~ubuntu-branches/debian/experimental/nuitka/experimental

Nuitka Developer Manual
~~~~~~~~~~~~~~~~~~~~~~~

.. image:: images/Nuitka-Logo-Symbol.png

.. contents::

.. raw:: pdf

   PageBreak oneColumn
   SetPageCounter 1

The purpose of this developer manual is to present the current design of Nuitka,
the project rules, and the motivations for choices made. It is intended to be a
guide to the source code, and to give explanations that don't fit into the
source code in comments form.

It should be used as a reference for the process of planning and documenting
decisions we made. Therefore we are e.g. presenting here the type inference
plans before implementing them. And we update them as we proceed.

It grows out of discussions and presentations made at conferences as well as
private conversations or discussions on the mailing list or bug tracker.

Milestones
==========

1. Feature parity with CPython, understand all the language construct and behave
   absolutely compatible.

   Feature parity has been reached for CPython 2.6 and 2.7. We do not target any
   older CPython release. For CPython 3.2, and CPython 3.3 it also has been
   reached. We do not target older CPython 3.1 and 3.0 releases.

   This milestone was reached.

2. Create the most efficient native code from this. This means to be fast with
   the basic Python object handling.

   This milestone was reached.

3. Then do constant propagation, determine as many values and useful constraints
   as possible at compile time and create more efficient code.

   This milestone is considered almost reached.

4. Type inference, detect and special case the handling of strings, integers,
   lists in the program.

   This milestone is considered in progress

5. Add interfacing to C code, so Nuitka can turn a ``ctypes`` binding into an
   efficient binding as written with C.

   This milestone is planned only.

6. Add hints module with a useful Python implementation that the compiler can
   use to learn about types from the programmer.

   This milestone is planned only.


Version Numbers
===============

For Nuitka we use a defensive version numbering system to indicate that it is
not yet ready for everything. We have defined milestones and the version numbers
should express which of these, we consider done.

- So far:

  Before milestone 1, we used "0.1.x" version numbers. After reaching it, we
  used "0.2.x" version numbers.

  Before milestone 2 and 3, we used "0.3.x" version numbers. After almost
  reaching 3, and beginning with 4, we use "0.4.x" version numbers. Due to an
  interface change, "0.5.x" version numbers are being used.

- Future:

  When we start to have sufficient amount of type inference in a stable release,
  that will be "0.6.x" version numbers. With ``ctypes`` bindings in a sufficient
  state it will be "0.7.x".

- Final:

  We will then round it up and call it "Nuitka 1.0" when this works as expected
  for a bunch of people. The plan is to reach this goal during 2015. This is
  based on positive assumptions that may not hold up though.

Of course, this may be subject to change.


Current State
=============

Nuitka top level works like this:

- ``nuitka.tree.Building`` outputs node tree
- ``nuitka.optimization`` enhances it as best as it can
- ``nuitka.finalization`` marks the tree for code generation
- ``nuitka.codegen.CodeGeneration`` creates code snippets and joins them
- ``nuitka.codegen.Generator`` knows how identifiers and code are constructed
- ``nuitka.MainControl`` keeps it all together

This design is intended to last.

Regarding Types, the state is:

- Types are always ``PyObject *``, implicitly.

- The only more specific use of type is "compile time constant", which can be
  used to predict some operations, conditions, etc.

- Every operation is expected to have ``PyObject *`` as result, if it is not a
  constant, then we know nothing about it. For some interfaces, e.g. iteration,
  there are initial attempts at abstracting it.

The limitation to only ``PyObject *`` will go away.

Coding Rules
============

These rules should generally be adhered when working on Nuitka code. It's not
library code and it's optimized for readability, and avoids all performance
optimization for itself.


Line Length
-----------

No more than 120 characters. Screens are wider these days, but most of the code
aims at keeping the lines below 80.


Indentation
-----------

No tabs, 4 spaces, no trailing white space.


Identifiers
-----------

Classes are camel case with leading upper case. Methods are with leading verb in
lower case, but also camel case. Around braces there are no spaces, but after
comma, there is spaces for better readability. Variables and arguments are
lower case with "_" as a separator.

.. code-block:: python

   class SomeClass:

      def doSomething(some_parameter):
         some_var = ("foo", "bar")

Base classes that are abstract have their name end with ``Base``, so that a meta
class can use that convention, and readers immediately know.

