful primarily if you have
a machine that's not already supported.  Gctest is a somewhat superficial
test of collector functionality.  Failure is indicated by a core dump or
a message to the effect that the collector is broken.  Gctest takes about
35 seconds to run on a SPARCstation 2. On a slower machine,
expect it to take a while.  It may use up to 8 MB of memory.  (The
multi-threaded version will use more.)  "Make test" will also, as
its last step, attempt to build and test the "cord" string library.
This will fail without an ANSI C compiler.

  The Makefile will generate a library gc.a which you should link against.
Typing "make cords" will add the cord library to gc.a.
Note that this requires an ANSI C compiler.

  It is suggested that if you need to replace a piece of the collector
(e.g. GC_mark_rts.c) you simply list your version ahead of gc.a on the
ld command line, rather than replacing the one in gc.a.  (This will
generate numerous warnings under some versions of AIX, but it still
works.)

  All include files that need to be used by clients will be put in the
include subdirectory.  (Normally this is just gc.h.  "Make cords" adds
"cord.h" and "ec.h".)

  The collector currently is designed to run essentially unmodified on
the following machines (most of the operating systems mentioned are
trademarks of their respective holders):

        Sun 3
        Sun 4 under SunOS 4.X or Solaris2.X (with or without threads)
        Vax under 4.3BSD, Ultrix
        Intel 386 or 486 under most operating systems, but not MSDOS.
            (Win32S is somewhat supported, so it is possible to
            build applications for Windows 3.1.  There exists a port
            to DOS + 32 bit extender for at least one 32 bit extender.
            However, I don't have source for this.)
        Sequent Symmetry  (single threaded)
        Encore Multimax   (single threaded)
        MIPS M/120 (and presumably M/2000) (RISC/os 4.0 with BSD libraries)
        IBM PC/RT  (Berkeley UNIX)
        IBM RS/6000
        HP9000/300
        HP9000/700
        DECstations under Ultrix
        DEC Alpha running OSF/1
        SGI workstations under IRIX 4 & 5
        Sony News
        Apple MacIntosh under A/UX or MacOS
        Commodore Amiga (see README.amiga)
        NeXT machines

  In a few cases (Amiga, OS/2, Win32, MacOS) a separate makefile
or equivalent is supplied.

  Dynamic libraries are completely supported only under SunOS
(and even that support is not functional on the last Sun 3 release),
IRIX 5, Win32 (not Win32S) and OSF/1 on DEC AXP machines.
On other machines we recommend that you do one of the following:

  1) Add dynamic library support (and send us the code).
  2) Use static versions of the libraries.
  3) Arrange for dynamic libraries to use the standard malloc.
     This is still dangerous if the library stores a pointer to a
     garbage collected object.  But nearly all standard interfaces
     prohibit this, because they deal correctly with pointers
     to stack allocated objects.  (Strtok is an exception.  Don't
     use it.)

  In all cases we assume that pointer alignment is consistent with that
enforced by the standard C compilers.  If you use a nonstandard compiler
you may have to adjust the alignment parameters defined in gc_private.h.

  A port to a machine that is not byte addressed, or does not use 32 bit
or 64 bit addresses will require a major effort.  A port to MSDOS is hard,
unless you are willing to assume an 80386 or better, and that only flat
32 bit pointers will ever need to be seen by the collector.

  For machines not already mentioned, or for nonstandard compilers, the
following are likely to require change:

1.  The parameters in config.h.
      The parameters that will usually require adjustment are
   STACKBOTTOM,  ALIGNMENT and DATASTART.  Setjmp_test
   prints its guesses of the first two.
      DATASTART should be an expression for computing the
   address of the beginning of the data segment.  This can often be
   &etext.  But some memory management units require that there be
   some unmapped space between the text and the data segment.  Thus
   it may be more complicated.   On UNIX systems, this is rarely
   documented.  But the adb "$m" command may be helpful.  (Note
   that DATASTART will usually be a function of &etext.  Thus a
   single experiment is usually insufficient.)
     STACKBOTTOM is used to initialize GC_stackbottom, which
   should be a sufficient approximation to the coldest stack address.
   On some machines, it is difficult to obtain such a value that is
   valid across a variety of MMUs, OS releases, etc.  A number of
   alternatives exist for using the collector in spite of this.  See the
   discussion in config.h.h immediately preceding the various
   definitions of STACKBOTTOM.

