1 Data Structures
2 ===============
3
4 The fundamental "value type" is a pair of references: one pointing to the
5 referenced object represented by the interchangeable value; one referring to
6 the context of the referenced object, typically the object through which the
7 referenced object was acquired as an attribute.
8
9 Value Layout
10 ------------
11
12 0 1
13 object context
14 reference reference
15
16 Values and Contexts
17 -------------------
18
19 Values are acquired through name lookups and attribute access, yielding
20 the appropriate object reference together with a context reference as
21 indicated in the following table:
22
23 Type of Access Context Notes
24 -------------- ------- -----
25
26 Local name Preserved Functions provide no context
27
28 Global name Preserved Modules provide no context
29
30 Class-originating Accessor Class accessor preserves the stored
31 attribute -or- context; instance accessor overrides
32 Preserved the stored context if it is null or
33 belongs to the instance's class
34 hierarchy
35
36 Instance-originating Preserved Methods retain their original context
37 attribute
38
39 There may be some scope for simplifying the above, to the detriment of Python
40 compatibility, since the unbound vs. bound methods situation can be confusing.
41
42 Acquiring Values
43 ----------------
44
45 According to the table describing value acquisition, different instructions
46 must implement different operations when acquiring values:
47
48 Instruction Purpose Context Operations
49 ----------- ------- ------------------
50
51 LoadConst Load class, function, Combine null context with loaded
52 module, constant object
53
54 LoadAddress Load attribute from Classes, functions and modules
55 known object cause the loaded attribute to be
56 retrieved unchanged; whereas
57 constants (representing instances)
58 cause the constant to override the
59 attribute's own context (since all
60 attributes should belong to the
61 constant's class hierarchy)
62
63 LoadAddressContext Override loaded context with a
64 predetermined object (provided
65 that the object and context are
66 compatible, which can be tested at
67 compile-time)
68
69 LoadAttr Load attribute from Attributes with null contexts or
70 instance contexts compatible with the
71 instance cause loaded attributes
72 to combine the instance as context
73 with the object from the
74 attribute; other attributes have
75 their context preserved
76
77 LoadAttrIndex Load attribute from Functions and modules as unknown
78 object the unknown object accessor cause
79 the loaded attribute to be
80 retrieved unchanged; classes and
81 instances cause the LoadAttr rules
82 to apply (class compatibility
83 applies)
84
85 A certain amount of run-time testing might be required for both LoadAttr and
86 LoadAttrIndex instructions. However, with certain restrictions in place around
87 class attributes, some simplifications are possible:
88
89 * Since only class-originating attributes may cause context overriding, and
90 since class attributes may only be defined within class definitions, the
91 attributes whose context may be modified should be known at compile-time.
92 (These will be those attributes whose context agrees with their parent
93 class.)
94
95 * By recording a special context value for attributes whose context can be
96 overridden, this value can be tested efficiently at run-time where the
97 appropriate conditions are satisfied. (This special context value or
98 indicator will be present in the object table record for the attribute.)
99
100 * It should be possible to move the instance compatibility condition testing
101 to compile-time by testing the compatibility of the origin of an attribute
102 with the class on which it is stored. However, some compatibility testing
103 will still be required if invoking methods via classes, since the instance
104 will be specified in the argument list instead of being presented in an
105 attribute lookup instruction.
106
107 Storing Values
108 --------------
109
110 According to the table describing value acquisition, different instructions
111 must implement different operations when acquiring values:
112
113 Instruction Purpose Context Operations
114 ----------- ------- ------------------
115
116 StoreAddress Store attribute in a Preserve context; note that no
117 known object test for class attribute
118 assignment should be necessary
119 since this instruction should only
120 be generated for module globals
121
122 StoreAttr Store attribute in an Preserve context; note that no
123 instance test for class attribute
124 assignment should be necessary
125 since this instruction should only
126 be generated for self accesses
127
128 StoreAttrIndex Store attribute in an Preserve context; since the index
129 unknown object lookup could yield a class
130 attribute, a test of the nature of
131 the nature of the structure is
132 necessary in order to prevent
133 assignments to classes
134
135 Objects
136 -------
137
138 Since classes, functions and instances are all "objects", each must support
139 certain features and operations in the same way.
