paul@15 | 1 | Namespace Definition
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paul@15 | 2 | ====================
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paul@15 | 3 |
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paul@15 | 4 | Module attributes are defined either at the module level or by global
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paul@15 | 5 | statements.
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paul@15 | 6 |
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paul@15 | 7 | Class attributes are defined only within class statements.
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paul@15 | 8 |
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paul@15 | 9 | Instance attributes are defined only by assignments to attributes of self
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paul@15 | 10 | within __init__ methods.
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paul@15 | 11 |
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paul@11 | 12 | Data Structures
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paul@11 | 13 | ===============
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paul@11 | 14 |
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paul@11 | 15 | Since classes, functions and instances are all "objects", each must support
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paul@11 | 16 | certain features and operations in the same way.
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paul@11 | 17 |
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paul@11 | 18 | The __class__ Attribute
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paul@11 | 19 | -----------------------
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paul@11 | 20 |
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paul@11 | 21 | All objects support the __class__ attribute:
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paul@11 | 22 |
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paul@11 | 23 | Class: refers to the type class (type.__class__ also refers to the type class)
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paul@11 | 24 | Function: refers to the function class
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paul@11 | 25 | Instance: refers to the class instantiated to make the object
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paul@11 | 26 |
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paul@11 | 27 | Invocation
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paul@11 | 28 | ----------
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paul@11 | 29 |
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paul@11 | 30 | The following actions need to be supported:
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paul@11 | 31 |
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paul@11 | 32 | Class: create instance, call __init__ with instance, return object
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paul@11 | 33 | Function: call function body, return result
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paul@11 | 34 | Instance: call __call__ method, return result
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paul@11 | 35 |
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paul@11 | 36 | Structure Layout
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paul@11 | 37 | ----------------
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paul@11 | 38 |
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paul@11 | 39 | A suitable structure layout might be something like this:
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paul@11 | 40 |
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paul@11 | 41 | 0 1 2 3 4
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paul@11 | 42 | classcode invocation __class__ attribute ...
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paul@11 | 43 | reference reference reference
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paul@11 | 44 |
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paul@11 | 45 | Here, the classcode refers to the attribute lookup table for the object. Since
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paul@11 | 46 | classes and instances share the same classcode, they might resemble the
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paul@11 | 47 | following:
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paul@11 | 48 |
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paul@11 | 49 | Class C:
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paul@11 | 50 |
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paul@11 | 51 | 0 1 2 3 4
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paul@11 | 52 | code for C __new__ class type attribute ...
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paul@11 | 53 | reference reference reference
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paul@11 | 54 |
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paul@11 | 55 | Instance of C:
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paul@11 | 56 |
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paul@11 | 57 | 0 1 2 3 4
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paul@11 | 58 | code for C C.__call__ class C attribute ...
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paul@11 | 59 | reference reference reference
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paul@11 | 60 | (if exists)
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paul@11 | 61 |
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paul@11 | 62 | The __new__ reference would lead to code consisting of the following
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paul@11 | 63 | instructions:
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paul@11 | 64 |
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paul@11 | 65 | create instance for C
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paul@11 | 66 | call C.__init__(instance, ...)
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paul@11 | 67 | return instance
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paul@11 | 68 |
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paul@11 | 69 | If C has a __call__ attribute, the invocation "slot" of C instances would
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paul@11 | 70 | refer to the same thing as C.__call__.
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paul@11 | 71 |
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paul@11 | 72 | For functions, the same general layout applies:
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paul@11 | 73 |
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paul@11 | 74 | Function f:
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paul@11 | 75 |
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paul@11 | 76 | 0 1 2 3 4
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paul@11 | 77 | code for code class attribute ...
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paul@11 | 78 | function reference function reference
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paul@11 | 79 | reference
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paul@11 | 80 |
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paul@11 | 81 | Here, the code reference would lead to code for the function.
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paul@11 | 82 |
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paul@11 | 83 | Invocation Operation
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paul@11 | 84 | --------------------
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paul@11 | 85 |
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paul@11 | 86 | Consequently, regardless of the object an invocation is always done as
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paul@11 | 87 | follows:
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paul@11 | 88 |
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paul@11 | 89 | get invocation reference (at object+1)
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paul@11 | 90 | jump to reference
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paul@11 | 91 |
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paul@11 | 92 | Additional preparation is necessary before the above code: positional
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paul@11 | 93 | arguments must be saved to the parameter stack, and keyword arguments must be
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paul@11 | 94 | resolved and saved to the appropriate position in the parameter stack.