2.  mach_dep.c.
      The most important routine here is one to mark from registers.
    The distributed file includes a generic hack (based on setjmp) that
    happens to work on many machines, and may work on yours.  Try
    compiling and running setjmp_test.c to see whether it has a chance of
    working.  (This is not correct C, so don't blame your compiler if it
    doesn't work.  Based on limited experience, register window machines
    are likely to cause trouble.  If your version of setjmp claims that
    all accessible variables, including registers, have the value they
    had at the time of the longjmp, it also will not work.  Vanilla 4.2 BSD
    makes such a claim.  SunOS does not.)
      If your compiler does not allow in-line assembly code, or if you prefer
    not to use such a facility, mach_dep.c may be replaced by a .s file
    (as we did for the MIPS machine and the PC/RT).
      At this point enough architectures are supported by mach_dep.c
    that you will rarely need to do more than adjust for assembler
    syntax.

3.  os_dep.c (and gc_priv.h).
        Several kinds of operating system dependent routines reside here.
      Many are optional.  Several are invoked only through corresponding
      macros in gc_priv.h, which may also be redefined as appropriate.
      The routine GC_register_data_segments is crucial.  It registers static
    data areas that must be traversed by the collector. (User calls to
    GC_add_roots may sometimes be used for similar effect.)
      Routines to obtain memory from the OS also reside here.
    Alternatively this can be done entirely by the macro GET_MEM
    defined in gc_priv.h.  Routines to disable and reenable signals
    also reside here if they are need by the macros DISABLE_SIGNALS
    and ENABLE_SIGNALS defined in gc_priv.h.
      In a multithreaded environment, the macros LOCK and UNLOCK
    in gc_priv.h will need to be suitably redefined.
      The incremental collector requires page dirty information, which
    is acquired through routines defined in os_dep.c.  Unless directed
    otherwise by config.h, these are implemented as stubs that simply
    treat all pages as dirty.  (This of course makes the incremental
    collector much less useful.)

4.  dyn_load.c
    This provides a routine that allows the collector to scan data
    segments associated with dynamic libraries.  Often it is not
    necessary to provide this routine unless user-written dynamic
    libraries are used.

  For a different version of UN*X or different machines using the
Motorola 68000, Vax, SPARC, 80386, NS 32000, PC/RT, or MIPS architecture,
it should frequently suffice to change definitions in config.h.


THE C INTERFACE TO THE ALLOCATOR

  The following routines are intended to be directly called by the user.
Note that usually only GC_malloc is necessary.  GC_clear_roots and GC_add_roots
calls may be required if the collector has to trace from nonstandard places
(e.g. from dynamic library data areas on a machine on which the
collector doesn't already understand them.)  On some machines, it may
be desirable to set GC_stacktop to a good approximation of the stack base.
(This enhances code portability on HP PA machines, since there is no
good way for the collector to compute this value.)  Client code may include
"gc.h", which defines all of the following, plus a few others.

1)  GC_malloc(nbytes)
    - allocate an object of size nbytes.  Unlike malloc, the object is
      cleared before being returned to the user.  Gc_malloc will
      invoke the garbage collector when it determines this to be appropriate.
      GC_malloc may return 0 if it is unable to acquire sufficient
      space from the operating system.  This is the most probable
      consequence of running out of space.  Other possible consequences
      are that a function call will fail due to lack of stack space,
      or that the collector will fail in other ways because it cannot
      maintain its internal data structures, or that a crucial system
      process will fail and take down the machine.  Most of these
      possibilities are independent of the malloc implementation.