Function calls use keyword argument preferably. These are slower in CPython, but
more readable:

.. code-block:: python

   return Generator.getSequenceCreationCode(
        sequence_kind       = sequence_kind,
        element_identifiers = identifiers,
        context             = context
   )

The ``=`` are all aligned to the longest parameter names without extra spaces
for it.

When the names don't add much value, sequential calls should be done, but
ideally with one value per line:

.. code-block:: python

   context.setLoopContinueTarget(
       handler_start_target,
       continue_name
   )

Here, ``setLoopContinueTarget`` will be so well known that the reader is
expected to know the argument names and their meaning, but it would be still
better to add them.

Contractions should span across multiple lines for increased readability:

.. code-block:: python

   result = [
       "PyObject *decorator_%d" % (d + 1)
       for d in
       range(decorator_count)
   ]


Module/Package Names
--------------------

Normal modules are named in camel case with leading upper case, because their of
role as singleton classes. The difference between a module and a class is small
enough and in the source code they are also used similarly.

For the packages, no real code is allowed in them and they must be lower case,
like e.g. ``nuitka`` or ``codegen``. This is to distinguish them from the
modules.

Packages shall only be used to group packages. In ``nuitka.codegen`` the code
generation packages are located, while the main interface is
``nuitka.codegen.CodeGeneration`` and may then use most of the entries as local
imports.

The use of a global package ``nuitka``, originally introduced by Nicolas, makes
the packaging of Nuitka with ``distutils`` etc. easier and lowers the
requirements on changes to the ``sys.path`` if necessary.

.. note::

   There are not yet enough packages inside Nuitka, feel free to propose changes
   as you see fit.

Names of modules should be plurals if they contain classes. Example is ``Nodes``
contains ``Node`` classes.


Prefer list contractions over built-ins
---------------------------------------

This concerns ``map``, ``filter``, and ``apply``. Usage of these built-ins is
highly discouraged within Nuitka source code. Using them is considered worth a
warning by "PyLint" e.g. "Used builtin function 'map'". We should use list
contractions instead, because they are more readable.

List contractions are a generalization for all of them. We love readability and
with Nuitka as a compiler will there won't be any performance difference at all.

There are cases where a list contraction are faster because you can avoid to
make a function call. And there may be cases, where map is faster, if a function
must be called. These calls can be very expensive in CPython, and if you
introduce a function, just for ``map``, then it might be slower.

But of course, Nuitka is the project to free us from what is faster and to allow
us to use what is more readable, so whatever is faster, we don't care. We make
all options equally fast and let people choose.

For Nuitka the choice is list contractions as these are more easily changed and
readable.

Look at this code examples from Python:

.. code-block:: python

   class A:
       def getX(self):
           return 1
       x = property(getX)

   class B(A):
      def getX(self):
         return 2


   A().x == 1 # True
   B().x == 1 # True (!)

This pretty much is what makes properties bad. One would hope ``B().x`` to be
``2``, but instead it's not changed. Because of the way properties take the
functions and not members, and because they then are not part of the class, they
cannot be overloaded without re-declaring them.

Overloading is then not at all obvious anymore. Now imagine having a setter and
only overloading the getter. How to update the property easily?

So, that's not likable about them. And then we are also for clarity in these
internal APIs too. Properties try and hide the fact that code needs to run and
may do things. So lets not use them.

For an external API you may exactly want to hide things, but internally that has
no use, and in Nuitka, every API is internal API. One exception may be the
``hints`` module, which will gladly use such tricks for an easier write syntax.


The "git flow" model
====================

* The flow was is used for releases and occasionally subsequent hot fixes.

  A few feature branches were used so far. It allows for quick delivery of fixes
  to both the stable and the development version, supported by a git plugin,
  that can be installed via "apt-get install git-flow" on latest Debian Testing
  at least.

* Stable (master branch)

  The stable version, is expected to pass all the tests at all times and is
  fully supported. As soon as bugs are discovered, they are fixed as hot fixes,
  and then merged to develop by the "git flow" automatically.

* Development (develop branch)

  The future release, supposedly in almost ready for release state at nearly all
  times, but this is as strict. It is not officially supported, and may have
  problems and at times inconsistencies. Normally this branch is supposed to not
  be rebased. For severe problems it may be done though.

* Factory (default feature branch)

  Code under construction. We publish commits there, that may not hold up in
  testing, and before it enters develop branch. Factory may have severe
  regressions frequently, and commits become rebased all the time.

* Feature Branches

  On these long lived developments that extend for multiple release cycles or
  contain changes that break Nuitka temporarily. They need not be functional at
  all.


