49. General MPS types

49.1. Introduction

.intro: See impl.h.mpmtypes.

49.2. 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.

49.3. Concrete types

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).

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.
ResFAIL	Something went wrong which doesn’t fall into any of the other categories. The exact meaning depends on the call. See documentation.
ResRESOURCE	A needed resource could not be obtained. Which resource depends on the call. See also ResMEMORY, which is a special case of this.
ResMEMORY	Needed memory (committed memory, not address space) could not be obtained.
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.)
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.
ResIO	An I/O error occurred. Exactly what depends on the function.
ResCOMMIT_LIMIT	The arena’s commit limit would have been exceeded as a result of allocation.
ResPARAM	An invalid parameter was passed. Normally reserved for parameters passed from the client.

.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.

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.

MPS_T_WORD Word

.word: Word is an unsigned integral type which matches the size of the machine word, that is, the natural size of the machine registers and addresses.

.word.use: Word should be used where an unsigned integer is required that might range as large as the machine word.

.word.source: Word is derived from the macro MPS_T_WORD which is declared in impl.h.mpstd according to the target platform (design.mps.config.pf.word).

.word.conv.c: Word is converted to mps_word_t in the MPS C Interface.

.word.ops: WordIsAligned(), WordAlignUp(), WordAlignDown() and WordRoundUp().

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.

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.

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).

Note

Should Count be used to count things that aren’t represented by objects (for example, a level)? I would say yes. gavinm 1998-07-21

Note

Only where it can be determined that the maximum count is less than the number of objects. pekka 1998-07-21

Word Accumulation

.accumulation: Accumulation is an arithmetic type which is large enough to hold accumulated totals of objects of bytes (for example, total number of objects allocated, total number of bytes allocated).

.accumulation.type: Currently it is double, but the reason for the interface is so that we can more easily change it if we want to (if we decide we need more accuracy for example).

.accumulation.use: Currently the only way to use an Accumulation is to reset it (by calling AccumulatorReset) and accumulate amounts into it (by calling Accumulate). There is no way to read it at the moment, but that’s okay, because no one seems to want to.

.accumulation.future: Probably we should have methods which return the accumulation into an unsigned long, and also a double; these functions should return Bool to indicate whether the accumulation can fit in the requested type. Possibly we could have functions which returned scaled accumulations. For example, AccumulatorScale(a, d) would divide the Accumulation a by double d and return the double result if it fitted into a double.

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).

Note

No other implementation exists at present. pekka 1998-09-07

.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.

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 may not be true for all existing code.

.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.

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.

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.

.shift.conv.c: Shift is converted to mps_shift_t in the MPS C Interface.

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

Word RefSet

.refset: RefSet is a conservative approximation to a set of references. See design.mps.refset.

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 a short?

Note

This documentation should be expanded and moved to its own document, then referenced from the implementation more thoroughly.

Size Epoch

.epoch: An Epoch is a count of the number of flips that have occurred. 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.

unsigned TraceId

.traceid: A TraceId is an unsigned integer which is less than TRACE_MAX. Each running trace has a different TraceId which is used to index into tables and bitfields used to remember the state of that trace.

unsigned TraceSet

.traceset: A TraceSet is a bitset of TraceId, represented in the obvious way:

member(ti, ts) ⇔ ((1<<ti) & ts) != 0

TraceSet is used to represent colour in the Tracer.

Note

Expand on this.

unsigned AccessSet

.access-set: An AccessSet is a bitset of Access modes, which are AccessREAD and AccessWRITE. AccessNONE is the empty AccessSet.

unsigned Attr

.attr: Pool attributes. A bitset of pool or pool class attributes, which are:

Attribute	Description
AttrFMT	Contains formatted objects.
AttrSCAN	Contains references and must be scanned.
AttrPM_NO_READ	May not be read protected.
AttrPM_NO_WRITE	May not be write protected.
AttrALLOC	Supports the PoolAlloc interface.
AttrFREE	Supports the PoolFree interface.
AttrBUF	Supports the allocation buffer interface.
AttrBUF_RESERVE	Supports the reserve/commit protocol on allocation buffers.
AttrBUF_ALLOC	Supports the alloc protocol on allocation buffers.
AttrGC	Is garbage collecting, that is, parts may be reclaimed.
AttrINCR_RB	Is incremental, requiring a read barrier.
AttrINCR_WB	Is incremental, requiring a write barrier.

There is an attribute field in the pool class (PoolClassStruct) which declares the attributes of that class. These attributes are only used for consistency checking at the moment.

Note

It’s no longer true that they are only used for consistency checking – drj 1998-05-07

int RootVar

.rootvar: The type RootVar is the type of the discriminator for the union within RootStruct.

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:

space[3].pool[5].buffer[2]

Why? Consistency checking, debugging, and logging. Not well thought out.

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.

MPS_T_ULONGEST ULongest

.ulongest: ULongest is the longest unsigned integer on the platform. (We used to use unsigned long but this assumption is violated by 64-bit Windows.) This type should be used for calculations where any integer might be passed. Notably, it is used in WriteF() to print any integer.

49.4. Abstract types

.adts: The following types are abstract data types, implemented as pointers to structures. For example, Ring is a pointer to a RingStruct. They are described elsewhere.

AP, Arena, Buffer, Format, LD, Lock, PoolClass, Pool, Ring, Root, ScanState, Seg, Space, Thread, Trace, VM.

void *Pointer

.pointer: The type Pointer is the same as void *, and exists to sanctify functions such as PointerAdd().