The collector uses a large amount of conditional compilation in order to
deal with platform dependencies.  This violates a number of known coding
standards.  On the other hand, it seems to be the only practical way to
support this many platforms without excessive code duplication.

A few guidelines have mostly been followed in order to keep this manageable:

1) #if and #ifdef directives are properly indented whenever easily possible.
All known C compilers allow whitespace between the "#" and the "if" to make
this possible.  ANSI C also allows white space before the "#", though we
avoid that.  It has the known disadvantages that it differs from the normal
GNU conventions, and that it makes patches larger than otherwise necessary.
In my opinion, it's still well worth it, for the same reason that we indent
ordinary "if" statements.

2) Whenever possible, tests are performed on the macros defined in gcconfig.h
instead of directly testing platform-specific predefined macros.  This makes
it relatively easy to adapt to new compilers with a different set of
predefined macros.  Currently these macros generally identify platforms
instead of features.  In many cases, this is a mistake.

Many of the tested configuration macros are at least somewhat defined in
either include/private/gcconfig.h or in Makefile.direct.  Here is an attempt
at documenting these macros:  (Thanks to Walter Bright for suggesting
this.  This is a work in progress)

MACRO           EXPLANATION
-----           -----------

GC_DEBUG        Tested by gc.h.  Causes all-upper-case macros to
                expand to calls to debug versions of collector routines.

GC_NO_THREAD_REDIRECTS  Tested by gc.h.  Prevents redirection of thread
                creation routines etc. to GC_ versions.  Requires the
                programmer to explicitly handle thread registration.

GC_NO_THREAD_DECLS      Tested by gc.h.  MS Windows only.  Do not declare
                Windows thread creation routines and do not include windows.h.

GC_UNDERSCORE_STDCALL   Tested by gc.h.  Explicitly prefix exported/imported
                WINAPI (__stdcall) symbols with '_' (underscore).  Could be
                used with MinGW (for x86) compiler (in conjunction with
                GC_DLL) to follow MS conventions for __stdcall symbols naming.

_ENABLE_ARRAYNEW
                #define'd by the Digital Mars C++ compiler when
                operator new[] and delete[] are separately
                overloadable. Used in gc_cpp.h.

_DLL            Tested by gc_config_macros.h. Defined by Visual C++ if runtime
                dynamic libraries are in use.  Used (only if none of GC_DLL,
                GC_NOT_DLL, __GNUC__ are defined) to test whether
                __declspec(dllimport) needs to be added to declarations
                to support the case in which the collector is in a DLL.

GC_DLL          Defined by user if dynamic libraries are being built
                or used.  Also set by gc.h if _DLL is defined (except for
                mingw) while GC_NOT_DLL and __GNUC__ are both undefined.
                This is the macro that is tested internally to determine
                whether the GC is in its own dynamic library.  May need
                to be set by clients before including gc.h.  Note that
                inside the GC implementation it indicates that the
                collector is in its own dynamic library, should export
                its symbols, etc.  But in clients it indicates that the
                GC resides in a different DLL, its entry points should
                be referenced accordingly, and precautions may need to
                be taken to properly deal with statically allocated
                variables in the main program.  Used for MS Windows.
                Also used by GCC v4+ (only when the dynamic shared library
                is being built) to hide internally used symbols.

GC_NOT_DLL      User-settable macro that overrides _DLL, e.g. if runtime
                dynamic libraries are used, but the collector is in a static
                library.  Tested by gc_config_macros.h.


These define arguments influence the collector configuration:
FIND_LEAK causes GC_find_leak to be initially set.
  This causes the collector to assume that all inaccessible
  objects should have been explicitly deallocated, and reports exceptions.
  Finalization and the test program are not usable in this mode.

SUNOS5SIGS      Solaris-like signal handling.  This is probably misnamed,
                since it really doesn't guarantee much more than Posix.
                Currently set only for Solaris2.X, HPUX, and DRSNX.  Should
                probably be set for some other platforms.

PCR             Set if the collector is being built as part of the Xerox
                Portable Common Runtime.