Checking the Source
===================

The static checking for errors is currently done with "PyLint". In the future,
Nuitka itself will gain the ability to present its findings in a similar way,
but this is not a priority, and we are not there yet.

So, we currently use "PyLint" with options defined in a script.

.. code-block:: sh

   ./misc/check-with-pylint --hide-todos

Ideally the above command gives no warnings, but that has never been true so
far. This has not yet been reached. The existing warnings often still serve as a
kind of "TODO" items. We are not white listing them, because they indicate a
problem that should be solved.

If you submit a patch, it would be good if you checked that it doesn't introduce
new warnings, but that is not strictly required. it will happen before release,
and that is considered enough. You probably are already aware of the beneficial
effects.

Running the Tests
=================

This section describes how to run Nuitka tests.

Running all Tests
-----------------

The top level access to the tests is as simple as this:

.. code-block:: sh

   ./misc/check-release

For fine grained control, it has the following options::

  -h, --help            show this help message and exit
  --skip-basic-tests    The basic tests, execute these to check if Nuitka is
                        healthy. Default is True.
  --skip-syntax-tests   The syntax tests, execute these to check if Nuitka
                        handles Syntax errors fine. Default is True.
  --skip-program-tests  The programs tests, execute these to check if Nuitka
                        handles programs, e.g. import recursions, etc. fine.
                        Default is True.
  --skip-reflection-test
                        The reflection test compiles Nuitka with Nuitka, and
                        then Nuitka with the compile Nuitka and compares the
                        outputs. Default is True.
  --skip-cpython26      The standard CPython2.6 test suite. Execute this for
                        all corner cases to be covered. With Python 2.7 this
                        covers exception behavior quite well. Default is True.
  --skip-cpython27      The standard CPython2.7 test suite. Execute this for
                        all corner cases to be covered. With Python 2.6 these
                        are not run. Default is True.
  --skip-cpython32      The standard CPython3.2 test suite. Execute this for all
                        corner cases to be covered. With Python 2.x these are
                        not run. Default is True.


You will only run the CPython test suites, if you have the submodules of the
Nuitka git repository checked out. Otherwise, these will be skipped
automatically with a warning that they are not available.

The policy is generally, that ``./misc/check-release`` running and passing all
the tests on Linux and Windows shall be considered sufficient for a release.

Basic Tests
-----------

You can run the "basic" tests like this:

.. code-block:: sh

   ./tests/basics/run_all.py search

These tests normally give sufficient coverage to assume that a change is
correct, if these "basic" tests pass. The most important constructs and
built-ins are excercised.

To control the Python version used for testing, you can set the ``PYTHON``
environment variable to e.g. "python3.2", or execute the "run_all.py" with the
intended version, it is portable across all supported Python versions.

Syntax Tests
------------

Then there are "syntax" tests, i.e. language constructs that need to give a
syntax error.

It sometimes so happens that Nuitka must do this itself, because the
``ast.parse`` doesn't see the problem and raises no ``SyntaxError`` of its
own.

Using the ``global`` statementon a function argument is an example of
this. These tests make sure that the errors of Nuitka and CPython are totally
the same for this:

.. code-block:: sh

   ./tests/syntax/run_all.py search

Program Tests
-------------

Then there are small programs tests, that e.g. exercise all kinds of import
tricks and are designed to reveal problems with inter-module behavior. These can
be run like this:

.. code-block:: sh

   ./tests/programs/run_all.py search

Compile Nuitka with Nuitka
--------------------------

And there is the "compile itself" or "reflected" test. This test makes Nuitka
compile itself and compare the resulting C++ when running compiled to
non-compiled, which helps to find in-determinism.

The test compiles every module of Nuitka into an extension module and all of
Nuitka into a single binary.

That test case also gives good coverage of the ``import`` mechanisms, because
Nuitka uses a lot of packages.

.. code-block:: sh

   ./tests/reflected/compile_itself.py


Design Descriptions
===================

These should be a lot more and contain graphics from presentations given. It
will be filled in, but not now.

Nuitka Logo
-----------

The logo was submitted by "dr. Equivalent". It's source is contained in
``misc/Logo`` where 3 variants of the logo in SVG are placed.

* Symbol only (symbol)

  .. image:: images/Nuitka-Logo-Symbol.png

* Text next to symbol (horizontal)

  .. image:: images/Nuitka-Logo-Horizontal.png

* Text beneath symbol (vertical)

  .. image:: images/Nuitka-Logo-Vertical.png

From these logos, PNG images, and "favicons", and are derived.