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paul@11 | 95 |
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paul@11 | 96 | Attribute Operations
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paul@11 | 97 | --------------------
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paul@11 | 98 |
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paul@11 | 99 | Attribute access needs to go through the attribute lookup table.
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paul@21 | 100 |
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paul@21 | 101 | Instruction Evaluation Model
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paul@21 | 102 | ============================
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paul@21 | 103 |
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paul@21 | 104 | Programs use a value stack where evaluated instructions may save their
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paul@21 | 105 | results. A value stack pointer indicates the top of this stack. In addition, a
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paul@21 | 106 | separate stack is used to record the invocation frames. All stack pointers
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paul@21 | 107 | refer to the next address to be used by the stack, not the address of the
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paul@21 | 108 | uppermost element.
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paul@21 | 109 |
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paul@21 | 110 | Frame Stack Value Stack
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paul@21 | 111 | ----------- ----------- Address of Callable
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paul@21 | 112 | -------------------
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paul@21 | 113 | previous ...
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paul@21 | 114 | current ------> callable -----> identifier
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paul@21 | 115 | arg1 reference to code
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paul@21 | 116 | arg2
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paul@21 | 117 | arg3
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paul@21 | 118 | local4
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paul@21 | 119 | local5
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paul@21 | 120 | ...
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paul@21 | 121 |
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paul@21 | 122 | Loading local names is a matter of performing frame-relative accesses to the
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paul@21 | 123 | value stack.
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paul@21 | 124 |
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paul@21 | 125 | Invocations and Argument Evaluation
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paul@21 | 126 | -----------------------------------
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paul@21 | 127 |
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paul@21 | 128 | When preparing for an invocation, the caller first sets the invocation frame
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paul@21 | 129 | pointer. Then, positional arguments are added to the stack such that the first
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paul@21 | 130 | argument positions are filled. A number of stack locations for the remaining
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paul@21 | 131 | arguments specified in the program are then reserved. The names of keyword
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paul@21 | 132 | arguments are used (in the form of table index values) to consult the
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paul@21 | 133 | parameter table and to find the location in which such arguments are to be
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paul@21 | 134 | stored.
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paul@21 | 135 |
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paul@21 | 136 | fn(a, b, d=1, e=2, c=3) -> fn(a, b, c, d, e)
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paul@21 | 137 |
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paul@21 | 138 | Value Stack
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paul@21 | 139 | -----------
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paul@21 | 140 |
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paul@21 | 141 | ... ... ... ...
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paul@21 | 142 | fn fn fn fn
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paul@21 | 143 | a a a a
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paul@21 | 144 | b b b b
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paul@21 | 145 | ___ ___ ___ --> 3
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paul@21 | 146 | ___ --> 1 1 | 1
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paul@21 | 147 | ___ | ___ --> 2 | 2
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paul@21 | 148 | 1 ----------- 2 ----------- 3 -----------
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paul@21 | 149 |
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paul@21 | 150 | Conceptually, the frame can be considered as a collection of attributes, as
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paul@21 | 151 | seen in other kinds of structures:
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paul@21 | 152 |
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paul@21 | 153 | Frame for invocation of fn:
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paul@21 | 154 |
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paul@21 | 155 | 0 1 2 3 4 5
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paul@21 | 156 | code a b c d e
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paul@21 | 157 | reference
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paul@21 | 158 |
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paul@21 | 159 | However, where arguments are specified positionally, such "attributes" are not
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paul@21 | 160 | set using a comparable approach to that employed with other structures.
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paul@21 | 161 | Keyword arguments are set using an attribute-like mechanism, though, where the
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paul@21 | 162 | position of each argument discovered using the parameter table.
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paul@21 | 163 |
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paul@21 | 164 | Tuples, Frames and Allocation
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paul@21 | 165 | -----------------------------
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paul@21 | 166 |
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paul@21 | 167 | Using the approach where arguments are treated like attributes in some kind of
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paul@21 | 168 | structure, we could choose to allocate frames in places other than a stack.
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paul@21 | 169 | This would produce something somewhat similar to a plain tuple object.
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paul@23 | 170 |
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paul@23 | 171 | Optimisations
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paul@23 | 172 | =============
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paul@23 | 173 |
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paul@29 | 174 | Some optimisations around constant objects might be possible; these depend on
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paul@29 | 175 | the following:
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paul@29 | 176 |
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paul@29 | 177 | * Reliable tracking of assignments: where assignment operations occur, the
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paul@29 | 178 | target of the assignment should be determined if any hope of optimisation
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paul@29 | 179 | is to be maintained. Where no guarantees can be made about the target of
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paul@29 | 180 | an assignment, no assignment-related information should be written to
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paul@29 | 181 | potential targets.
