1 Concepts
2 ========
3
4 This document describes the underlying concepts employed in micropython.
5
6 * Namespaces and attribute definition
7 * Contexts and values
8 * Tables, attributes and lookups
9 * Objects and structures
10 * Parameters and lookups
11 * Instantiation
12 * Register usage
13 * List and tuple representations
14
15 Namespaces and Attribute Definition
16 ===================================
17
18 Namespaces are any objects which can retain attributes.
19
20 * Module attributes are defined either at the module level or by global
21 statements.
22 * Class attributes are defined only within class statements.
23 * Instance attributes are defined only by assignments to attributes of self
24 within __init__ methods.
25
26 These restrictions apply because such attributes are thus explicitly declared,
27 permitting the use of tables (described below). Module and class attributes
28 can also be finalised in this way in order to permit certain optimisations.
29
30 An additional restriction required for the current implementation of tables
31 (as described below) applies to class definitions: each class must be defined
32 using a unique name; repeated definition of classes having the same name is
33 thus not permitted. This restriction arises from the use of the "full name" of
34 a class as a key to the object table, where the full name is a qualified path
35 via the module hierarchy ending with the name of the class.
36
37 See rejected.txt for complicating mechanisms which could be applied to
38 mitigate the effects of these restrictions on optimisations.
39
40 Contexts and Values
41 ===================
42
43 Values are used as the common reference representation in micropython: as
44 stored representations of attributes (of classes, instances, modules, and
45 other objects supporting attribute-like entities) as well as the stored values
46 associated with names in functions and methods.
47
48 Unlike other implementations, micropython does not create things like bound
49 method objects for individual instances. Instead, all objects are referenced
50 using a context, reference pair:
51
52 Value Layout
53 ------------
54
55 0 1
56 context object
57 reference reference
58
59 Specific implementations might reverse this ordering for optimisation
60 purposes.
61
62 Rationale
63 ---------
64
65 To reduce the number of created objects whilst retaining the ability to
66 support bound method invocations. The context indicates the context in which
67 an invocation is performed, typically the owner of the method.
68
69 Usage
70 -----
71
72 The context may be inserted as the first argument when a value is involved in
73 an invocation. This argument may then be omitted from the invocation if its
74 usage is not appropriate.
75
76 See invocation.txt for details.
77
78 Context Value Types
79 -------------------
80
81 The following types of context value exist:
82
83 Type Usage Transformations
84 ---- ----- ---------------
85
86 Replaceable With functions (not methods) May be replaced with an
87 instance or a class when a
88 value is stored on an
89 instance or class
90
91 Placeholder With classes May not be replaced
92
93 Instance With instances (and constants) May not be replaced
94 or functions as methods
95
96 Class With functions as methods May be replaced when a
97 value is loaded from a
98 class attribute via an
99 instance
100
101 Contexts in Acquired Values
102 ---------------------------
103
104 There are four classes of instructions which provide values:
105
106 Instruction Purpose Context Operations
107 ----------- ------- ------------------
108
109 1) LoadConst Load module, constant Use loaded object with itself
110 as context
111
112 2) LoadFunction Load function Combine replaceable context
113 with loaded object
114
115 3) LoadClass Load class Combine placeholder context
116 with loaded object
117
118 4) LoadAddress* Load attribute from Preserve or override stored
119 LoadAttr* class, module, context (as described in
120 instance assignment.txt)
121
122 In order to comply with traditional Python behaviour, contexts may or may not
123 represent the object from which an attribute has been acquired.
124
125 See assignment.txt for details.
126
127 Contexts in Stored Values
128 -------------------------
129
130 There are two classes of instruction for storing values:
131
132 Instruction Purpose Context Operations
133 ----------- ------- ------------------
134
135 1) StoreAddress Store attribute in a Preserve context; note that no
136 known object test for class attribute
137 assignment should be necessary
138 since this instruction should only
139 be generated for module globals
140
141 StoreAttr Store attribute in an Preserve context; note that no
142 instance test for class attribute
143 assignment should be necessary
144 since this instruction should only
145 be generated for self accesses
146
147 StoreAttrIndex Store attribute in an Preserve context; since the index
148 unknown object lookup could yield a class
149 attribute, a test of the nature of
150 the nature of the structure is
151 necessary in order to prevent
152 assignments to classes
153
154 2) StoreAddressContext Store attribute in a Override context if appropriate;
155 known object if the value has a replaceable
156 context, permit the target to
157 take ownership of the value
158
159 See assignment.txt for details.
160
161 Tables, Attributes and Lookups
162 ==============================
163
164 Attribute lookups, where the exact location of an object attribute is deduced,
165 are performed differently in micropython than in other implementations.
