micropython

TO_DO.txt

706:7d5601304487
2013-10-24 Paul Boddie Added a general typed instance class to represent instantiation results. syspython-as-target
     1 Name usage types: as parameters, as base classes, as callables. This potentially restricts
     2 attribute usage effects because names mentioned as base classes are not propagated and
     3 made freely available for use in attribute accesses.
     4 
     5 Low-Level Instructions and Macro Instructions
     6 =============================================
     7 
     8 Have contexts and values stored separately in memory. This involves eliminating DataValue
     9 and storing attributes using two words.
    10 
    11 Migrate macro instructions such as the *Index instructions to library code implemented
    12 using low-level instructions.
    13 
    14 Consider introducing classic machine level instructions (word addition, subtraction, and
    15 so on) in order to implement all current RSVP instructions.
    16 
    17 Move common code sequences to a library routine, such as the context checking that occurs
    18 in functions and methods.
    19 
    20 Dataflow Optimisations
    21 ======================
    22 
    23 Assignments, particularly now that no result register exists, may cause StoreTemp/LoadTemp
    24 instruction pairs to be produced and these could be eliminated.
    25 
    26 Ambiguous/Multiple Class/Function Definitions
    27 =============================================
    28 
    29 Classes and functions are not supposed to have multiple definitions, where one code path
    30 may define one form of a class or function with a given name and another code path may
    31 define another form with that name. Currently, such multiple definitions are treated like
    32 "unions" in the object table.
    33 
    34   Consider functions as well as classes which are supported using "shadow" names for the
    35   second and subsequent definitions of classes in the same namespace.
    36 
    37 Class and Module Attribute Assignment
    38 =====================================
    39 
    40 Allow unrestricted class and module assignment (but not new external binding of
    41 attributes), eliminating run-time checks on object types in instructions like
    42 StoreAttrIndex. This may involve less specific objects being identified during inspection.
    43 
    44   Potentially re-evaluate class bases in order to see if they are non-constant.
    45 
    46 Verify that the context information is correctly set, particularly for the unoptimised
    47 cases.
    48 
    49   Update docs/assignment.txt.
    50 
    51 Prevent assignments within classes, such as method aliasing, from causing the source of an
    52 assignment from being automatically generated. Instead, only external references should be
    53 registered.
    54 
    55 Prevent "from <module> import ..." statements from registering references to such local
    56 aliases such that they cause the source of each alias to be automatically generated.
    57 
    58 Consider attribute assignment observations, along with the possibility of class and module
    59 attribute assignment.
    60 
    61   (Note direct assignments as usual, indirect assignments via the attribute usage
    62   mechanism. During attribute collection and inference, add assigned values to all
    63   inferred targets.)
    64 
    65   (Since class attributes can be assigned, StoreAttrIndex would no longer need to reject
    66   static attributes, although this might still be necessary where attribute usage analysis
    67   has not been performed.)
    68 
    69   Potentially consider changing static attribute details to use object-relative offsets in
    70   order to simplify the instruction implementations. This might allow us to eliminate the
    71   static attribute flag for attributes in the object table, at least at run-time.
    72 
    73 Dynamic Attribute Access
    74 ========================
    75 
    76 Consider explicit accessor initialisation:
    77 
    78   attr = accessor("attr")
    79   getattr(C, attr)
    80 
    81 Attribute Usage
    82 ===============
    83 
    84 To consider: is it useful to distinguish between attribute name sets when the same names
    85 are mentioned, but where one path through the code sets different values on attributes
    86 than another? The _attrtypes collapses observations in order to make a list of object
    87 types for a name, and the final set of names leading to such type deductions might be a
    88 useful annotation to be added alongside _attrcombined.
    89 
    90   (Update the reports to group identical sets of attribute names.)
    91 
    92 Attribute usage on attributes might be possible if one can show that the expression of an
    93 attribute access is constant and that the attribute target is also constant or only refers
    94 to a single type. For example, in the sys module:
    95 
    96   stderr = file()
    97 
    98 If no work is done to associate the result of the invocation with the stderr name, then
    99 one could instead at least attempt to determine whether stderr is assigned only once. If
   100 so, it might be possible to record attribute usage on references to the name. For example:
   101 
   102   sys.stderr.write(...) # sys.stderr supports write -> {file, ...}
   103 
   104 Interface/Type Generalisation
   105 -----------------------------
   106 
   107 Consolidate interface observations by taking all cached table accesses and determining
   108 which usage patterns lead to the same types. For example, if full usage of {a, b} and
   109 {a, b, c} leads to A and B in both cases, either {a, b} can be considered as partial usage
   110 of the complete interface {a, b, c}, or the latter can be considered as an
   111 overspecification of the former.
   112 
   113 Consider type deduction and its consequences where types belong to the same hierarchy
   114 and where a guard could be generated for the most general type.
   115 
   116 Consider permitting multiple class alternatives where the attributes are all identical.
   117 
   118 Support class attribute positioning similar to instance attribute positioning, potentially
   119 (for both) based on usage observations. For example, if __iter__ is used on two classes,
   120 the class attribute could be exposed at a similar relative position to the class (and
   121 potentially accessible using a LoadAttr-style instruction).
   122 
   123 **** Constant attribute users need not maintain usage since they are already resolved. ****
   124 
   125 Self-Related Usage
   126 ------------------
   127 
   128 Perform attribute usage on attributes of self as names, potentially combining observations
   129 across methods.
   130 
   131 Additional Guards
   132 -----------------
   133 
   134 Consider handling branches of values within namespaces in order to support more precise value usage.
   135 
   136 Loop entry points and other places where usage becomes more specific might be used as
   137 places to impose guards. See tests/attribute_access_type_restriction_loop_list.py for an
   138 example. (Such information is already shown in the reports.)