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paul@29 | 182 |
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paul@29 | 183 | * Objects acting as "containers" of attributes must be regarded as "safe":
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paul@29 | 184 | where assignments are recorded as occurring on an attribute, it must be
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paul@29 | 185 | guaranteed that no other unforeseen ways exist to assign to such
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paul@29 | 186 | attributes.
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paul@29 | 187 |
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paul@29 | 188 | The discussion below presents certain rules which must be imposed to uphold
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paul@29 | 189 | the above requirements.
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paul@29 | 190 |
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paul@28 | 191 | Constant Attributes
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paul@28 | 192 | -------------------
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paul@28 | 193 |
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paul@23 | 194 | Where attributes of modules, classes and instances are only set once and are
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paul@23 | 195 | effectively constant, it should be possible to circumvent the attribute lookup
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paul@28 | 196 | mechanism and to directly reference the attribute value. This technique may
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paul@28 | 197 | only be considered applicable in the following cases for "container" objects:
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paul@28 | 198 |
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paul@28 | 199 | 1. For modules, provided that assignments to module attributes are only
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paul@28 | 200 | permitted within the module itself either at the top-level or via names
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paul@28 | 201 | declared as globals. Thus, the following would not be permitted:
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paul@28 | 202 |
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paul@28 | 203 | another_module.this_module.attr = value
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paul@28 | 204 |
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paul@29 | 205 | In the above, this_module is a reference to the current module.
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paul@28 | 206 |
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paul@28 | 207 | 2. For classes, provided that assignments to class attributes are only
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paul@28 | 208 | permitted within the class definition, outside methods. This would mean
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paul@28 | 209 | that classes would be "sealed" at definition time (like functions).
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paul@28 | 210 |
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paul@28 | 211 | Unlike the property of function locals that they may only sensibly be accessed
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paul@28 | 212 | within the function in which they reside, these cases demand additional
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paul@28 | 213 | controls or assumptions on or about access to the stored data. Meanwhile, it
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paul@28 | 214 | would be difficult to detect eligible attributes on arbitrary instances due to
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paul@28 | 215 | the need for some kind of type inference or abstract execution.
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paul@28 | 216 |
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paul@28 | 217 | Where (1) and (2) apply, any attribute with only one recorded assignment on it
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paul@28 | 218 | can be considered a constant attribute and this eligible for this
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paul@28 | 219 | optimisation, the consequence of which would be the replacement of a
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paul@28 | 220 | LoadAttrIndex instruction (which needs to look up an attribute using the
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paul@28 | 221 | run-time details of the "container" and the compile-time details of the
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paul@28 | 222 | attribute) with a LoadAttr instruction.
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paul@28 | 223 |
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paul@29 | 224 | Additional Controls
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paul@29 | 225 | -------------------
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paul@29 | 226 |
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paul@29 | 227 | For the above cases for "container" objects, the following controls would need
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paul@29 | 228 | to apply:
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paul@29 | 229 |
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paul@29 | 230 | 1. Modules would need to be immutable after initialisation. However, during
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paul@29 | 231 | initialisation, there remains a possibility of another module attempting
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paul@29 | 232 | to access the original module. For example, if ppp/__init__.py contained
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paul@29 | 233 | the following...
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paul@29 | 234 |
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paul@29 | 235 | x = 1
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paul@29 | 236 | import ppp.qqq
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paul@29 | 237 | print x
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paul@29 | 238 |
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paul@29 | 239 | ...and if ppp/qqq.py contained the following...
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paul@29 | 240 |
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paul@29 | 241 | import ppp
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paul@29 | 242 | ppp.x = 2
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paul@29 | 243 |
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paul@29 | 244 | ...then the value 2 would be printed. Since modules are objects which are
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paul@29 | 245 | registered globally in a program, it would be possible to set attributes
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paul@29 | 246 | in the above way.
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paul@29 | 247 |
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paul@29 | 248 | 2. Classes would need to be immutable after initialisation. However, since
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paul@29 | 249 | classes are objects, any reference to a class after initialisation could
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paul@29 | 250 | be used to set attributes on the class.
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paul@29 | 251 |
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paul@29 | 252 | Solutions:
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paul@29 | 253 |
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paul@29 | 254 | 1. Insist on global scope for module attribute assignments.
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paul@29 | 255 |
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paul@29 | 256 | 2. Insist on local scope within classes.