166 Instead of providing attribute dictionaries, in which attributes are found,
167 attributes are located at fixed places in object structures (described below)
168 and their locations are stored using a special representation known as a
169 table.
170
171 For a given program, a table can be considered as being like a matrix mapping
172 classes to attribute names. For example:
173
174 class A:
175 # instances have attributes x, y
176
177 class B(A):
178 # introduces attribute z for instances
179
180 class C:
181 # instances have attributes a, b, z
182
183 This would provide the following table, referred to as an object table in the
184 context of classes and instances:
185
186 Class/attr a b x y z
187
188 A 1 2
189 B 1 2 3
190 C 1 2 3
191
192 A limitation of this representation is that instance attributes may not shadow
193 class attributes: if an attribute with a given name is not defined on an
194 instance, an attribute with the same name cannot be provided by the class of
195 the instance or any superclass of the instance's class.
196
197 The table can be compacted using a representation known as a displacement
198 list (referred to as an object list in this context):
199
200 Classes with attribute offsets
201
202 classcode A
203 attrcode a b x y z
204
205 B
206 a b x y z
207
208 C
209 a b x y z
210
211 List . . 1 2 1 2 3 1 2 . . 3
212
213 Here, the classcode refers to the offset in the list at which a class's
214 attributes are defined, whereas the attrcode defines the offset within a
215 region of attributes corresponding to a single attribute of a given name.
216
217 Attribute Locations
218 -------------------
219
220 The locations stored in table/list elements are for instance attributes
221 relative to the location of the instance, whereas those for class attributes
222 and modules are absolute addresses (although these could also be changed to
223 object-relative locations). Thus, each occupied table cell has the following
224 structure:
225
226 attrcode, uses-absolute-address, address (or location)
227
228 This could be given instead as follows:
229
230 attrcode, is-class-or-module, location
231
232 Since uses-absolute-address corresponds to is-class-or-module, and since there
233 is a need to test for classes and modules to prevent assignment to attributes
234 of such objects, this particular information is always required.
235
236 Comparing Tables as Matrices with Displacement Lists
237 ----------------------------------------------------
238
239 Although displacement lists can provide reasonable levels of compaction for
240 attribute data, the element size is larger than that required for a simple
241 matrix: the attribute code (attrcode) need not be stored since each element
242 unambiguously refers to the availability of an attribute for a particular
243 class or instance of that class, and so the data at a given element need not
244 be tested for relevance to a given attribute access operation.
245
246 Given a program with 20 object types and 100 attribute types, a matrix would
247 occupy the following amount of space:
248
249 number of object types * number of attribute types * element size
250 = 20 * 100 * 1 (assuming that a single location is sufficient for an element)
251 = 2000
252
253 In contrast, given a compaction to 40% of the matrix size (without considering
254 element size) in a displacement list, the amount of space would be as follows:
255
256 number of elements * element size
257 = 40% * (20 * 100) * 2 (assuming that one additional location is required)
258 = 1600
259
260 Consequently, the principal overhead of using a displacement list is likely to
261 be in the need to check element relevance when retrieving values from such a
262 list.
263
264 Objects and Structures
265 ======================
266
267 As well as references, micropython needs to have actual objects to refer to.
268 Since classes, functions and instances are all objects, it is desirable that
269 certain common features and operations are supported in the same way for all
270 of these things. To permit this, a common data structure format is used.
271
272 Header.................................................... Attributes.................
273
274 Identifier Identifier Address Identifier Size Object Object ...
275
276 0 1 2 3 4 5 6 7
277 classcode attrcode/ invocation funccode size __class__ attribute ...
278 instance reference reference reference
279 status
280
281 Classcode
282 ---------
283
284 Used in attribute lookup.
285
286 Here, the classcode refers to the attribute lookup table for the object (as
287 described above). Classes and instances share the same classcode, and their
288 structures reflect this. Functions all belong to the same type and thus employ
289 the classcode for the function built-in type, whereas modules have distinct
290 types since they must support different sets of attributes.