2)  GC_malloc_atomic(nbytes)
    - allocate an object of size nbytes that is guaranteed not to contain any
      pointers.  The returned object is not guaranteed to be cleeared.
      (Can always be replaced by GC_malloc, but results in faster collection
      times.  The collector will probably run faster if large character
      arrays, etc. are allocated with GC_malloc_atomic than if they are
      statically allocated.)

3)  GC_realloc(object, new_size)
    - change the size of object to be new_size.  Returns a pointer to the
      new object, which may, or may not, be the same as the pointer to
      the old object.  The new object is taken to be atomic iff the old one
      was.  If the new object is composite and larger than the original object,
      then the newly added bytes are cleared (we hope).  This is very likely
      to allocate a new object, unless MERGE_SIZES is defined in gc_priv.h.
      Even then, it is likely to recycle the old object only if the object
      is grown in small additive increments (which, we claim, is generally bad
      coding practice.)

4)  GC_free(object)
    - explicitly deallocate an object returned by GC_malloc or
      GC_malloc_atomic.  Not necessary, but can be used to minimize
      collections if performance is critical.  Probably a performance
      loss for very small objects (<= 8 bytes).

5)  GC_expand_hp(bytes)
    - Explicitly increase the heap size.  (This is normally done automatically
      if a garbage collection failed to GC_reclaim enough memory.  Explicit
      calls to GC_expand_hp may prevent unnecessarily frequent collections at
      program startup.)

6)  GC_clear_roots()
    - Reset the collectors idea of where static variables containing pointers
      may be located to the empty set of locations.  No statically allocated
      variables will be traced from after this call, unless there are
      intervening GC_add_roots calls.  The collector will still trace from
      registers and the program stack.

7)  GC_add_roots(low_address, high_address_plus_1)
    - Add [low_address, high_address) as an area that may contain root pointers
      and should be traced by the collector.  The static data and bss segments
      are considered by default, and should not be added unless GC_clear_roots
      has been called.  The number of root areas is currently limited to 50.
      This is intended as a way to register data areas for dynamic libraries,
      or to replace the entire data ans bss segments by smaller areas that are
      known to contain all the roots.

8) GC_enable_incremental()
    - Enables generational and incremental collection.  Useful for large
      heaps on machines that provide access to page dirty information.
      Some dirty bit implementations may interfere with debugging
      (by catching address faults) and place restrictions on heap arguments
      to system calls (since write faults inside a system call may not be
      handled well).

9) Several routines to allow for registration of finalization code.
   User supplied finalization code may be invoked when an object becomes
   unreachable.  To call (*f)(obj, x) when obj becomes inaccessible, use
    GC_register_finalizer(obj, f, x, 0, 0);
   For more sophisticated uses, and for finalization ordering issues,
   see gc.h.

  The global variable GC_free_space_divisor may be adjusted up from its
default value of 4 to use less space and more collection time, or down for
the opposite effect.  Setting it to 1 or 0 will effectively disable collections
and cause all allocations to simply grow the heap.

  The variable GC_non_gc_bytes, which is normally 0, may be changed to reflect
the amount of memory allocated by the above routines that should not be
considered as a candidate for collection.  Careless use may, of course, result
in excessive memory consumption.

  Some additional tuning is possible through the parameters defined
near the top of gc_private.h.

  If only GC_malloc is intended to be used, it might be appropriate to define:

#define malloc(n) GC_malloc(n)
#define calloc(m,n) GC_malloc((m)*(n))

  For small pieces of VERY allocation intensive code, gc_inl.h
includes some allocation macros that may be used in place of GC_malloc
and friends.

  All externally visible names in the garbage collector start with "GC_".
To avoid name conflicts, client code should avoid this prefix, except when
accessing garbage collector routines or variables.

  There are provisions for allocation with explicit type information.
This is rarely necessary.  Details can be found in gc_typed.h.

THE C++ INTERFACE TO THE ALLOCATOR:

  The Ellis-Hull C++ interface to the collector is included in
the collector distribution.  If you intend to use this, type
"make c++" after the initial build of the collector is complete.
See gc_c++.h for the difinition of the interface.  This interface
tries to approximate the Ellis-Detlefs C++ garbage collection
proposal without compiler changes.

