.. mode: -*- rst -*- General MPS types ================= :Tag: design.mps.type :Author: Richard Brooksby :Date: 1996-10-23 :Status: complete document :Revision: $Id: //info.ravenbrook.com/project/mps/master/design/type.txt#37 $ :Copyright: See `Copyright and License`_. :Index terms: pair: general types; design Introduction ------------ _`.intro`: See impl.h.mpmtypes. Rationale --------- Some types are declared to resolve a point of design, such as the best type to use for array indexing. Some types are declared so that the intention of code is clearer. For example, ``Byte`` is necessarily ``unsigned char``, but it's better to say ``Byte`` in your code if it's what you mean. Concrete types -------------- ``typedef unsigned AccessSet`` _`.access-set`: An ``AccessSet`` is a bitset of ``Access`` modes, which are ``AccessREAD`` and ``AccessWRITE``. ``AccessSetEMPTY`` is the empty ``AccessSet``. ``typedef struct AddrStruct *Addr`` _`.addr`: ``Addr`` is the type used for "managed addresses", that is, addresses of objects managed by the MPS. _`.addr.def`: ``Addr`` is defined as ``struct AddrStruct *``, but ``AddrStruct`` is never defined. This means that ``Addr`` is always an incomplete type, which prevents accidental dereferencing, arithmetic, or assignment to other pointer types. _`.addr.use`: ``Addr`` should be used whenever the code needs to deal with addresses. It should not be used for the addresses of memory manager data structures themselves, so that the memory manager remains amenable to working in a separate address space. Be careful not to confuse ``Addr`` with ``void *``. _`.addr.ops`: Limited arithmetic is allowed on addresses using ``AddrAdd()`` and ``AddrOffset()`` (impl.c.mpm). Addresses may also be compared using the relational operators ``==``, ``!=``, ``<``, ``<=``, ``>``, and ``>=``. _`.addr.ops.mem`: We need efficient operators similar to ``memset()``, ``memcpy()``, and ``memcmp()`` on ``Addr``; these are called ``AddrSet()``, ``AddrCopy()``, and ``AddrComp()``. When ``Addr`` is compatible with ``void *``, these are implemented through the functions ``mps_lib_memset()``, ``mps_lib_memcpy()``, and ``mps_lib_memcmp()`` functions in the plinth (impl.h.mpm). _`.addr.conv.c`: ``Addr`` is converted to ``mps_addr_t`` in the MPS C Interface. ``mps_addr_t`` is defined to be the same as ``void *``, so using the MPS C Interface confines the memory manager to the same address space as the client data. _`.addr.readonly`: For read-only addresses, see `.readonlyaddr`_. ``typedef Word Align`` _`.align`: ``Align`` is an unsigned integral type which is used to represent the alignment of managed addresses. All alignments are positive powers of two. ``Align`` is large enough to hold the maximum possible alignment. _`.align.use`: ``Align`` should be used whenever the code needs to deal with the alignment of a managed address. _`.align.conv.c`: ``Align`` is converted to ``mps_align_t`` in the MPS C Interface. ``typedef unsigned Attr`` _`.attr`: Pool attributes. A bitset of pool class attributes, which are: =================== =================================================== Attribute Description =================== =================================================== ``AttrGC`` Is garbage collecting, that is, parts may be reclaimed. Used to decide which segments are condemned. ``AttrMOVINGGC`` Is moving, that is, objects may move in memory. Used to update the set of zones that might have moved and so implement location dependency. =================== =================================================== There is an attribute field in the pool class (``PoolClassStruct``) which declares the attributes of that class. See design.mps.pool.field.attr_. .. _design.mps.pool.field.attr: pool#.field.attr ``typedef int Bool`` _`.bool`: The ``Bool`` type is mostly defined so that the intention of code is clearer. In C, Boolean expressions evaluate to ``int``, so ``Bool`` is in fact an alias for ``int``. _`.bool.value`: ``Bool`` has two values, ``TRUE`` and ``FALSE``. These are defined to be ``1`` and ``0`` respectively, for compatibility with C Boolean expressions (so one may set a ``Bool`` to the result of a C Boolean expression). _`.bool.use`: ``Bool`` is a type which should be used when a Boolean value is intended, for example, as the result of a function. Using a Boolean type in C is a tricky thing. Non-zero values are "true" (when used as control conditions) but are not all equal to ``TRUE``. Use with care. _`.bool.check`: ``BoolCheck()`` simply checks whether the argument is ``TRUE`` (``1``) or ``FALSE`` (``0``). _`.bool.check.inline`: The inline macro version of ``BoolCheck`` casts the ``int`` to ``unsigned`` and checks that it is ``<= 1``. This is safe, well-defined, uses the argument exactly once, and generates reasonable code. _`.bool.check.inline.smaller`: In fact we can expect that the "inline" version of ``BoolCheck()`` to be smaller than the equivalent function call. On IA-32 for example, a function call will be 3 instructions (total 9 bytes), the inline code for ``BoolCheck()`` will be 1 instruction (total 3 bytes) (both