The exact ImageMagick commands are in ``misc/make-doc.py``, but are not executed
each time, the commands are also replicated here:

.. code-block:: sh

   convert -background none misc/Logo/Nuitka-Logo-Symbol.svg images/Nuitka-Logo-Symbol.png
   convert -background none misc/Logo/Nuitka-Logo-Vertical.svg images/Nuitka-Logo-Vertical.png
   convert -background none misc/Logo/Nuitka-Logo-Horizontal.svg images/Nuitka-Logo-Horizontal.png

   optipng -o2 images/Nuitka-Logo-Symbol.png
   optipng -o2 images/Nuitka-Logo-Vertical.png
   optipng -o2 images/Nuitka-Logo-Horizontal.png


Choice of the Target Language
-----------------------------

* Choosing the target language, is an important decision

  * The portability of Nuitka is decided here

* Other factors:

  * How difficult is it to generate the code?
  * Does the Python C-API have bindings?
  * Is that language known?
  * Does the language aid to find bugs?

* These candidates were considered

  * C++03, C++11, Ada, C

.. table:: Requirement to Language matrix:

   =====================  =====   ======  =========   =========
   Requirement\\Language  C       C++03   C++11       Ada
   =====================  =====   ======  =========   =========
   Portable               Yes     Yes     No [1]_     Yes
   ---------------------  -----   ------  ---------   ---------
   Knowledge              Yes     Yes     No [2]_     Yes
   ---------------------  -----   ------  ---------   ---------
   Python C-API           Yes     Yes     Yes         No [3]_
   ---------------------  -----   ------  ---------   ---------
   Runtime checks         No      No      No          Yes [4]_
   ---------------------  -----   ------  ---------   ---------
   Code Generation        Tough   Hard    Easy        Harder
   =====================  =====   ======  =========   =========


_`1`:: C++11 is not fully supported by all compilers.

_`2`:: Not a whole lot of people have C++11 knowledge. My *only* C++11 code was
that in Nuitka.

_`3`:: The Python C-API for Ada would have to be created by us, possible just
big project by itself.

_`4`:: Run time checks exist only for Ada in that quality. I miss automatic
``CONSTRAINT_ERROR`` exceptions, for data structures with validity indicators,
where in other languages, I need to check myself.

The *decision for C* is ultimately:

* for portability
* for language knowledge
* for control over created code.

All of these are important advantages.

For C++11 initially spoke easy code generation:

* variadic templates
* raw strings

Yet, as it turns out, variadic templates do not help at all with evaluation
order, so that code that used it, needed to be changed to generating instances
of their code. And raw strings turned out to be not as perfect as one wants to
be, and solving the problem with C++03 is feasible too, even if not pretty.

For C++03 initially spoke less explicit code generation:

* Destructors can ensure cleanups happen
* Local objects could e.g. repair the stack frames

For Ada would have spoken the time savings through run time checks, which would
have shortened some debugging sessions quite some. But building the Python C-API
bindings on our own, and potentially incorrectly, would have eaten that up.

Later, it was found that using C++ for exceptions is tremendously inefficient,
and must be avoided. In order to do this, a more C style code generation is
needed, where even less things are done with C++, e.g. the cleanup of temporary
variables inside a statement will be done manually instead.

The current status is C-ish. That is, with very few classes remaining, the
syntax used is C++ still, but we are approaching being pure C.


Use of Scons internally
-----------------------

Nuitka does not involve Scons in its user interface at all; Scons is purely used
internally. Nuitka itself, being pure Python, will run without any build process
just fine.

Nuitka prepares ".build" folders with lots of files and tasks scons to execute
the final build.

.. note::

   When we speak of "standalone" mode, this is handled outside of Scons, and
   after it, creating the ".dist" folder. This is done in ``nuitka.MainControl``
   module.

For interfacing to Scons, there is the module ``nuitka.build.SconsInterface``
that will support calling scons - potentially from an inline copy, mainly on
Windows or when using source releases - and passing arguments to it. These
arguments are passed as ``key=value``, and decoded in the scons file of Nuitka.

The scons file is named ``SingleExe.scons`` for lack of better name. It's really
wrong now, but we have yet to find a better name. It once expressed the
intention to be used to create executables, but the same works for modules too,
as in terms of building, and to Scons, things really are the same.

The scons file supports operation in multiple modes for many things, and modules
is just one of them. It runs outside of Nuitka process scope, even with a
different Python version potentially, so all the information must be passed on
the command line.

