1 Optimisations
2 =============
3
4 Some optimisations around constant objects might be possible; these depend on
5 the following:
6
7 * Reliable tracking of assignments: where assignment operations occur, the
8 target of the assignment should be determined if any hope of optimisation
9 is to be maintained. Where no guarantees can be made about the target of
10 an assignment, no assignment-related information should be written to
11 potential targets.
12
13 * Objects acting as "containers" of attributes must be regarded as "safe":
14 where assignments are recorded as occurring on an attribute, it must be
15 guaranteed that no other unforeseen ways exist to assign to such
16 attributes.
17
18 The discussion below presents certain rules which must be imposed to uphold
19 the above requirements.
20
21 Safe Containers
22 ---------------
23
24 Where attributes of modules, classes and instances are only set once and are
25 effectively constant, it should be possible to circumvent the attribute lookup
26 mechanism and to directly reference the attribute value. This technique may
27 only be considered applicable for the following "container" objects, subject
28 to the noted constraints:
29
30 1. For modules, "safety" is enforced by ensuring that assignments to module
31 attributes are only permitted within the module itself either at the
32 top-level or via names declared as globals. Thus, the following would not
33 be permitted:
34
35 another_module.this_module.attr = value
36
37 In the above, this_module is a reference to the current module.
38
39 2. For classes, "safety" is enforced by ensuring that assignments to class
40 attributes are only permitted within the class definition, outside
41 methods. This would mean that classes would be "sealed" at definition time
42 (like functions).
43
44 Unlike the property of function locals that they may only sensibly be accessed
45 within the function in which they reside, these cases demand additional
46 controls or assumptions on or about access to the stored data. Meanwhile, it
47 would be difficult to detect eligible attributes on arbitrary instances due to
48 the need for some kind of type inference or abstract execution.
49
50 Constant Attributes
51 -------------------
52
53 When accessed via "safe containers", as described above, any attribute with
54 only one recorded assignment on it can be considered a constant attribute and
55 this eligible for optimisation, the consequence of which would be the
56 replacement of a LoadAttrIndex instruction (which needs to look up an
57 attribute using the run-time details of the "container" and the compile-time
58 details of the attribute) with a LoadAttr instruction.
59
60 However, some restrictions exist on assignment operations which may be
61 regarded to cause only one assignment in the lifetime of a program:
62
63 1. For module attributes, only assignments at the top-level outside loop
64 statements can be reliably assumed to cause only a single assignment.
65
66 2. For class attributes, only assignments at the top-level within class
67 definitions and outside loop statements can be reliably assumed to cause
68 only a single assignment.
69
70 All assignments satisfying the "safe container" requirements, but not the
71 requirements immediately above, should each be recorded as causing at least
72 one assignment.
73
74 Additional Controls
75 -------------------
76
77 For the above cases for "container" objects, the following controls would need
78 to apply:
79
80 1. Modules would need to be immutable after initialisation. However, during
81 initialisation, there remains a possibility of another module attempting
82 to access the original module. For example, if ppp/__init__.py contained
83 the following...
84
85 x = 1
86 import ppp.qqq
87 print x
88
89 ...and if ppp/qqq.py contained the following...
90
91 import ppp
92 ppp.x = 2
93
94 ...then the value 2 would be printed. Since modules are objects which are
95 registered globally in a program, it would be possible to set attributes
96 in the above way.
97
98 2. Classes would need to be immutable after initialisation. However, since
99 classes are objects, any reference to a class after initialisation could
100 be used to set attributes on the class.
101
102 Solutions:
103
104 1. Insist on global scope for module attribute assignments.
105
106 2. Insist on local scope within classes.
107
108 Both of the above measures need to be enforced at run-time, since an arbitrary
109 attribute assignment could be attempted on any kind of object, yet to uphold
110 the properties of "safe containers", attempts to change attributes of such
111 objects should be denied. Since foreseen attribute assignment operations have
112 certain properties detectable at compile-time, it could be appropriate to
113 generate special instructions (or modified instructions) during the
114 initialisation of modules and classes for such foreseen assignments, whilst
115 employing normal attribute assignment operations in all other cases. Indeed,
116 the StoreAttr instruction, which is used to set attributes in "safe
117 containers" would be used exclusively for this purpose; the StoreAttrIndex
118 instruction would be used exclusively for all other attribute assignments.
119
120 To ensure the "sealing" of modules and classes, entries in the attribute
121 lookup table would encode whether a class or module is being accessed, so
122 that the StoreAttrIndex instruction could reject such accesses.
123
124 Constant Attribute Values
125 -------------------------
126
127 Where an attribute value is itself regarded as constant, is a "safe container"
128 and is used in an operation accessing its own attributes, the value can be
129 directly inspected for optimisations or employed in the generated code. For
130 the attribute values themselves, only objects of a constant nature may be
131 considered suitable for this particular optimisation:
132
133 * Classes
134 * Modules
135 * Instances defined as constant literals
136
137 This is because arbitrary objects (such as most instances) have no
138 well-defined form before run-time and cannot be investigated further at
139 compile-time or have a representation inserted into the generated code.
