| 1 | The collector uses a large amount of conditional compilation in order to
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| 2 | deal with platform dependencies. This violates a number of known coding
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| 3 | standards. On the other hand, it seems to be the only practical way to
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| 4 | support this many platforms without excessive code duplication.
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| 5 |
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| 6 | A few guidelines have mostly been followed in order to keep this manageable:
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| 7 |
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| 8 | 1) #if and #ifdef directives are properly indented whenever easily possible.
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| 9 | All known C compilers allow whitespace between the "#" and the "if" to make
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| 10 | this possible. ANSI C also allows white space before the "#", though we
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| 11 | avoid that. It has the known disadvantages that it differs from the normal
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| 12 | GNU conventions, and that it makes patches larger than otherwise necessary.
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| 13 | In my opinion, it's still well worth it, for the same reason that we indent
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| 14 | ordinary "if" statements.
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| 15 |
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| 16 | 2) Whenever possible, tests are performed on the macros defined in gcconfig.h
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| 17 | instead of directly testing patform-specific predefined macros. This makes it
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| 18 | relatively easy to adapt to new compilers with a different set of predefined
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| 19 | macros. Currently these macros generally identify platforms instead of
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| 20 | features. In many cases, this is a mistake.
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| 21 |
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| 22 | 3) The code currently avoids #elif, eventhough that would make it more
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| 23 | readable. This was done since #elif would need to be understood by ALL
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| 24 | compilers used to build the collector, and that hasn't always been the case.
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| 25 | It makes sense to reconsider this decision at some point, since #elif has been
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| 26 | standardized at least since 1989.
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| 27 |
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| 28 | Many of the tested configuration macros are at least somewhat defined in
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| 29 | either include/private/gcconfig.h or in Makefile.direct. Here is an attempt
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| 30 | at defining some of the remainder: (Thanks to Walter Bright for suggesting
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| 31 | this. This is a work in progress)
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| 32 |
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| 33 | MACRO EXPLANATION
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| 34 | ----- -----------
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| 35 |
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| 36 | __DMC__ Always #define'd by the Digital Mars compiler. Expands
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| 37 | to the compiler version number in hex, i.e. 0x810 is
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| 38 | version 8.1b0
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| 39 |
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| 40 | _ENABLE_ARRAYNEW
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| 41 | #define'd by the Digital Mars C++ compiler when
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| 42 | operator new[] and delete[] are separately
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| 43 | overloadable. Used in gc_cpp.h.
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| 44 |
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| 45 | _MSC_VER Expands to the Visual C++ compiler version. Assumed to
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| 46 | not be defined for other compilers (at least if they behave
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| 47 | appreciably differently).
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| 48 |
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| 49 | _DLL Defined by Visual C++ if dynamic libraries are being built
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| 50 | or used. Used to test whether __declspec(dllimport) or
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| 51 | __declspec(dllexport) needs to be added to declarations
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| 52 | to support the case in which the collector is in a dll.
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| 53 |
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| 54 | GC_DLL User-settable macro that forces the effect of _DLL. Set
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| 55 | by gc.h if _DLL is defined and GC_NOT_DLL is undefined.
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| 56 | This is the macro that is tested internally to determine
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| 57 | whether the GC is in its own dynamic library. May need
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| 58 | to be set by clients before including gc.h. Note that
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| 59 | inside the GC implementation it indicates that the
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| 60 | collector is in its own dynamic library, should export
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| 61 | its symbols, etc. But in clients it indicates that the
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| 62 | GC resides in a different DLL, its entry points should
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| 63 | be referenced accordingly, and precautions may need to
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| 64 | be taken to properly deal with statically allocated
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| 65 | variables in the main program. Used only for MS Windows.
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| 66 |
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| 67 | GC_NOT_DLL User-settable macro that overrides _DLL, e.g. if dynamic
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| 68 | libraries are used, but the collector is in a static library.
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| 69 |
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| 70 | __STDC__ Assumed to be defined only by compilers that understand
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| 71 | prototypes and other C89 features. Its value is generally
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| 72 | not used, since we are fine with most nonconforming extensions.
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| 73 |
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| 74 | SUNOS5SIGS Solaris-like signal handling. This is probably misnamed,
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| 75 | since it really doesn't guarantee much more than Posix.
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| 76 | Currently set only for Solaris2.X, HPUX, and DRSNX. Should
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| 77 | probably be set for some other platforms.
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| 78 |
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| 79 | PCR Set if the collector is being built as part of the Xerox
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| 80 | Portable Common Runtime.
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| 81 |
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| 82 | SRC_M3 Set if the collector is being built as a replacement of the
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| 83 | one in the DEC/Compaq SRC Modula-3 runtime. I suspect this
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| 84 | was last used around 1994, and no doubt broke a long time ago.
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| 85 | It's there primarily incase someone wants to port to a similar
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| 86 | system.
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| 87 |
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| 88 | USE_COMPILER_TLS Assume the existence of __thread-style thread-local
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| 89 | storage. Set automatically for thread-local allocation with
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| 90 | the HP/UX vendor compiler. Usable with gcc on sufficiently
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| 91 | up-to-date ELF platforms.
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| 92 |
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| 93 |
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| 94 |
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