What follows is the (lengthy) list of arguments that the scons file processes:

* ``source_dir``

  Where is the generated C++ source code. Scons will just compile everything it
  finds there. No list of files is passed.

* ``nuitka_src``

  Where do the include files and static C++ parts of Nuitka live. These provide
  e.g. the implementation of compiled function, generators, and other helper
  codes, this will point to where ``nuitka.build`` package lives normally.

* ``result_base``

  This is not a full name, merely the basename for the result to be produced,
  but with path included, and the suffix comes from module or executable mode.

* ``module_mode``

  Build a module instead of a program.

* ``debug_mode``

  Enable debug mode, which is a mode, where Nuitka tries to help identify errors
  in itself, and will generate less optimal code. This also asks for warnings,
  and makes the build fail if there are any.

* ``python_debug``

  Compile and link against Python debug mode, which does assertions and extra
  checks, to identify errors, mostly related to reference counting. May make the
  build fail, if no debug build library of CPython is available. On Windows it
  typically is not installed.

* ``optimize_mode``

  Optimization mode, enable as much as currently possible. This refers to
  building the binary.

* ``full_compat_mode``

  Full compatibility, even where it's stupid, i.e. do not provide information,
  even if available, in order to assert maximum compatibility. Intended to
  control level of compatability to absurd.

* ``experimental_mode``

  Do things that are not yet accepted to be safe.

* ``lto_mode``

  Make use of link time optimization of g++ compiler if available and known good
  with the compiler in question. So far, this was not found to make major
  differences.

* ``win_disable_console``

  Windows subsystem mode: Disable console for windows builds.

* ``unstriped_mode``

  Unstriped mode: Do not remove debug symbols.

* ``clang_mode``

  Clang compiler mode, default on MacOS X and FreeBSD, optional on Linux.

* ``mingw_mode``

  MinGW compiler mode, optional and interesting to Windows only.

* ``standalone_mode``

  Building a standalone distribution for the binary.

* ``show_scons``

  Show scons mode, output information about Scons operation. This will e.g. also
  output the actual compiler used, output from compilation process, and
  generally debug information relating to be build process.

* ``python_prefix``

  Home of Python to be compiled against, used to locate headers and libraries.

* ``target_arch``

  Target architecture to build.

* ``icon_path``

  The icon to use for Windows programs if given.


Locating Modules and Packages
------------------------------

The search for of modules used is driven by ``nuitka.Importing`` module.

* From the module documentation

  The actual import of a module may already execute code that changes
  things. Imagine a module that does ``os.system()``, it will be done. People
  often connect to databases, and these kind of things, at import time. Not a
  good style, but it's being done.

  Therefore CPython exhibits the interfaces in an ``imp`` module in standard
  library, which one can use those to know ahead of time, what file import would
  load. For us unfortunately there is nothing in CPython that is easily
  accessible and gives us this functionality for packages and search paths
  exactly like CPython does, so we implement here a multi step search process
  that is compatible.

  This approach is much safer of course and there is no loss. To determine if
  it's from the standard library, one can abuse the attribute ``__file__`` of
  the ``os`` module like it's done in ``isStandardLibraryPath`` of this module.

* Role

  This module serves the recursion into modules and analysis if a module is a
  known one. It will give warnings for modules attempted to be located, but not
  found. These warnings are controlled by a while list inside the module.


Hooking for module ``import`` process
-------------------------------------

Currently, in created code, for every ``import`` a normal ``__import__()`` call
is executed. The "ModuleUnfreezer.cpp" (located in "nuitka/build/static_src")
provides the implementation of a ``sys.meta_path`` hook.

This one allows us to have the Nuitka provided module imported even when
imported by non-compiled code.

.. note::

   Of course it would make sense to compile time detect which module it is that
   is being imported and then to make it directly. At this time, we don't have
   this inter-module optimization yet, mid-term it should become easy to add.

Supporting ``__class__`` of Python3
-----------------------------------

In Python3 the handling of ``__class__`` and ``super`` is different from
Python2. It used to be a normal variable, and now the following things have
changed.

* The use of the ``super`` variable name triggers the addition of a closure
  variable ``__class__``, as can be witnessed by the following code:

  .. code-block:: python

     class X:
        def f1(self):
           print( locals() )

        def f2(self):
           print( locals() )
           super

     x = X()
     x.f1()
     x.f2()

  .. code-block:: python

     {'self': <__main__.X object at 0x7f1773762390>}
     {'self': <__main__.X object at 0x7f1773762390>, '__class__': <class '__main__.X'>}


* This value of ``__class__`` is also available in the child functions.