USE_COMPILER_TLS  Assume the existence of __thread-style thread-local
                storage.  Set automatically for thread-local allocation with
                the HP/UX vendor compiler.  Usable with gcc on sufficiently
                up-to-date ELF platforms.

IMPORTANT: Any of the _THREADS options must normally also be defined in
  the client before including gc.h.  This redefines thread primitives to
  invoke the GC_ versions instead.  Alternatively, linker-based symbol
  interception can be used on a few platforms.

GC_THREADS      Should set the appropriate one of the below macros,
  except GC_WIN32_PTHREADS, which must be set explicitly.  Tested by gc.h.

GC_SOLARIS_THREADS      Enables support for Solaris pthreads.
  Must also define _REENTRANT.

GC_IRIX_THREADS         Enables support for Irix pthreads.  See README.sgi.

GC_HPUX_THREADS         Enables support for HP/UX 11 pthreads.
  Also requires _REENTRANT or _POSIX_C_SOURCE=199506L. See README.hp.

GC_LINUX_THREADS        Enables support for Xavier Leroy's Linux threads
  or NPTL threads. See README.linux.  _REENTRANT may also be required.

GC_OSF1_THREADS         Enables support for Tru64 pthreads.

GC_FREEBSD_THREADS      Enables support for FreeBSD pthreads.
  Appeared to run into some underlying thread problems.

GC_NETBSD_THREADS       Enables support for NetBSD pthreads.

GC_OPENBSD_THREADS      Enables support for OpenBSD pthreads.

GC_DARWIN_THREADS       Enables support for Mac OS X pthreads.

GC_AIX_THREADS  Enables support for IBM AIX threads.

GC_DGUX386_THREADS      Enables support for DB/UX on I386 threads.
  See README.DGUX386.  (Probably has not been tested recently.)

GC_WIN32_THREADS        Enables support for win32 threads.  That makes sense
  for this Makefile only under Cygwin.

GC_WIN32_PTHREADS       Enables support for Ming32 pthreads.  This cannot be
  enabled automatically by GC_THREADS, which would assume Win32 native
  threads.

PTW32_STATIC_LIB        Causes the static version of the Mingw pthreads
  library to be used.  Requires GC_WIN32_PTHREADS.

GC_PTHREADS_PARAMARK    Causes pthread-based parallel mark implementation
  to be used even if GC_WIN32_PTHREADS is undefined.  (Useful for WinCE.)

ALL_INTERIOR_POINTERS   Allows all pointers to the interior of objects to be
  recognized.  (See gc_priv.h for consequences.)  Alternatively,
  GC_all_interior_pointers can be set at process initialization time.

SMALL_CONFIG    Tries to tune the collector for small heap sizes,
  usually causing it to use less space in such situations.
  Incremental collection no longer works in this case.

GC_DISABLE_INCREMENTAL  Turn off the incremental collection support.

NO_INCREMENTAL  Causes the gctest program to not invoke the incremental
  collector.  This has no impact on the generated library, only on the test
  program.  (This is often useful for debugging failures unrelated to
  incremental GC.)

LARGE_CONFIG    Tunes the collector for unusually large heaps.
  Necessary for heaps larger than about 4 GiB on most (64-bit) machines.
  Recommended for heaps larger than about 500 MiB.  Not recommended for
  embedded systems.

DONT_ADD_BYTE_AT_END    Meaningful only with ALL_INTERIOR_POINTERS or
  GC_all_interior_pointers = 1.  Normally ALL_INTERIOR_POINTERS
  causes all objects to be padded so that pointers just past the end of
  an object can be recognized.  This can be expensive.  (The padding
  is normally more than one byte due to alignment constraints.)
  DONT_ADD_BYTE_AT_END disables the padding.

NO_EXECUTE_PERMISSION   May cause some or all of the heap to not
  have execute permission, i.e. it may be impossible to execute
  code from the heap.  Currently this only affects the incremental
  collector on UNIX machines.  It may greatly improve its performance,
  since this may avoid some expensive cache synchronization.