sequences not including the test which is the same length in either case). _`.bool.check.inline.why`: As well as being smaller (see `.bool.check.inline.smaller`_) it is faster. On 1998-11-16 drj compared ``w3i3mv\hi\amcss.exe`` running with and without the macro for ``BoolCheck`` on the PC Aaron. "With" ran in 97.7% of the time (averaged over 3 runs). _`.bool.bitfield`: When a Boolean needs to be stored in a bitfield, the type of the bitfield must be ``unsigned:1``, not ``Bool:1``. (That's because the two values of the type ``Bool:1`` are ``0`` and ``-1``, which means that assigning ``TRUE`` would require a sign conversion.) To make it clear why this is done, ``misc.h`` provides the ``BOOLFIELD`` macro. _`.bool.bitfield.assign`: To avoid warnings about loss of data from GCC with the ``-Wconversion`` option, ``misc.h`` provides the ``BOOLOF`` macro for coercing a value to an unsigned single-bit field. _`.bool.bitfield.check`: A Boolean bitfield cannot have an incorrect value, and if you call ``BoolCheck()`` on such a bitfield then GCC 4.2 issues the warning "comparison is always true due to limited range of data type". When avoiding such a warning, reference this tag. ``typedef unsigned BufferMode`` _`.buffermode`: ``BufferMode`` is a bitset of buffer attributes. See design.mps.buffer_. It is a sum of the following: .. _design.mps.buffer: buffer ======================== ============================================== Mode Description ======================== ============================================== ``BufferModeATTACHED`` Buffer is attached to a region of memory. ``BufferModeFLIPPED`` Buffer has been flipped. ``BufferModeLOGGED`` Buffer remains permanently trapped, so that all reserve and commit events can be logged. ``BufferModeTRANSITION`` Buffer is in the process of being detached. ======================== ============================================== ``typedef unsigned char Byte`` _`.byte`: ``Byte`` is an unsigned integral type corresponding to the unit in which most sizes are measured, and also the units of ``sizeof``. _`.byte.use`: ``Byte`` should be used in preference to ``char`` or ``unsigned char`` wherever it is necessary to deal with bytes directly. _`.byte.source`: ``Byte`` is a just pedagogic version of ``unsigned char``, since ``char`` is the unit of ``sizeof``. ``typedef Word Clock`` _`.clock`: ``Clock`` is an unsigned integral type representing clock time since some epoch. _`.clock.use`: A ``Clock`` value is returned by the plinth function ``mps_clock``. It is used to make collection scheduling decisions and to calibrate the time stamps on events in the telemetry stream. _`.clock.units`: The plinth function ``mps_clocks_per_sec`` defines the units of a ``Clock`` value. _`.clock.conv.c`: ``Clock`` is converted to ``mps_clock_t`` in the MPS C Interface. ``typedef unsigned Compare`` _`.compare`: ``Compare`` is the type of tri-state comparison values. ================== ==================================================== Value Description ================== ==================================================== ``CompareLESS`` A value compares less than another value. ``CompareEQUAL`` Two values compare the same. ``CompareGREATER`` A value compares greater than another value. ================== ==================================================== ``typedef Word Count`` _`.count`: ``Count`` is an unsigned integral type which is large enough to hold the size of any collection of objects in the MPS. _`.count.use`: ``Count`` should be used for a number of objects (control or managed) where the maximum number of objects cannot be statically determined. If the maximum number can be statically determined then the smallest unsigned integer with a large enough range may be used instead (although ``Count`` may be preferable for clarity). _`.count.use.other`: ``Count`` may also be used to count things that aren't represented by objects (for example, levels), but only where it can be determined that the maximum count is less than the number of objects. ``typedef Size Epoch`` _`.epoch`: An ``Epoch`` is a count of the number of flips that have occurred, in which objects may have moved. It is used in the implementation of location dependencies. ``Epoch`` is converted to ``mps_word_t`` in the MPS C Interface, as a field of ``mps_ld_s``. ``typedef unsigned FindDelete`` _`.finddelete`: ``FindDelete`` represents an instruction to one of the *find* methods of a ``Land`` as to what it should do if it finds a suitable block. See design.mps.land_. It takes one of the following values: .. _design.mps.land: land ==================== ================================================== Value Description ==================== ================================================== ``FindDeleteNONE`` Don't delete after finding. ``FindDeleteLOW`` Delete from low end of block. ``FindDeleteHIGH`` Delete from high end of block. ``FindDeleteENTIRE`` Delete entire block. ==================== ================================================== ``typedef void (*Fun)(void)`` _`.fun`: ``Fun`` is the type of a pointer to a function about which