1 Object contexts:
2
3 Object Context
4 ------ -------
5 function overridable
6 method defining/originating class
7 class null
8 instance self
9 module null
10
11 Assignment types:
12
13 Assignment of stored value to... Effect on context
14 -------------------------------- -----------------
15 local preserved
16 global (module) preserved
17 class overridden (if overridable)
18 instance preserved
19
20 Assignment to a namespace preserves the context except for some class
21 assignments
22
23 Assigning to classes:
24
25 Assignment of... Effect on context
26 ---------------- -----------------
27 function (overridable context) overridden by class (becomes method)
28 method (existing context) preserved
29 class (null context) preserved
30 instance (self context) preserved
31 module (null context) preserved
32
33 With run-time restrictions on assignment targets:
34
35 Assignment of stored value to... Effect on context Optimised instruction Unoptimised instruction
36 -------------------------------- ----------------- --------------------- -----------------------
37 local preserved StoreName
38 global (module) preserved StoreAddress StoreAttrIndex
39 instance preserved StoreAttr StoreAttrIndex
40 class -> class StoreAddressContext StoreAttrIndex
41
42 Assignment of new class attributes is only permitted during initialisation
43 unless attribute usage observations are employed to detect possible attribute
44 assignments
45
46 Remember that once a function is assigned to a class, it will have that class
47 as context (as an unbound method)
48
49 Access types:
50
51 Access to stored value from... Effect on context Optimised instruction Unoptimised instruction
52 ------------------------------ ----------------- --------------------- -----------------------
53 local preserved LoadName
54 global (module) preserved LoadAddress LoadAttrIndex
55 class preserved LoadAddress LoadAttrIndex
56 class via instance -> instance LoadAddressContext(Cond) LoadAttrIndexContext(Cond)
57 instance preserved LoadAttr LoadAttrIndex
58
59 Access to a namespace may not preserve the stored context
60
61 Access to class attributes via instances:
62
63 Access to stored value with... Effect on context
64 ------------------------------ -----------------
65 compatible class as context -> instance
66 incompatible class as context preserved
67 null context preserved
68 other context (instance) preserved
69
70 Optimisation possibilities for class attribute access via instances:
71
72 Class Class attribute Context of attribute Instruction
73 ----- --------------- -------------------- -----------
74 known constant preserved LoadAddress
75 known constant -> instance LoadAddressContext
76 known not constant preserved LoadAddress (attribute may always be preserved)
77 known not constant -> instance LoadAddressContext (attribute may always be overridden)
78 known not constant not known LoadAddressContextCond (perform context check)
79 not known not known preserved LoadAttrIndex (attribute may have preserved context in all classes)
80 * not known not known -> instance LoadAttrIndexContext (attribute may have overridden context in all classes)
81 not known not known not known LoadAttrIndexContextCond (perform context check for class attribute access)
82
83 * Since knowing that an instance will replace the context might also mean
84 knowing the nature of the attribute, LoadAttrIndexContext is not likely to
85 be useful in practice.
86
87 Since the object table encodes sufficient information (an instance must be
88 compatible to access the class attribute, and compatibility information is
89 stored), an explicit compatibility test may not always be required at
90 run-time
91
92 Definite compile-time knowledge about instance usage is likely to be
93 restricted to literal constants: in general, no assertions can be made about
94 an accessor never being a class, module or constant and therefore being an
95 instance