GC_NO_OPERATOR_NEW_ARRAY        Declares that the C++ compiler does not
  support the  new syntax "operator new[]" for allocating and deleting arrays.
  See gc_cpp.h for details.  No effect on the C part of the collector.
  This is defined implicitly in a few environments.  Must also be defined
  by clients that use gc_cpp.h.

REDIRECT_MALLOC=<X>     Causes malloc to be defined as alias for X.
  Unless the following macros are defined, realloc is also redirected
  to GC_realloc, and free is redirected to GC_free.
  Calloc and strdup are redefined in terms of the new malloc.  X should
  be either GC_malloc or GC_malloc_uncollectable, or
  GC_debug_malloc_replacement.  (The latter invokes GC_debug_malloc
  with dummy source location information, but still results in
  properly remembered call stacks on Linux/X86 and Solaris/SPARC.
  It requires that the following two macros also be used.)
  The former is occasionally useful for working around leaks in code
  you don't want to (or can't) look at.  It may not work for
  existing code, but it often does.  Neither works on all platforms,
  since some ports use malloc or calloc to obtain system memory.
  (Probably works for UNIX, and win32.)  If you build with DBG_HDRS_ALL,
  you should only use GC_debug_malloc_replacement as a malloc
  replacement.

REDIRECT_REALLOC=<X>    Causes GC_realloc to be redirected to X.
  The canonical use is REDIRECT_REALLOC=GC_debug_realloc_replacement,
  together with REDIRECT_MALLOC=GC_debug_malloc_replacement to
  generate leak reports with call stacks for both malloc and realloc.
  This also requires REDIRECT_FREE.

REDIRECT_FREE=<X>       Causes free to be redirected to X.  The canonical use
  is REDIRECT_FREE=GC_debug_free.

IGNORE_FREE     Turns calls to free into a no-op.  Only useful with
  REDIRECT_MALLOC.

NO_DEBUGGING    Removes GC_dump and the debugging routines it calls.
  Reduces code size slightly at the expense of debuggability.

DEBUG_THREADS   Turn on printing additional thread-support debugging
  information.

JAVA_FINALIZATION       Makes it somewhat safer to finalize objects out of
  order by specifying a nonstandard finalization mark procedure  (see
  finalize.c).  Objects reachable from finalizable objects will be marked
  in a separate post-pass, and hence their memory won't be reclaimed.
  Not recommended unless you are implementing a language that specifies
  these semantics.  Since 5.0, determines only the initial value
  of GC_java_finalization variable.

FINALIZE_ON_DEMAND      Causes finalizers to be run only in response
  to explicit GC_invoke_finalizers() calls.
  In 5.0 this became runtime adjustable, and this only determines the
  initial value of GC_finalize_on_demand.

ATOMIC_UNCOLLECTABLE    Includes code for GC_malloc_atomic_uncollectable.
  This is useful if either the vendor malloc implementation is poor,
  or if REDIRECT_MALLOC is used.

MARK_BIT_PER_GRANULE    Requests that a mark bit (or often byte)
  be allocated for each allocation granule, as opposed to each object.
  This often improves speed, possibly at some cost in space and/or
  cache footprint.  Normally it is best to let this decision be
  made automatically depending on platform.

MARK_BIT_PER_OBJ        Requests that a mark bit be allocated for each
  object instead of allocation granule.  The opposite of
  MARK_BIT_PER_GRANULE.

HBLKSIZE=<ddd>  Explicitly sets the heap block size (where ddd is a power of
  2 between 512 and 16384).  Each heap block is devoted to a single size and
  kind of object.  For the incremental collector it makes sense to match
  the most likely page size.  Otherwise large values result in more
  fragmentation, but generally better performance for large heaps.

USE_MMAP        Use MMAP instead of sbrk to get new memory.
  Works for Linux, FreeBSD, Cygwin, Solaris and Irix.

USE_MUNMAP      Causes memory to be returned to the OS under the right
  circumstances.  This currently disables VM-based incremental collection
  (except for Win32 with GetWriteWatch() available).
  Works under some Unix, Linux and Windows versions.
  Requires USE_MMAP except for Windows.