140
141 Class Attributes and Access via Instances
142 -----------------------------------------
143
144 Unlike module attributes, class attributes can be accessed in a number of
145 different ways:
146
147 * Using the class itself:
148
149 C.x = 123
150 cls = C; cls.x = 234
151
152 * Using a subclass of the class (for reading attributes):
153
154 class D(C):
155 pass
156 D.x # setting D.x would populate D, not C
157
158 * Using instances of the class or a subclass of the class (for reading
159 attributes):
160
161 c = C()
162 c.x # setting c.x would populate c, not C
163
164 Since assignments are only achieved using direct references to the class, and
165 since class attributes should be defined only within the class initialisation
166 process, the properties of class attributes should be consistent with those
167 desired.
168
169 Method Access via Instances
170 ---------------------------
171
172 It is desirable to optimise method access, even though most method calls are
173 likely to occur via instances. It is possible, given the properties of methods
174 as class attributes to investigate the kind of instance that the self
175 parameter/local refers to within each method: it should be an instance either
176 of the class in which the method is defined or a compatible class, although
177 situations exist where this might not be the case:
178
179 * Explicit invocation of a method:
180
181 d = D() # D is not related to C
182 C.f(d) # calling f(self) in C
183
184 If blatant usage of incompatible instances were somehow disallowed, it would
185 still be necessary to investigate the properties of an instance's class and
186 its relationship with other classes. Consider the following example:
187
188 class A:
189 def f(self): ...
190
191 class B:
192 def f(self): ...
193 def g(self):
194 self.f()
195
196 class C(A, B):
197 pass
198
199 Here, instances of B passed into the method B.g could be assumed to provide
200 access to B.f when self.f is resolved at compile-time. However, instances of C
201 passed into B.g would instead provide access to A.f when self.f is resolved at
202 compile-time (since the method resolution order is C, A, B instead of just B).
203
204 One solution might be to specialise methods for each instance type, but this
205 could be costly. Another less ambitious solution might only involve the
206 optimisation of such internal method calls if an unambiguous target can be
207 resolved.
208
209 Narrowing Type Candidates using Attribute Names
210 -----------------------------------------------
211
212 Within functions, it might be useful to record attribute accesses on local
213 names and to collect the names in order to help resolve targets in the
214 generated code. For example:
215
216 def f(x, y):
217 x.method(y)
218 if x.something:
219 ...
220 return x.attr
221
222 Here, the object referenced via x should have "method", "something" and "attr"
223 as attributes. We could impose a test for a type providing these attributes at
224 the earliest opportunity and then specialise the attribute accesses, perhaps
225 also invocations, in the generated code. AttributeError occurrences would need
226 to be considered, however, potentially disqualifying certain attributes from
227 any optimisations, and control flow would also need to be considered. The
228 result would resemble the following:
229
230 def f(x, y):
231 if not isinstance(x, C) and x is not C:
232 raise TypeError
233 x.method(y)
234 if x.something:
235 ...
236 return x.attr
237
238 With more specific information about the attributes used with a given name,
239 combinations of attributes can be provided instead of single attributes when
240 recording attribute usage. For example, from the above:
241
242 x uses method -> indicates potential usage of C.method, D1.method, ...
243 x uses something -> indicates potential usage of C.something,
244 D2.something, ...
245 x uses attr -> indicates potential usage of C.attr, D3.attr, ...
246
247 These unspecific declarations of attribute usage can be replaced with the
248 following:
249
250 x uses method, something, attr -> indicates potential usage of C, D4, ...
251 (each providing all of the stated
252 attributes)
253
254 Reducing Object Table Scope
255 ---------------------------
256
257 Where attributes may be used in a program but never accessed via the object
258 table-dependent instructions, such attributes could be omitted from the object
259 table.
260
261 Implemented Optimisation Types
262 ==============================
263
264 Optimisation Prerequisites and Effect
265 ------------ ------------------------
266
267 constant_storage value instruction references a constant;
268 (guidance) storage instruction references a constant;
269 | indicate whether both instructions satisfy the
270 | preconditions and should be removed/omitted
271
272 constant_accessor value instruction references a constant;
273 (guidance) | target name provided (for use in producing an
274 | address access instruction)
275
276 known_target value instruction references a constant;
277 (guidance) | target and context are provided (no instructions
278 | are removed)
279
280 self_access value instruction references "self" in a method;
281 (guidance) specified attribute name always has the same
282 position;
283 | indicate whether an appropriate instruction can
284 | be generated for the access
285
286 temp_storage value instruction is a simple input operation;
287 (elimination) value instruction is the last instruction;
288 (guidance) | remove the value instruction, provide the value
289 | instruction in place of a temporary storage
290 | reference
291
292 no_operations input to the current instruction loads from the
293 (guidance) destination of the current instruction;
294 | indicate that the current instruction should be
295 | omitted
296
297 unused_results value instruction is a simple input operation;
298 (elimination) value instruction is the final instruction of a
299 discarded expression;
300 | remove the value instruction
301
302 unused_objects attribute is defined using a name which is not
303 (inspection) used in an active region of the program or is
304 defined within a class which is not used by
305 the program, object is unambiguously
306 referenced by such attributes
307 | remove such attributes and objects