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paul@29 | 257 |
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paul@29 | 258 | Both of the above measures need to be enforced at run-time, since an arbitrary
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paul@29 | 259 | attribute assignment could be attempted on any kind of object, yet to uphold
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paul@29 | 260 | the properties of "safe containers", attempts to change attributes of such
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paul@29 | 261 | objects should be denied. Since foreseen attribute assignment operations have
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paul@29 | 262 | certain properties detectable at compile-time, it could be appropriate to
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paul@29 | 263 | generate special instructions (or modified instructions) during the
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paul@29 | 264 | initialisation of modules and classes for such foreseen assignments, whilst
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paul@29 | 265 | employing normal attribute assignment operations in all other cases. Indeed,
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paul@29 | 266 | the StoreAttr instruction, which is used to set attributes in "safe
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paul@29 | 267 | containers" would be used exclusively for this purpose; the StoreAttrIndex
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paul@29 | 268 | instruction would be used exclusively for all other attribute assignments.
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paul@29 | 269 |
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paul@28 | 270 | Constant Attribute Values
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paul@28 | 271 | -------------------------
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paul@28 | 272 |
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paul@29 | 273 | Where an attribute value is itself regarded as constant, is a "safe container"
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paul@29 | 274 | and is used in an operation accessing its own attributes, the value can be
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paul@29 | 275 | directly inspected for optimisations or employed in the generated code. For
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paul@29 | 276 | the attribute values themselves, only objects of a constant nature may be
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paul@28 | 277 | considered suitable for this particular optimisation:
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paul@28 | 278 |
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paul@28 | 279 | * Classes
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paul@28 | 280 | * Modules
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paul@28 | 281 | * Instances defined as constant literals
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paul@28 | 282 |
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paul@28 | 283 | This is because arbitrary objects (such as most instances) have no
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paul@28 | 284 | well-defined form before run-time and cannot be investigated further at
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paul@28 | 285 | compile-time or have a representation inserted into the generated code.
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paul@29 | 286 |
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paul@29 | 287 | Class Attributes and Access via Instances
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paul@29 | 288 | -----------------------------------------
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paul@29 | 289 |
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paul@29 | 290 | Unlike module attributes, class attributes can be accessed in a number of
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paul@29 | 291 | different ways:
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paul@29 | 292 |
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paul@29 | 293 | * Using the class itself:
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paul@29 | 294 |
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paul@29 | 295 | C.x = 123
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paul@29 | 296 | cls = C; cls.x = 234
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paul@29 | 297 |
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paul@29 | 298 | * Using a subclass of the class (for reading attributes):
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paul@29 | 299 |
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paul@29 | 300 | class D(C):
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paul@29 | 301 | pass
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paul@29 | 302 | D.x # setting D.x would populate D, not C
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paul@29 | 303 |
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paul@29 | 304 | * Using instances of the class or a subclass of the class (for reading
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paul@29 | 305 | attributes):
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paul@29 | 306 |
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paul@29 | 307 | c = C()
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paul@29 | 308 | c.x # setting c.x would populate c, not C
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paul@29 | 309 |
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paul@29 | 310 | Since assignments are only achieved using direct references to the class, and
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paul@29 | 311 | since class attributes should be defined only within the class initialisation
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paul@29 | 312 | process, the properties of class attributes should be consistent with those
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paul@29 | 313 | desired.
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paul@29 | 314 |
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paul@29 | 315 | Method Access via Instances
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paul@29 | 316 | ---------------------------
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paul@29 | 317 |
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paul@29 | 318 | It is desirable to optimise method access, even though most method calls are
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paul@29 | 319 | likely to occur via instances. It is possible, given the properties of methods
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paul@29 | 320 | as class attributes to investigate the kind of instance that the self
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paul@29 | 321 | parameter/local refers to within each method: it should be an instance either
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paul@29 | 322 | of the class in which the method is defined or a compatible class, although
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paul@29 | 323 | situations exist where this might not be the case:
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paul@29 | 324 |
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paul@29 | 325 | * Explicit invocation of a method:
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paul@29 | 326 |
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paul@29 | 327 | d = D() # D is not related to C
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paul@29 | 328 | C.f(d) # calling f(self) in C
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paul@29 | 329 |
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paul@29 | 330 | Optimising Function Invocations
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paul@29 | 331 | -------------------------------
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paul@29 | 332 |
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paul@29 | 333 | Where an attribute value is itself regarded as constant and is a function,
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paul@29 | 334 | knowledge about the parameters of the function can be employed to optimise the
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paul@29 | 335 | preparation of the invocation frame.
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