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+Copyright 1988, 1989 Hans-J. Boehm, Alan J. Demers
+Copyright (c) 1991-1996 by Xerox Corporation.  All rights reserved.
+Copyright (c) 1996-1998 by Silicon Graphics.  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 4.13alpha2 of a conservative garbage collector for C and C++.
+
+You might find a more recent version of this at
+
+http://reality.sgi.com/boehm/gc.html
+
+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
+(weiser@parc.xerox.com).  The Encore Multimax modifications were supplied by
+Kevin Kenny (kenny@m.cs.uiuc.edu).  The adaptation to the RT is largely due
+to Vernon Lee (scorpion@rice.edu), 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 (wunder@hp-ses.sde.hp.com).
+Robert Brazile (brazile@diamond.bbn.com) originally supplied the ULTRIX code.
+Al Dosser (dosser@src.dec.com) and Regis Cridlig (Regis.Cridlig@cl.cam.ac.uk)
+subsequently provided updates and information on variation between ULTRIX
+systems.  Parag Patel (parag@netcom.com) supplied the A/UX code.
+Jesper Peterson(jep@mtiame.mtia.oz.au) and
+Michel Schinz supplied the Amiga port.
+Thomas Funke (thf@zelator.in-berlin.de(?)) and
+Brian D.Carlstrom (bdc@clark.lcs.mit.edu) supplied the NeXT ports.
+Douglas Steel (doug@wg.icl.co.uk) provided ICL DRS6000 code.
+Bill Janssen (janssen@parc.xerox.com) supplied the SunOS dynamic loader
+specific code. Manuel Serrano (serrano@cornas.inria.fr) supplied linux and
+Sony News specific code.  Al Dosser provided Alpha/OSF/1 code.  He and
+Dave Detlefs(detlefs@src.dec.com) also provided several generic bug fixes.
+Alistair G. Crooks(agc@uts.amdahl.com) supplied the NetBSD and 386BSD ports.
+Jeffrey Hsu (hsu@soda.berkeley.edu) provided the FreeBSD port.
+Brent Benson (brent@jade.ssd.csd.harris.com) ported the collector to
+a Motorola 88K processor running CX/UX (Harris NightHawk).
+Ari Huttunen (Ari.Huttunen@hut.fi) generalized the OS/2 port to
+nonIBM development environments (a nontrivial task).
+Patrick Beard (beard@cs.ucdavis.edu) provided the initial MacOS port.
+David Chase, then at Olivetti Research, suggested several improvements.
+Scott Schwartz (schwartz@groucho.cse.psu.edu) 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.)
+
+Credits for some more recent modifications are given in the modification
+history at the end of this file.
+
+    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).
+
+and
+
+Boehm H., "Simple GC-safe Compilation", Proceedings
+of the ACM SIGPLAN '96 Conference on Programming Language Design and
+Implementation.
+
+(Both are also available from
+http://reality.sgi.com/employees/boehm_mti/papers/, 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 collecting 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.
+The same is true for heap pointers if the collector is compiled with
+ ALL_INTERIOR_POINTERS defined, as is now the default.
+
+Compiling without ALL_INTERIOR_POINTERS may reduce accidental retention
+of garbage objects, by requiring pointers from the heap to to the beginning
+of an object.  But this no longer appears to be a significant
+issue for most programs.
+
+There are a number of routines which modify the pointer recognition
+algorithm.  GC_register_displacement allows certain interior pointers
+to be recognized even if ALL_INTERIOR_POINTERS is nor defined.
+GC_malloc_ignore_off_page allows some pointers into the middle of large objects
+to be disregarded, greatly reducing the probablility of accidental
+retention of large objects.  For most purposes it seems best to compile
+with ALL_INTERIOR_POINTERS and to use GC_malloc_ignore_off_page if
+you get collector warnings from allocations of very large objects.
+See README.debugging for details.
+
+  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 to garbage collectable memory.
+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 always 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, IRIX 5.X and 6.X,
+HP/UX, Alpha OSF/1, Linux, and win32 is included and used by default.  (See
+README.win32 for win32 details.)  On other systems pointers from dynamic
+library data areas may not be considered by the collector.
+
+  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 may be deferred during collection,
+and during uninterruptible parts of the allocation process.  Unlike
+standard ANSI C mallocs, it can 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 assume this.)  To obtain this level  of signal
+safety, remove the definition of -DNO_SIGNALS in Makefile.  This incurs
+a minor performance penalty, and hence is no longer the default.
+
+  The allocator/collector can also be configured for thread-safe operation.
+(Full signal safety can also be achieved, 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
+		work.)
+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
+machines that use a flat 32-bit or 64-bit address space.
+That includes the vast majority of Workstations and X86 (X >= 3) PCs.
+(The list here was deleted because it was getting too long and constantly
+out of date.)
+  It does NOT run under plain 16-bit DOS or Windows 3.X.  There are however
+various packages (e.g. win32s, djgpp) that allow flat 32-bit address
+applications to run under those systemsif the have at least an 80386 processor,
+and several of those are compatible with the collector.
+
+  In a few cases (Amiga, OS/2, Win32, MacOS) a separate makefile
+or equivalent is supplied.  Many of these have separate README.system
+files.
+
+  Dynamic libraries are completely supported only under SunOS
+(and even that support is not functional on the last Sun 3 release),
+IRIX 5&6, HP-PA, 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_priv.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 plain MSDOS
+or win16 is hard.
+
+  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 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_t.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
+    on Vaxen 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 many 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 cleared.
+      (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_malloc_ignore_off_page(bytes)
+	- identical to GC_malloc, but the client promises to keep a pointer to
+	  the somewhere within the first 256 bytes of the object while it is
+	  live.  (This pointer should nortmally be declared volatile to prevent
+	  interference from compiler optimizations.)  This is the recommended
+	  way to allocate anything that is likely to be larger than 100Kbytes
+	  or so.  (GC_malloc may result in failure to reclaim such objects.)
+
+7)  GC_set_warn_proc(proc)
+	- Can be used to redirect warnings from the collector.  Such warnings
+	  should be rare, and should not be ignored during code development.
+      
+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_priv.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_cpp.h for the definition 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_cpp.cc and gc_cpp.h should be suitably modified.
+
+4. Many current C++ compilers have deficiencies that
+break some of the functionality.  See the comments in gc_cpp.h
+for suggested workarounds.
+
+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 in Makefile
+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.  The collector will
+no longer reclaim inaccessible memory; in this form it is purely a
+debugging tool.
+  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.)  Also note that the test
+program is not designed to run meaningfully in FIND_LEAK mode.
+Use "make gc.a" to build the collector.
+
+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 achieved 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 boehm@mti.sgi.com
+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 boehm@mti.sgi.com.  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 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.
+
+To do:
+ - I have a backlog of unintegrated contributed platform-specific changes.
+ - 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.
+ - Integrate MIT and DEC pthreads ports.
+