140
141 The __class__ Attribute
142 -----------------------
143
144 All objects support the __class__ attribute:
145
146 Class: refers to the type class (type.__class__ also refers to the type class)
147 Function: refers to the function class
148 Instance: refers to the class instantiated to make the object
149
150 Invocation
151 ----------
152
153 The following actions need to be supported:
154
155 Class: create instance, call __init__ with instance, return object
156 Function: call function body, return result
157 Instance: call __call__ method, return result
158
159 Structure Layout
160 ----------------
161
162 A suitable structure layout might be something like this:
163
164 Identifier Identifier Address Details Type Object ...
165
166 0 1 2 3 4 5 6
167 classcode attrcode invocation invocation __class__ attribute ...
168 reference #args, reference reference
169 defaults
170 reference
171
172 Here, the classcode refers to the attribute lookup table for the object. Since
173 classes and instances share the same classcode, they might resemble the
174 following:
175
176 Class C:
177
178 0 1 2 3 4 5 6
179 code for C attrcode __new__ __new__ class type attribute ...
180 for C reference #args, reference reference
181 defaults
182 reference
183
184 Instance of C:
185
186 0 1 2 3 4 5 6
187 code for C attrcode C.__call__ C.__call__ class C attribute ...
188 for C reference #args, reference reference
189 (if exists) defaults
190 reference
191
192 The __new__ reference would lead to code consisting of the following
193 instructions:
194
195 create instance for C
196 call C.__init__(instance, ...)
197 return instance
198
199 If C has a __call__ attribute, the invocation "slot" of C instances would
200 refer to the same thing as C.__call__.
201
202 For functions, the same general layout applies:
203
204 Function f:
205
206 0 1 2 3 4 5 6
207 code for attrcode code code class attribute ...
208 function for reference #args, function (default)
209 function defaults reference reference
210 reference
211
212 Here, the code reference would lead to code for the function. Note that the
213 function locals are completely distinct from this structure and are not
214 comparable to attributes. Instead, attributes are reserved for default
215 parameter values.
216
217 For modules, there is no meaningful invocation reference:
218
219 Module m:
220
221 0 1 2 3 4 5 6
222 code for m attrcode (unused) (unused) module type attribute ...
223 for m reference (global)
224 reference
225
226 Both classes and modules have code in their definitions, but this would be
227 generated in order and not referenced externally.
228
229 Invocation Operation
230 --------------------
231
232 Consequently, regardless of the object an invocation is always done as
233 follows:
234
235 get invocation reference (at object+1)
236 jump to reference
237
238 Additional preparation is necessary before the above code: positional
239 arguments must be saved to the parameter stack, and keyword arguments must be
240 resolved and saved to the appropriate position in the parameter stack.
241
242 Attribute Operations
243 --------------------
244
245 Attribute access needs to go through the attribute lookup table. Some
246 optimisations are possible and are described in the appropriate section.
247
248 One important aspect of attribute access is the appropriate setting of the
249 context in the acquired attribute value. From the table describing the
250 acquisition of values, it is clear that the principal exception is that where
251 a class-originating attribute is accessed on an instance. Consequently, the
252 following algorithm could be employed once an attribute has been located:
253
254 1. If the attribute's context is a special value, indicating that it should
255 be replaced upon instance access, then proceed to the next step;
256 otherwise, acquire both the context and the object as they are.
257
258 2. If the accessor is an instance, use that as the value's context, acquiring
259 only the object from the attribute.
260
261 Where accesses can be determined ahead of time (as discussed in the
262 optimisations section), the above algorithm may not necessarily be employed in
263 the generated code for some accesses.
264
265 Instance/Class Compatibility
266 ----------------------------
267
268 Although it would be possible to have a data structure mapping classes to
269 compatible classes, which in the case of context (or self argument)
270 suitability in invocations would involve a mapping from a class to itself plus
271 its descendants, the need to retain the key to such a data structure for each
272 class might introduce a noticeable overhead. Such a structure would
273 effectively be a matrix with each dimension indexed using the same sequence of
274 codes for each of the classes in a program.
275
276 An alternative might be to insert descendants as special attributes into the
277 object/attribute lookup table. This would also require an extra key to be
278 retained, since each class would have its own attribute code.