291
292 Attrcode
293 --------
294
295 Used to test instances for membership of classes (or descendants of classes).
296
297 Since, in traditional Python, classes are only ever instances of some generic
298 built-in type, support for testing such a relationship directly has been
299 removed and the attrcode is not specified for classes: the presence of an
300 attrcode indicates that a given object is an instance. In addition, support
301 has also been removed for testing modules in the same way, meaning that the
302 attrcode is also not specified for modules.
303
304 See the "Testing Instance Compatibility with Classes (Attrcode)" section below
305 for details of attrcodes.
306
307 Invocation Reference
308 --------------------
309
310 Used when an object is called.
311
312 This is the address of the code to be executed when an invocation is performed
313 on the object.
314
315 Funccode
316 --------
317
318 Used to look up argument positions by name.
319
320 The strategy with keyword arguments in micropython is to attempt to position
321 such arguments in the invocation frame as it is being constructed.
322
323 See the "Parameters and Lookups" section for more information.
324
325 Size
326 ----
327
328 Used to indicate the size of an object including attributes.
329
330 Attributes
331 ----------
332
333 For classes, modules and instances, the attributes in the structure correspond
334 to the attributes of each kind of object. For functions, however, the
335 attributes in the structure correspond to the default arguments for each
336 function, if any.
337
338 Structure Types
339 ---------------
340
341 Class C:
342
343 0 1 2 3 4 5 6 7
344 classcode (unused) __new__ funccode size class type attribute ...
345 for C reference for reference reference
346 instantiator
347
348 Instance of C:
349
350 0 1 2 3 4 5 6 7
351 classcode attrcode C.__call__ funccode size class C attribute ...
352 for C for C reference for reference reference
353 (if exists) C.__call__
354
355 Function f:
356
357 0 1 2 3 4 5 6 7
358 classcode attrcode code funccode size class attribute ...
359 for for reference function (default)
360 function function reference reference
361
362 Module m:
363
364 0 1 2 3 4 5 6 7
365 classcode attrcode (unused) (unused) (unused) module type attribute ...
366 for m for m reference (global)
367 reference
368
369 The __class__ Attribute
370 -----------------------
371
372 All objects support the __class__ attribute and this is illustrated above with
373 the first attribute.
374
375 Class: refers to the type class (type.__class__ also refers to the type class)
376 Function: refers to the function class
377 Instance: refers to the class instantiated to make the object
378
379 Lists and Tuples
380 ----------------
381
382 The built-in list and tuple sequences employ variable length structures using
383 the attribute locations to store their elements, where each element is a
384 reference to a separately stored object.
385
386 Testing Instance Compatibility with Classes (Attrcode)
387 ------------------------------------------------------
388
389 Although it would be possible to have a data structure mapping classes to
390 compatible classes, such as a matrix indicating the subclasses (or
391 superclasses) of each class, the need to retain the key to such a data
392 structure for each class might introduce a noticeable overhead.
393
394 Instead of having a separate structure, descendant classes of each class are
395 inserted as special attributes into the object table. This requires an extra
396 key to be retained, since each class must provide its own attribute code such
397 that upon an instance/class compatibility test, the code may be obtained and
398 used in the object table.
399
400 Invocation and Code References
401 ------------------------------
402
403 Modules: there is no meaningful invocation reference since modules cannot be
404 explicitly called.
405
406 Functions: a simple code reference is employed pointing to code implementing
407 the function. Note that the function locals are completely distinct from this
408 structure and are not comparable to attributes. Instead, attributes are
409 reserved for default parameter values, although they do not appear in the
410 object table described above, appearing instead in a separate parameter table
411 described below.
412
413 Classes: given that classes must be invoked in order to create instances, a
414 reference must be provided in class structures. However, this reference does
415 not point directly at the __init__ method of the class. Instead, the
416 referenced code belongs to a special initialiser function, __new__, consisting
417 of the following instructions:
418
419 create instance for C
420 call C.__init__(instance, ...)
421 return instance
422
423 Instances: each instance employs a reference to any __call__ method defined in
424 the class hierarchy for the instance, thus maintaining its callable nature.