nothing more is known. _`.fun.use`: ``Fun`` should be used where it's necessary to handle a function in a polymorphic way without calling it. For example, if you need to write a function ``g`` which passes another function ``f`` through to a third function ``h``, where ``h`` knows the real type of ``f`` but ``g`` doesn't. ``typedef Word Index`` _`.index`: ``Index`` is an unsigned integral type which is large enough to hold any array index. _`.index.use`: ``Index`` should be used where the maximum size of the array cannot be statically determined. If the maximum size can be determined then the smallest unsigned integer with a large enough range may be used instead. ``typedef unsigned LocusPrefKind`` _`.locusprefkind`: The type ``LocusPrefKind`` expresses a preference for addresses within an address space. It takes one of the following values: ==================== ==================================== Kind Description ==================== ==================================== ``LocusPrefHIGH`` Prefer high addresses. ``LocusPrefLOW`` Prefer low addresses. ``LocusPrefZONESET`` Prefer addresses in specified zones. ==================== ==================================== ``typedef unsigned MessageType`` _`.messagetype`: ``MessageType`` is the type of a message. See design.mps.message_. It takes one of the following values: .. _design.mps.message: message =========================== =========================================== Message type Description =========================== =========================================== ``MessageTypeFINALIZATION`` A block is finalizable. ``MessageTypeGC`` A garbage collection finished. ``MessageTypeGCSTART`` A garbage collection started. =========================== =========================================== ``typedef unsigned Rank`` _`.rank`: ``Rank`` is an enumeration which represents the rank of a reference. The ranks are: ============= ===== ================================================== Rank Index Description ============= ===== ================================================== ``RankAMBIG`` 0 The reference is ambiguous. That is, it must be assumed to be a reference, but not updated in case it isn't. ``RankEXACT`` 1 The reference is exact, and refers to an object. ``RankFINAL`` 2 The reference is exact and final, so special action is required if only final or weak references remain to the object. ``RankWEAK`` 3 The reference is exact and weak, so should be deleted if only weak references remain to the object. ============= ===== ================================================== ``Rank`` is stored with segments and roots, and passed around. ``Rank`` is converted to ``mps_rank_t`` in the MPS C Interface. The ordering of the ranks is important. It is the order in which the references must be scanned in order to respect the properties of references of the ranks. Therefore they are declared explicitly with their integer values. .. note:: Could ``Rank`` be an ``unsigned short`` or ``unsigned char``? .. note:: This documentation should be expanded and moved to its own document, then referenced from the implementation more thoroughly. ``typedef unsigned RankSet`` _`.rankset`: ``RankSet`` is a set of ranks, represented as a bitset. ``typedef const struct AddrStruct *ReadonlyAddr`` _`.readonlyaddr`: ``ReadonlyAddr`` is the type used for managed addresses that an interface promises it will only read through, never write. Otherwise it is identical to ``Addr``. ``typedef Addr Ref`` _`.ref`: ``Ref`` is a reference to a managed object (as opposed to any old managed address). ``Ref`` should be used where a reference is intended. .. note:: This isn't too clear -- richard ``typedef Word RefSet`` _`.refset`: ``RefSet`` is a conservative approximation to a set of references. See design.mps.collection.refsets_. .. _design.mps.collection.refsets: collection#.refsets ``typedef int Res`` _`.res`: ``Res`` is the type of result codes. A result code indicates the success or failure of an operation, along with the reason for failure. Like Unix error codes, the meaning of the code depends on the call that returned it. These codes are just broad categories with mnemonic names for various sorts of problems. =================== =================================================== Result code Description =================== =================================================== ``ResOK`` The operation succeeded. Return parameters may only be updated if OK is returned, otherwise they must be left untouched. ``ResCOMMIT_LIMIT`` The arena's commit limit would have been exceeded as a result of allocation. ``ResFAIL`` Something went wrong which doesn't fall into any of the other categories. The exact meaning depends on the call. See documentation. ``ResIO`` An I/O error occurred. Exactly what depends on the function. ``ResLIMIT`` An internal limitation was reached. For example, the maximum number of somethings was reached. We should avoid returning this by not including static limitations in our code, as far as possible. (See rule.impl.constrain and rule.impl.limits.) ``ResMEMORY`` Needed memory (committed memory, not address