Commit | Line | Data |
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abdf50aa MW |
1 | * Instance layout |
2 | ||
3 | This is fairly easy. The superclasses are partitioned into chains. | |
4 | Each chain is named after its head class (i.e., the class with no | |
5 | successor). | |
6 | ||
7 | ** Things in instance layouts | |
8 | ||
9 | An instance layout contains a chunk for each component chain. | |
10 | ||
11 | struct CLASS__ilayout { | |
12 | struct CLASS__ichain_CHAINn NICKn; | |
13 | /* ... */ | |
14 | }; | |
15 | ||
16 | An ilayout is a C structure consisting of an ichain for each of the | |
17 | class's chains, with the primary chain first. The others are in | |
18 | direct-superclass order. | |
19 | ||
20 | ** Instance slots | |
21 | ||
22 | An islots structure is a C structure consisting of a class's instance | |
23 | slots, in order. | |
24 | ||
25 | struct CLASS__islots { | |
26 | TYPEn SLOTn; | |
27 | /* ... */ | |
28 | }; | |
29 | ||
30 | If a class defines no slots then it has no islots structure. | |
31 | ||
32 | ** Instance chains | |
33 | ||
34 | struct CLASS__ichain_CHAIN { | |
35 | const struct CLASS__vt_CHAIN *_vt; | |
36 | struct SUPERn__islots NICKn; | |
37 | /* ... */ | |
38 | }; | |
39 | ||
40 | A ichain is a C structure consisting of: | |
41 | ||
42 | * A pointer `_vt' to the chain's vtable structure. | |
43 | ||
44 | * An islots substructure, named after the class's nick for each class | |
45 | on the chain, least-specific first. | |
46 | ||
47 | Because of the chain invariant, all of a class's ichains are prefixes of | |
48 | the corresponding ichains of any of its subclasses. | |
49 | ||
50 | The type CLASS is an alias for the class's primary ichain | |
51 | CLASS__ichain_CHAIN. One needs to do a cross-chain upcast to find slots | |
52 | in non-primary chains. | |
53 | ||
54 | * Vtable layout | |
55 | ||
56 | This is more complicated. The vtable for a chain doesn't just contain | |
57 | things directly relevant to the classes on the chain: because a vtable | |
58 | is (assumed) immutable, we can have copies of values from other chains | |
59 | where this is convenient. | |
60 | ||
61 | Note that effective methods are customized for particular classes: they | |
62 | can assume that their argument points to a specific ichain of a an | |
63 | instance of a specific class. This makes conversions in effective | |
64 | methods very cheap. By including apparently effective-method pointers | |
65 | for messages defined in other chains, we can speed up dispatch. | |
66 | ||
67 | ** Things in a vtable chain | |
68 | ||
69 | There are three kinds of items to store in a vtable chain. | |
70 | ||
71 | * Class pointers | |
72 | * The base offset | |
73 | * Chain offsets | |
74 | * Effective method pointers | |
75 | ||
76 | struct CLASS__vt_CHAIN { | |
77 | struct METACLASS__ichain_sod_object *_class; | |
78 | size_t _base; | |
79 | struct METACLASS__ichain_METACHAINn *_cls_NICKn; | |
80 | ptrdiff_t _off_CHAINn; | |
81 | struct SUPERn__vtmsgs NICKn; | |
82 | }; | |
83 | ||
84 | A class has a separate vtable chain for each of its chains. | |
85 | ||
86 | ** The base offset | |
87 | ||
88 | There is a single member _base which is the offset of the chain's ichain | |
89 | in the overall ilayout structure. This lets you find the bottom of the | |
90 | ilayout given a pointer to any ichain as | |
91 | ||
92 | (CLASS__ilayout *)((char *)p - p->_vt._base) | |
93 | ||
94 | ** Class pointers | |
95 | ||
96 | The class's metaclass may have multiple chains. For each chain of the | |
97 | metaclass, there is a separate pointer to that metaclass's ichain, named | |
98 | _cls_NICKn after the metaclass's chain head. Exception: _cls_cls is | |
99 | called _class instead. | |
100 | ||
101 | ** Chain offsets | |
102 | ||
103 | For each other chain, there is a member _off_NICKn named after the | |
104 | chain's head giving the offset of that ichain from the current chain's | |
105 | ichain. (There's a long way around, exploring the class's layout | |
106 | information, but this provides a much easier way of doing cross-chain | |
107 | upcasts.) | |
108 | ||
109 | ** Effective method pointers | |
110 | ||
111 | For each class, there may be a structure | |
112 | ||
113 | struct CLASS__vtmsgs { | |
114 | TYPEn (*MSGn)(ARGnn *, ...); | |
115 | /* ... */ | |
116 | }; | |
117 | ||
118 | of pointers to effective methods for the messages defined by the class. | |
119 | If a class defines no messages then it won't have a vtmsgs structure. | |
120 | ||
121 | ** Layout order | |
122 | ||
123 | The first two items are always _class and _base. After that: | |
124 | ||
125 | * for each class in the chain, from least to most specific, | |
126 | ||
127 | * for each of that class's superclasses, in reverse class-precedence- | |
128 | list order, which has not yet been processed: | |
129 | ||
130 | * if the class is in a chain which hasn't been seen before (it must be | |
131 | the chain head!), emit a chain offset for it; | |
132 | ||
133 | * if the class has a metaclass chain which hasn't been seen before, | |
134 | emit a class pointer for it; | |
135 | ||
136 | * if the class has a vtmsgs structure, emit it. | |
137 | ||
138 | * Questions | |
139 | ||
140 | Are class-slot initializers inherited? No. We have instance | |
141 | initializers on metaclasses for that. |