Cautions:
1. Arrays allocated without new placement syntax are
allocated as uncollectable objects.  They are traced by the
collector, but will not be reclaimed.

2. Failure to use "make c++" in combination with (1) will
result in arrays allocated using the default new operator.
This is likely to result in disaster without linker warnings.

3. If your compiler supports an overloaded new[] operator,
then gc_c++.cc and gc_c++.h should be suitably modified.


USE AS LEAK DETECTOR:

  The collector may be used to track down leaks in C programs that are
intended to run with malloc/free (e.g. code with extreme real-time or
portability constraints).  To do so define FIND_LEAK somewhere in
gc_priv.h.  This will cause the collector to invoke the report_leak
routine defined near the top of reclaim.c whenever an inaccessible
object is found that has not been explicitly freed.
  Productive use of this facility normally involves redefining report_leak
to do something more intelligent.  This typically requires annotating
objects with additional information (e.g. creation time stack trace) that
identifies their origin.  Such code is typically not very portable, and is
not included here, except on SPARC machines.
  If all objects are allocated with GC_DEBUG_MALLOC (see next section),
then the default version of report_leak will report the source file
and line number at which the leaked object was allocated.  This may
sometimes be sufficient.  (On SPARC/SUNOS4 machines, it will also report
a cryptic stack trace.  This can often be turned into a sympolic stack
trace by invoking program "foo" with "callprocs foo".  Callprocs is
a short shell script that invokes adb to expand program counter values
to symbolic addresses.  It was largely supplied by Scott Schwartz.)
  Note that the debugging facilities described in the next section can
sometimes be slightly LESS effective in leak finding mode, since in
leak finding mode, GC_debug_free actually results in reuse of the object.
(Otherwise the object is simply marked invalid.)

DEBUGGING FACILITIES:

  The routines GC_debug_malloc, GC_debug_malloc_atomic, GC_debug_realloc,
and GC_debug_free provide an alternate interface to the collector, which
provides some help with memory overwrite errors, and the like.
Objects allocated in this way are annotated with additional
information.  Some of this information is checked during garbage
collections, and detected inconsistencies are reported to stderr.

  Simple cases of writing past the end of an allocated object should
be caught if the object is explicitly deallocated, or if the
collector is invoked while the object is live.  The first deallocation
of an object will clear the debugging info associated with an
object, so accidentally repeated calls to GC_debug_free will report the
deallocation of an object without debugging information.  Out of
memory errors will be reported to stderr, in addition to returning
NIL.

  GC_debug_malloc checking  during garbage collection is enabled
with the first call to GC_debug_malloc.  This will result in some
slowdown during collections.  If frequent heap checks are desired,
this can be acheived by explicitly invoking GC_gcollect, e.g. from
the debugger.

  GC_debug_malloc allocated objects should not be passed to GC_realloc
or GC_free, and conversely.  It is however acceptable to allocate only
some objects with GC_debug_malloc, and to use GC_malloc for other objects,
provided the two pools are kept distinct.  In this case, there is a very
low probablility that GC_malloc allocated objects may be misidentified as
having been overwritten.  This should happen with probability at most
one in 2**32.  This probability is zero if GC_debug_malloc is never called.

  GC_debug_malloc, GC_malloc_atomic, and GC_debug_realloc take two
additional trailing arguments, a string and an integer.  These are not
interpreted by the allocator.  They are stored in the object (the string is
not copied).  If an error involving the object is detected, they are printed.