space) could not be obtained. ``ResPARAM`` An invalid parameter was passed. Normally reserved for parameters passed from the client. ``ResRESOURCE`` A needed resource could not be obtained. Which resource depends on the call. See also ``ResMEMORY``, which is a special case of this. ``ResUNIMPL`` The operation, or some vital part of it, is unimplemented. This might be returned by functions which are no longer supported, or by operations which are included for future expansion, but not yet supported. =================== =================================================== _`.res.use`: ``Res`` should be returned from any function which might fail. Any other results of the function should be passed back in "return" parameters (pointers to locations to fill in with the results). .. note:: This is documented elsewhere, I think -- richard _`.res.use.spec`: The most specific code should be returned. ``typedef unsigned RootMode`` _`.rootmode`: ``RootMode`` is an unsigned integral type which is used to represent an attribute of a root: ============================= ========================================= Root mode Description ============================= ========================================= ``RootModeCONSTANT`` Client program will not change the root after it is registered. ``RootModePROTECTABLE`` Root is protectable: the MPS may place a barrier on any page containing any part of the root. ``RootModePROTECTABLE_INNER`` Root is protectable: the MPS may place a barrier on any page completely covered by part of the root. ============================= ========================================= _`.rootmode.const.unused`: ``RootModeCONSTANT`` has no effect. This mode was introduced in the hope of being able to maintain a remembered set for the root without needing a write barrier, but it can't work as described, since you can't reliably create a valid registered constant root that contains any references. (If you add the references before registering the root, they may have become invalid; but you can't add them afterwards because the root is supposed to be constant.) _`.rootmode.conv.c`: ``RootMode`` is converted to ``mps_rm_t`` in the MPS C Interface. ``typedef unsigned RootVar`` _`.rootvar`: The type ``RootVar`` is the type of the discriminator for the union within ``RootStruct``. ``typedef unsigned Serial`` _`.serial`: A ``Serial`` is a number which is assigned to a structure when it is initialized. The serial number is taken from a field in the parent structure, which is incremented. Thus, every instance of a structure has a unique "name" which is a path of structures from the global root. For example, "the third arena's fifth pool's second buffer". Why? Consistency checking, debugging, and logging. Not well thought out. ``typedef unsigned Shift`` _`.shift`: ``Shift`` is an unsigned integral type which can hold the amount by which a ``Word`` can be shifted. It is therefore large enough to hold the word width (in bits). _`.shift.use`: ``Shift`` should be used whenever a shift value (the right-hand operand of the ``<<`` or ``>>`` operators) is intended, to make the code clear. It should also be used for structure fields which have this use. .. note:: Could ``Shift`` be an ``unsigned short`` or ``unsigned char``? ``typedef unsigned long Sig`` _`.sig`: ``Sig`` is the type of signatures, which are written into structures when they are created, and invalidated when they are destroyed. They provide a limited form of run-time type checking and dynamic scope checking. See design.mps.sig_. .. _design.mps.sig: sig ``typedef Word Size`` _`.size`: ``Size`` is an unsigned integral type large enough to hold the size of any object which the MPS might manage. _`.size.byte`: ``Size`` should hold a size calculated in bytes. .. warning:: This is violated by ``GenParams.capacity`` (which is measured in kilobytes). _`.size.use`: ``Size`` should be used whenever the code needs to deal with the size of managed memory or client objects. It should not be used for the sizes of the memory manager's own data structures, so that the memory manager is amenable to working in a separate address space. Be careful not to confuse it with ``size_t``. _`.size.ops`: ``SizeIsAligned()``, ``SizeAlignUp()``, ``SizeAlignDown()`` and ``SizeRoundUp()``. _`.size.conv.c`: ``Size`` is converted to ``size_t`` in the MPS C Interface. This constrains the memory manager to the same address space as the client data. ``typedef unsigned TraceId`` _`.traceid`: A ``TraceId`` is an unsigned integer which is less than ``TraceLIMIT``. Each running trace has a different ``TraceId`` which is used to index into the tables and bitfields that record the state of that trace. See design.mps.trace.instance.limit_. .. _design.mps.trace.instance.limit: trace#.instance.limit ``typedef unsigned TraceSet`` _`.traceset`: A ``TraceSet`` is a bitset of ``TraceId``, represented in the obvious way:: member(ti, ts) ⇔ ((1<`_. Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met: 1. Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer. 2. 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