   139 
   140 Strict Interfaces/Types
   141 -----------------------
   142 
   143 Make the gathering of usage parameterisable according to the optimisation level so that a
   144 choice can be made between control-flow-dependent observations and the simple collection
   145 of all attributes used with a name (producing a more static interface observation).
   146 
   147 AttributeError
   148 --------------
   149 
   150 Consider attribute usage observations being suspended or optional inside blocks where
   151 AttributeError may be caught (although this doesn't anticipate such exceptions being
   152 caught outside a function altogether). For example:
   153 
   154   y = a.y
   155   try:
   156       z = a.z # z is an optional attribute
   157   except AttributeError:
   158       z = None
   159 
   160 Frame Optimisations
   161 ===================
   162 
   163 Stack frame replacement where a local frame is unused after a call, such as in a tail call
   164 situation.
   165 
   166 Local assignment detection plus frame re-use. Example: slice.__init__ calls
   167 xrange.__init__ with the same arguments which are unchanged in xrange.__init__. There is
   168 therefore no need to build a new frame for this call, although in some cases the locals
   169 frame might need expanding.
   170 
   171 Reference tracking where objects associated with names are assigned to attributes of other
   172 objects may assist in allocation optimisations. Recording whether an object referenced by
   173 a name is assigned to an attribute, propagated to another name and assigned to an
   174 attribute, or passed to another function or method might, if such observations were
   175 combined, allow frame-based or temporary allocation to occur.
   176 
   177 Instantiation Deduction
   178 =======================
   179 
   180 Consider handling Const, List and Tuple in micropython.inspect in order to produce
   181 instances of specific classes. Then, consider adding support for guard
   182 removal/verification where known instances are involved. For example:
   183 
   184   l = []
   185   l.append(123) # type deductions are filtered using instantiation knowledge
   186 
   187 Currently, this is done only for Const values in the context of attribute accesses during
   188 inspection.
   189 
   190 Handling CallFunc in a similar way is more challenging. Consider the definitions in the sys module:
   191 
   192   stderr = file()
   193 
   194 It must first be established that file only ever refers to the built-in file class, and
   195 only then can the assumption be made that stderr in this case refers to instances of file.
   196 If file can also refer to other objects, potential filtering operations are more severely
   197 limited.
   198 
   199 Invocation-Related Deduction
   200 ============================
   201 
   202 Where an attribute access (either in conjunction with usage observations or independently)
   203 could refer to a number of different targets, but where the resulting attribute is then
   204 used in an invocation, filtering of the targets could be done to eliminate any targets
   205 that are not callable. Guards would need introducing to prevent inappropriate operations
   206 from occurring at run-time.
   207 
   208 Inlining
   209 ========
   210 
   211 Where a function or method call can always be determined, the body of the target could be
   212 inlined - copied into place - within the caller. If the target is only ever called by a
   213 single caller it could be moved into place. This could enhance deductions based on
   214 attribute usage since observations from the inlined function would be merged into the
   215 caller.
   216 
   217 Distinguish between frame sharing and inlining: where a called function does not rebind
   218 its names, and where the frame of the caller is compatible, the frame of the caller might
   219 be shared with the called function even if a branch and return is still involved.
   220 
   221 Suitable candidates for inlining, frame sharing or enhanced analysis might be lambdas and
   222 functions containing a single statement.
   223 
   224 Function Specialisation
   225 =======================
   226 
   227 Specialisation of certain functions, such as isinstance(x, cls) where cls is a known
   228 constant.
   229 
   230 Structure and Object Table Optimisations
   231 ========================================
   232 
   233 Fix object table entries for attributes not provided by any known object, or provide an
   234 error, potentially overridden by options. For example, the augmented assignment methods
   235 are not supported by the built-in objects and thus the operator module functions cause
   236 the compilation to fail. Alternatively, just supply the methods since something has to do
   237 so in the builtins.
   238 
   239 Consider attribute merging where many attributes are just aliases for the same underlying
   240 definition.
   241 
   242 Consider references to defaults as occurring only within the context of a particular
   243 function, thus eliminating default value classes if such functions are not themselves
   244 invoked.
   245 
   246 Scope Handling
   247 ==============
   248 
   249 Consider merging the InspectedModule.store tests with the scope conflict handling.
   250 
   251 Consider labelling _scope on assignments and dealing with the assignment of removed
   252 attributes, possibly removing the entire assignment, and distinguishing between such cases
   253 and unknown names.
   254 
   255 Check name origin where multiple branches could yield multiple scope interpretations:
   256 
   257   try:
   258       set # built-in name
   259   except NameError:
   260       from sets import Set as set # local definition of name
   261 
   262   set # could be confused by the local definition at run-time
   263 
   264 Object Coverage
   265 ===============
   266 
   267 Support __init__ traversal (and other implicit names) more effectively.
   268 
   269 Importing Modules
   270 =================
   271 
   272 Consider supporting relative imports, even though this is arguably a misfeature.
   273 
   274 Other
   275 =====
   276 
   277 Check context_value initialisation (avoiding or handling None effectively).
   278 
   279 Consider better "macro" support where new expressions need to be generated and processed.
   280 
   281 Detect TestIdentity results involving constants, potentially optimising status-affected
   282 instructions:
   283 
   284   TestIdentity(x, y) # where x is always y
   285   JumpIfFalse(...)   # would be removed (never false)
   286   JumpIfTrue(...)    # changed to Jump(...)
   287 
   288 Status-affected blocks could be optimised away for such constant-related results.
   289 
   290 Caching of structure and attribute usage information for incremental compilation.