  The macros GC_MALLOC, GC_MALLOC_ATOMIC, GC_REALLOC, GC_FREE, and
GC_REGISTER_FINALIZER are also provided.  These require the same arguments
as the corresponding (nondebugging) routines.  If gc.h is included
with GC_DEBUG defined, they call the debugging versions of these
functions, passing the current file name and line number as the two
extra arguments, where appropriate.  If gc.h is included without GC_DEBUG
defined, then all these macros will instead be defined to their nondebugging
equivalents.  (GC_REGISTER_FINALIZER is necessary, since pointers to
objects with debugging information are really pointers to a displacement
of 16 bytes form the object beginning, and some translation is necessary
when finalization routines are invoked.  For details, about what's stored
in the header, see the definition of the type oh in debug_malloc.c)

INCREMENTAL/GENERATIONAL COLLECTION:

The collector normally interrupts client code for the duration of
a garbage collection mark phase.  This may be unacceptable if interactive
response is needed for programs with large heaps.  The collector
can also run in a "generational" mode, in which it usually attempts to
collect only objects allocated since the last garbage collection.
Furthermore, in this mode, garbage collections run mostly incrementally,
with a small amount of work performed in response to each of a large number of
GC_malloc requests.

This mode is enabled by a call to GC_enable_incremental().

Incremental and generational collection is effective in reducing
pause times only if the collector has some way to tell which objects
or pages have been recently modified.  The collector uses two sources
of information:

1. Information provided by the VM system.  This may be provided in
one of several forms.  Under Solaris 2.X (and potentially under other
similar systems) information on dirty pages can be read from the
/proc file system.  Under other systems (currently SunOS4.X) it is
possible to write-protect the heap, and catch the resulting faults.
On these systems we require that system calls writing to the heap
(other than read) be handled specially by client code.
See os_dep.c for details.

2. Information supplied by the programmer.  We define "stubborn"
objects to be objects that are rarely changed.  Such an object
can be allocated (and enabled for writing) with GC_malloc_stubborn.
Once it has been initialized, the collector should be informed with
a call to GC_end_stubborn_change.  Subsequent writes that store
pointers into the object must be preceded by a call to
GC_change_stubborn.

This mechanism performs best for objects that are written only for
initialization, and such that only one stubborn object is writable
at once.  It is typically not worth using for short-lived
objects.  Stubborn objects are treated less efficiently than pointerfree
(atomic) objects.

A rough rule of thumb is that, in the absence of VM information, garbage
collection pauses are proportional to the amount of pointerful storage
plus the amount of modified "stubborn" storage that is reachable during
the collection.

Initial allocation of stubborn objects takes longer than allocation
of other objects, since other data structures need to be maintained.

We recommend against random use of stubborn objects in client
code, since bugs caused by inappropriate writes to stubborn objects
are likely to be very infrequently observed and hard to trace.
However, their use may be appropriate in a few carefully written
library routines that do not make the objects themselves available
for writing by client code.


BUGS:

  Any memory that does not have a recognizable pointer to it will be
reclaimed.  Exclusive-or'ing forward and backward links in a list
doesn't cut it.
  Some C optimizers may lose the last undisguised pointer to a memory
object as a consequence of clever optimizations.  This has almost
never been observed in practice.  Send mail to [e-mail]
for suggestions on how to fix your compiler.
  This is not a real-time collector.  In the standard configuration,
percentage of time required for collection should be constant across
heap sizes.  But collection pauses will increase for larger heaps.
(On SPARCstation 2s collection times will be on the order of 300 msecs
per MB of accessible memory that needs to be scanned.  Your mileage
may vary.)  The incremental/generational collection facility helps,
but is portable only if "stubborn" allocation is used.
  Please address bug reports to [e-mail].  If you are
contemplating a major addition, you might also send mail to ask whether
it's already been done (or whether we tried and discarded it).

RECENT VERSIONS:

  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 excercised 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 Detelfs 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
  ([e-mail]).
- 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_private.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.
- 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
  [e-mail].)
- 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 ([e-mail])
- 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
  ([e-mail])).
- 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 [e-mail].
- 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 ([e-mail])
- 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
  <[e-mail]>.)
- John Ellis' additions to the C++ support:  From John:

* I completely rewrote the documentation in the interface gc_c++.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++ 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 Windoes NT Makefile to include C++ support in
  gc.lib.  Added C++ test as Makefile target.