425
426 Both classes and modules may contain code in their definitions - the former in
427 the "body" of the class, potentially defining attributes, and the latter as
428 the "top-level" code in the module, potentially defining attributes/globals -
429 but this code is not associated with any invocation target. It is thus
430 generated in order of appearance and is not referenced externally.
431
432 Invocation Operation
433 --------------------
434
435 Consequently, regardless of the object an invocation is always done as
436 follows:
437
438 get invocation reference from the header
439 jump to reference
440
441 Additional preparation is necessary before the above code: positional
442 arguments must be saved in the invocation frame, and keyword arguments must be
443 resolved and saved to the appropriate position in the invocation frame.
444
445 See invocation.txt for details.
446
447 Parameters and Lookups
448 ======================
449
450 Since Python supports keyword arguments when making invocations, it becomes
451 necessary to record the parameter names associated with each function or
452 method. Just as object tables record attributes positions on classes and
453 instances, parameter tables record parameter positions in function or method
454 parameter lists.
455
456 For a given program, a parameter table can be considered as being like a
457 matrix mapping functions/methods to parameter names. For example:
458
459 def f(x, y, z):
460 pass
461
462 def g(a, b, c):
463 pass
464
465 def h(a, x):
466 pass
467
468 This would provide the following table, referred to as a parameter table in
469 the context of functions and methods:
470
471 Function/param a b c x y z
472
473 f 1 2 3
474 g 1 2 3
475 h 1 2
476
477 Confusion can occur when functions are adopted as methods, since the context
478 then occupies the first slot in the invocation frame:
479
480 def f(x, y, z):
481 pass
482
483 f(x=1, y=2, z=3) -> f(<context>, 1, 2, 3)
484 -> f(1, 2, 3)
485
486 class C:
487 f = f
488
489 def g(x, y, z):
490 pass
491
492 c = C()
493
494 c.f(y=2, z=3) -> f(<context>, 2, 3)
495 c.g(y=2, z=3) -> C.g(<context>, 2, 3)
496
497 Just as with parameter tables, a displacement list can be prepared from a
498 parameter table:
499
500 Functions with parameter (attribute) offsets
501
502 funccode f
503 attrcode a b c x y z
504
505 g
506 a b c x y z
507
508 h
509 a b c x y z
510
511 List . . . 1 2 3 1 2 3 1 . . 2 . .
512
513 Here, the funccode refers to the offset in the list at which a function's
514 parameters are defined, whereas the attrcode defines the offset within a
515 region of attributes corresponding to a single parameter of a given name.
516
517 Instantiation
518 =============
519
520 When instantiating classes, memory must be reserved for the header of the
521 resulting instance, along with locations for the attributes of the instance.
522 Since the instance header contains data common to all instances of a class, a
523 template header is copied to the start of the newly reserved memory region.
524
525 Register Usage
526 ==============
527
528 During code generation, much of the evaluation produces results which are
529 implicitly recorded in the "active value" register, and various instructions
530 will consume the active value. In addition, some instructions will consume a
531 separate "active source value" from a register, typically those which are
532 assigning the result of an expression to an assignment target.
533
534 Since values often need to be retained for later use, a set of temporary
535 storage locations are typically employed. However, optimisations may reduce
536 the need to use such temporary storage where instructions which provide the
537 "active value" can be re-executed and will produce the same result.
538
539 List and Tuple Representations
540 ==============================
541
542 Since tuples have a fixed size, the representation of a tuple instance is
543 merely a header describing the size of the entire object, together with a
544 sequence of references to the object "stored" at each position in the
545 structure. Such references consist of the usual context and reference pair.
546
547 Lists, however, have a variable size and must be accessible via an unchanging
548 location even as more memory is allocated elsewhere to accommodate the
549 contents of the list. Consequently, the representation must resemble the
550 following:
551
552 Structure header for list (size == header plus special attribute)
553 Special attribute referencing the underlying sequence
554
555 The underlying sequence has a fixed size, like a tuple, but may contain fewer
556 elements than the size of the sequence permits:
557
558 Special header indicating the current size and allocated size
559 Element
560 ... <-- current size
561 (Unused space)
562 ... <-- allocated size
563
564 This representation permits the allocation of a new sequence when space is
565 exhausted in an existing sequence, with the new sequence address stored in the
566 main list structure. Since access to the contents of the list must go through
567 the main list structure, underlying allocation activities may take place
568 without the users of a list having to be aware of such activities.
