90 lines
4.0 KiB
Plaintext
90 lines
4.0 KiB
Plaintext
The collector uses a large amount of conditional compilation in order to
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deal with platform dependencies. This violates a number of known coding
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standards. On the other hand, it seems to be the only practical way to
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support this many platforms without excessive code duplication.
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A few guidelines have mostly been followed in order to keep this manageable:
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1) #if and #ifdef directives are properly indented whenever easily possible.
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All known C compilers allow whitespace between the "#" and the "if" to make
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this possible. ANSI C also allows white space before the "#", though we
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avoid that. It has the known disadvantages that it differs from the normal
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GNU conventions, and that it makes patches larger than otherwise necessary.
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In my opinion, it's still well worth it, for the same reason that we indent
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ordinary "if" statements.
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2) Whenever possible, tests are performed on the macros defined in gcconfig.h
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instead of directly testing patform-specific predefined macros. This makes it
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relatively easy to adapt to new compilers with a different set of predefined
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macros. Currently these macros generally identify platforms instead of
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features. In many cases, this is a mistake.
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3) The code currently avoids #elif, eventhough that would make it more
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readable. This was done since #elif would need to be understood by ALL
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compilers used to build the collector, and that hasn't always been the case.
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It makes sense to reconsider this decision at some point, since #elif has been
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standardized at least since 1989.
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Many of the tested configuration macros are at least somewhat defined in
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either include/private/gcconfig.h or in Makefile.direct. Here is an attempt
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at defining some of the remainder: (Thanks to Walter Bright for suggesting
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this. This is a work in progress)
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MACRO EXPLANATION
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----- -----------
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__DMC__ Always #define'd by the Digital Mars compiler. Expands
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to the compiler version number in hex, i.e. 0x810 is
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version 8.1b0
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_ENABLE_ARRAYNEW
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#define'd by the Digital Mars C++ compiler when
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operator new[] and delete[] are separately
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overloadable. Used in gc_cpp.h.
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_MSC_VER Expands to the Visual C++ compiler version. Assumed to
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not be defined for other compilers (at least if they behave
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appreciably differently).
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_DLL Defined by Visual C++ if dynamic libraries are being built
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or used. Used to test whether __declspec(dllimport) or
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__declspec(dllexport) needs to be added to declarations
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to support the case in which the collector is in a dll.
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GC_DLL User-settable macro that forces the effect of _DLL. Set
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by gc.h if _DLL is defined and GC_NOT_DLL is undefined.
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This is the macro that is tested internally to determine
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whether the GC is in its own dynamic library. May need
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to be set by clients before including gc.h. Note that
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inside the GC implementation it indicates that the
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collector is in its own dynamic library, should export
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its symbols, etc. But in clients it indicates that the
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GC resides in a different DLL, its entry points should
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be referenced accordingly, and precautions may need to
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be taken to properly deal with statically allocated
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variables in the main program. Used only for MS Windows.
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GC_NOT_DLL User-settable macro that overrides _DLL, e.g. if dynamic
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libraries are used, but the collector is in a static library.
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__STDC__ Assumed to be defined only by compilers that understand
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prototypes and other C89 features. Its value is generally
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not used, since we are fine with most nonconforming extensions.
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SUNOS5SIGS Solaris-like signal handling. This is probably misnamed,
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since it really doesn't guarantee much more than Posix.
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Currently set only for Solaris2.X, HPUX, and DRSNX. Should
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probably be set for some other platforms.
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PCR Set if the collector is being built as part of the Xerox
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Portable Common Runtime.
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SRC_M3 Set if the collector is being built as a replacement of the
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one in the DEC/Compaq SRC Modula-3 runtime. I suspect this
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was last used around 1994, and no doubt broke a long time ago.
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It's there primarily incase someone wants to port to a similar
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system.
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