MUNMAP_THRESHOLD=<value>        Set the desired memory blocks unmapping
  threshold (the number of sequential garbage collections for which
  a candidate block for unmapping should remain free).

GC_FORCE_UNMAP_ON_GCOLLECT      Set "unmap as much as possible on explicit GC"
  mode on by default.  The mode could be changed at run-time.  Has no effect
  unless unmapping is turned on.  Has no effect on implicitly-initiated
  garbage collections.

MMAP_STACKS (for Solaris threads)       Use mmap from /dev/zero rather than
  GC_scratch_alloc() to get stack memory.

PRINT_BLACK_LIST        Whenever a black list entry is added, i.e. whenever
  the garbage collector detects a value that looks almost, but not quite,
  like a pointer, print both the address containing the value, and the
  value of the near-bogus-pointer.  Can be used to identify regions of
  memory that are likely to contribute misidentified pointers.

KEEP_BACK_PTRS  Add code to save back pointers in debugging headers
  for objects allocated with the debugging allocator.  If all objects
  through GC_MALLOC with GC_DEBUG defined, this allows the client
  to determine how particular or randomly chosen objects are reachable
  for debugging/profiling purposes.  The gc_backptr.h interface is
  implemented only if this is defined.

GC_ASSERTIONS   Enable some internal GC assertion checking.  Currently
  this facility is only used in a few places.  It is intended primarily
  for debugging of the garbage collector itself, but could also...

DBG_HDRS_ALL    Make sure that all objects have debug headers.  Increases
  the reliability (from 99.9999% to 100% mod. bugs) of some of the debugging
  code (especially KEEP_BACK_PTRS).  Makes SHORT_DBG_HDRS possible.
  Assumes that all client allocation is done through debugging allocators.

SHORT_DBG_HDRS  Assume that all objects have debug headers.  Shorten
  the headers to minimize object size, at the expense of checking for
  writes past the end of an object.  This is intended for environments
  in which most client code is written in a "safe" language, such as
  Scheme or Java.  Assumes that all client allocation is done using
  the GC_debug_ functions, or through the macros that expand to these,
  or by redirecting malloc to GC_debug_malloc_replacement.
  (Also eliminates the field for the requested object size.)
  Occasionally could be useful for debugging of client code.  Slows down the
  collector somewhat, but not drastically.

SAVE_CALL_COUNT=<n>     Set the number of call frames saved with objects
  allocated through the debugging interface.  Affects the amount of
  information generated in leak reports.  Only matters on platforms
  on which we can quickly generate call stacks, currently Linux/(X86 & SPARC)
  and Solaris/SPARC and platforms that provide execinfo.h.
  Default is zero.  On X86, client
  code should NOT be compiled with -fomit-frame-pointer.

SAVE_CALL_NARGS=<n>     Set the number of functions arguments to be saved
  with each call frame.  Default is zero.  Ignored if we don't know how to
  retrieve arguments on the platform.

CHECKSUMS       Reports on erroneously clear dirty bits, and unexpectedly
  altered stubborn objects, at substantial performance cost.  Use only for
  debugging of the incremental collector.  Not compatible with USE_MUNMAP
  or threads.

GC_GCJ_SUPPORT  Includes support for gcj (and possibly other systems
  that include a pointer to a type descriptor in each allocated object).
  Building this way requires an ANSI C compiler.

USE_I686_PREFETCH       Causes the collector to issue Pentium III style
  prefetch instructions.  No effect except on X86 Linux platforms.
  Assumes a very recent gcc-compatible compiler and assembler.
  (Gas prefetcht0 support was added around May 1999.)
  Empirically the code appears to still run correctly on Pentium II
  processors, though with no performance benefit.  May not run on other
  X86 processors?  In some cases this improves performance by
  15% or so.

USE_3DNOW_PREFETCH      Causes the collector to issue AMD 3DNow style
  prefetch instructions.  Same restrictions as USE_I686_PREFETCH.
  Minimally tested.  Didn't appear to be an obvious win on a K6-2/500.

