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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 patform-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.

3) The code currently avoids #elif, eventhough that would make it more
readable.  This was done since #elif would need to be understood by ALL
compilers used to build the collector, and that hasn't always been the case.
It makes sense to reconsider this decision at some point, since #elif has been
standardized at least since 1989.

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 defining some of the remainder:  (Thanks to Walter Bright for suggesting
this.  This is a work in progress)

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

__DMC__ Always #define'd by the Digital Mars compiler. Expands
                to the compiler version number in hex, i.e. 0x810 is
                version 8.1b0

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

_MSC_VER        Expands to the Visual C++ compiler version.  Assumed to
                not be defined for other compilers (at least if they behave
                appreciably differently).

_DLL            Defined by Visual C++ if dynamic libraries are being built
                or used.  Used to test whether __declspec(dllimport) or
                __declspec(dllexport) needs to be added to declarations
                to support the case in which the collector is in a dll.

GC_DLL          User-settable macro that forces the effect of _DLL.  Set
                by gc.h if _DLL is defined and GC_NOT_DLL is 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 only for MS Windows.

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

__STDC__        Assumed to be defined only by compilers that understand
                prototypes and other C89 features.  Its value is generally
                not used, since we are fine with most nonconforming extensions.

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.

SRC_M3          Set if the collector is being built as a replacement of the
                one in the DEC/Compaq SRC Modula-3 runtime.  I suspect this
                was last used around 1994, and no doubt broke a long time ago.
                It's there primarily incase someone wants to port to a similar
                system.

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.