USE_PPC_PREFETCH        Causes the collector to issue PowerPC style
  prefetch instructions.  No effect except on PowerPC OS X platforms.
  Performance impact untested.

GC_USE_LD_WRAP  In combination with the old flags listed in README.linux
  causes the collector some system and pthread calls in a more transparent
  fashion than the usual macro-based approach.  Requires GNU ld, and
  currently probably works only with Linux.

GC_USE_DLOPEN_WRAP      Causes the collector to redefine malloc and
  intercepted pthread routines with their real names, and causes it to use
  dlopen and dlsym to refer to the original versions.  This makes it possible
  to build an LD_PRELOADable malloc replacement library.

THREAD_LOCAL_ALLOC      Defines GC_malloc(), GC_malloc_atomic() and
  GC_gcj_malloc() to use a per-thread set of free-lists. These then allocate
  in a way that usually does not involve acquisition of a global lock.
  Recommended for multiprocessors.  Requires explicit GC_INIT() call, unless
  REDIRECT_MALLOC is defined and GC_malloc is used first.

USE_COMPILER_TLS        Causes thread local allocation to use
  the compiler-supported "__thread" thread-local variables.  This is the
  default in HP/UX.  It may help performance on recent Linux installations.
  (It failed for me on RedHat 8, but appears to work on RedHat 9.)

PARALLEL_MARK   Allows the marker to run in multiple threads.  Recommended
  for multiprocessors.

DONT_USE_SIGNALANDWAIT (Win32 only)     Use an alternate implementation for
  marker threads (if PARALLEL_MARK defined) synchronization routines based
  on InterlockedExchange() (instead of AO_fetch_and_add()) and on multiple
  event objects (one per each marker instead of that based on Win32
  SignalObjectAndWait() using a single event object).  This is the default
  for WinCE.

GC_WINMAIN_REDIRECT (Win32 only)        Redirect (rename) an application
  WinMain to GC_WinMain; implement the "real" WinMain which starts a new
  thread to call GC_WinMain after initializing the GC.  Useful for WinCE.
  Incompatible with GC_DLL.

GC_REGISTER_MEM_PRIVATE (Win32 only)    Force to register MEM_PRIVATE R/W
  sections as data roots.  Might be needed for some WinCE 6.0+ custom builds.
  (May result in numerous "Data Abort" messages logged to WinCE debugging
  console.)  Incompatible with GCC toolchains for WinCE.

NO_GETENV       Prevents the collector from looking at environment variables.
  These may otherwise alter its configuration, or turn off GC altogether.
  I don't know of a reason to disable this, except possibly if the resulting
  process runs as a privileged user.  (This is on by default for WinCE.)

EMPTY_GETENV_RESULTS    Define to workaround a reputed Wine bug in getenv
  (getenv() may return an empty string instead of NULL for a missing entry).

USE_GLOBAL_ALLOC (Win32 only)   Use GlobalAlloc() instead of VirtualAlloc()
  to allocate the heap.  May be needed to work around a Windows NT/2000 issue.
  Incompatible with USE_MUNMAP.  See README.win32 for details.

MAKE_BACK_GRAPH         Enable GC_PRINT_BACK_HEIGHT environment variable.
  See README.environment for details.  Experimental. Limited platform
  support.  Implies DBG_HDRS_ALL.  All allocation should be done using
  the debug interface.

STUBBORN_ALLOC  Allows allocation of "hard to change" objects, and thus
  makes incremental collection easier.  Was enabled by default until 6.0.
  Rarely used, to my knowledge.

HANDLE_FORK     Attempts to make GC_malloc() work in a child process fork()'ed
  from a multithreaded parent.  Currently only supported by pthread_support.c.
  (Similar code should work on Solaris or Irix, but it hasn't been tried.)

TEST_WITH_SYSTEM_MALLOC         Causes gctest to allocate (and leak) large
  chunks of memory with the standard system malloc.  This will cause the root
  set and collected heap to grow significantly if malloc'ed memory is somehow
  getting traced by the collector.  This has no impact on the generated
  library; it only affects the test.

POINTER_MASK=<0x...>    Causes candidate pointers to be AND'ed with the given
  mask before being considered.  If either this or the following macro is
  defined, it will be assumed that all pointers stored in the heap need to be
  processed this way.  Stack and register pointers will be considered both
  with and without processing.  These macros are normally needed only to
  support systems that use high-order pointer tags.  EXPERIMENTAL.

POINTER_SHIFT=<n>       Causes the collector to left shift candidate pointers
  by the indicated amount before trying to interpret them.  Applied after
  POINTER_MASK. EXPERIMENTAL.  See also the preceding macro.

ENABLE_TRACE    Enables the GC_TRACE=addr environment setting to do its job.
  By default this is not supported in order to keep the marker as fast as
  possible.

DARWIN_DONT_PARSE_STACK         Causes the Darwin port to discover thread
  stack bounds in the same way as other pthread ports, without trying to
  walk the frames onthe stack.  This is recommended only as a fallback for
  applications that don't support proper stack unwinding.

USE_PROC_FOR_LIBRARIES  Causes the Linux collector to treat writable
  memory mappings (as reported by /proc) as roots, if it doesn't have
  other information about them.  It no longer traverses dynamic loader
  data structures to find dynamic library static data.  This may be
  required for applications that store pointers in mmapped segments without
  informing the collector.  But it typically performs poorly, especially
  since it will scan inactive but cached NPTL thread stacks completely.

NO_PROC_STAT    Causes the collector to avoid relying on Linux
  "/proc/self/stat".

NO_GETCONTEXT   Causes the collector to not assume the existence of the
  getcontext() function on linux-like platforms.  This currently happens
  implicitly on Darwin, Hurd, or ARM or MIPS hardware.  It is explicitly
  needed for some old versions of FreeBSD.

STATIC=static   Causes various GC_ symbols that could logically be declared
  static to be declared (this is the default if NO_DEBUGGING is specified).
  Reduces the number of visible symbols (letting the optimizer do its work
  better), which is probably cleaner, but may make some kinds of debugging
  and profiling harder.

GC_DLL  Build dynamic-link library (or dynamic shared object).  For Unix this
  causes the exported symbols to have 'default' visibility (ignored unless
  GCC v4+) and the internal ones to have 'hidden' visibility.

DONT_USE_USER32_DLL (Win32 only)        Don't use "user32" DLL import library
  (containing MessageBox() entry); useful for a static GC library.

GC_PREFER_MPROTECT_VDB  Choose MPROTECT_VDB manually in case of multiple
  virtual dirty bit strategies are implemented (at present useful on Win32
  to force MPROTECT_VDB strategy instead of the default GWW_VDB one).

GC_NO_DLLMAIN (Win32+DLL only)  Exclude DllMain-based thread registration
  support.

GC_IGNORE_GCJ_INFO      Disable GCJ-style type information (useful for
  debugging on WinCE).

GC_PRINT_VERBOSE_STATS  Permanently turn on verbose logging (useful for
  debugging and profiling on WinCE).

GC_DONT_EXPAND  Don't expand the heap unless explicitly requested or forced to.

GC_INITIAL_HEAP_SIZE=<value>    Set the desired default initial heap size
  in bytes.

GC_FREE_SPACE_DIVISOR=<value>   Set alternate default GC_free_space_divisor
  value.

GC_TIME_LIMIT=<milliseconds>    Set alternate default GC_time_limit value
  (setting this to GC_TIME_UNLIMITED will essentially disable incremental
  collection while leaving generational collection enabled).

GC_FULL_FREQ=<value>    Set alternate default number of partial collections
  between full collections (matters only if incremental collection is on).

NO_CANCEL_SAFE (Posix platforms with threads only)      Don't bother trying
  to make the collector safe for thread cancellation; cancellation is not
  used.  (Note that if cancellation is used anyway, threads may end up
  getting cancelled in unexpected places.)  Even without this option,
  PTHREAD_CANCEL_ASYNCHRONOUS is never safe with the collector.  (We could
  argue about its safety without the collector.)