* The parser marks up code objects usage of "super". It doesn't have to be a
  call, it can also be a local variable. If the ``super`` builtin is assigned to
  another name and that is used without arguments, it won't work unless
  ``__class__`` is taken as a closure variable.

* As can be seen in the CPython3.2 code, the closure value is added after the
  class creation is performed.

* It appears, that only functions locally defined to the class are affected and
  take the closure.

This left Nuitka with the strange problem, of how to emulate that.

The solution is this:

* Under Python3, usage of ``__class__`` as a reference in a function body that
  is not a class dictionary creation, marks it up via
  ``markAsClassClosureTaker``.

* Functions that are marked up, will be forced to reference variable to
  ``__class__``.

  .. note::

     This one should be optimized away later if not used. Currently we have "no
     unused closure variable" detection, but it would cover it.

* When recognizing calls to ``super`` without arguments, make the arguments into
  variable reference to ``__class__`` and potentially ``self`` (actually first
  argument name).

* Class dictionary definitions are added.

  These are special direct function calls, ready to propagate also "bases" and
  "metaclass" values, which need to be calculated outside.

  The function bodies used for classes will automatically store ``__class__`` as
  a shared local variable, if anything uses it. And if it's not assigned by user
  code, it doesn't show up in the "locals()" used for dictionary creation.

  Existing ``__class__`` local variable values are in fact provided as closure,
  and overridden with the built class , but they should be used for the closure
  giving, before the class is finished.

  So ``__class__`` will be local variable of the class body, until the class is
  built, then it will be the ``__class__`` itself.


Frame Stack
-----------

In Python, every function, class, and module has a frame. It creates created
when the scope it entered, and there is a stack of these at run time, which
becomes visible in tracebacks in case of exceptions.

The choice of Nuitka is to make this non-static elements of the node tree, that
are as such subject to optimization. In cases, where they are not needed, they
may be removed.

Consider the following code.

.. code-block:: python

   def f():
       if someNotRaisingCall():
           return somePotentiallyRaisingCall()
       else:
           return None

In this example, the frame is not needed for all the code, because the condition
checked wouldn't possibly raise at all. The idea is the make the frame guard
explicit and then to reduce its scope whenever possible.

So we start out with code like this one:

.. code-block:: python

   def f():
       with frame_guard( "f" ):
           if someNotRaisingCall():
               return somePotentiallyRaisingCall()
           else:
               return None

This is to be optimized into:

.. code-block:: python

   def f():
       if someNotRaisingCall():
           with frame_guard( "f" ):
               return somePotentiallyRaisingCall()
       else:
           return None


Notice how the frame guard taking is limited and may be avoided, or in best
cases, it might be removed completely. Also this will play a role when in-lining
function. The frame stack entry will then be automatically preserved without
extra care.

Parameter Parsing
-----------------

The parsing of parameters is very convoluted in Python, and doing it in an
compatible way is not that easy. This is a description of the required process,
for easier overview.

Input
+++++

The input is an argument ``tuple`` (the type is fixed), which contains the
positional arguments, and potentially an argument ``dict`` (type is fixed as
well, but could also be ``NULL``, indicating that there are no keyword
arguments.

Keyword dictionary
++++++++++++++++++

The keyword argument dictionary is checked first. Anything in there, that cannot
be associated, either raises an error, or is added to a potentially given star
dict argument. So there are two major cases.

* No star dict argument: Iterate over dictionary, and assign or raise errors.

  This check covers extra arguments given.

* With star dict argument: Iterate over dictionary, and assign or raise errors.

  Interesting case for optimization are no positional arguments, then no check
  is needed, and the keyword argument dictionary could be used as the star
  argument. Should it change, a copy is needed though.

What's noteworthy here, is that in comparison of the keywords, we can hope that
they are the same value as we use. The interning of strings increases chances
for non-compiled code to do that, esp. for short names.

We then can do a simple ``is`` comparison and only fall back to real string `==`
comparisons, after all of these failed. That means more code, but also a lot
faster code in the positive case.

Argument tuple
++++++++++++++

After this completed, the argument tuple is up for processing. The first thing
it needs to do is to check if it's too many of them, and then to complain.

For arguments in Python2, there is the possibility of them being nested, in
which case they cannot be provided in the keyword dictionary, and merely should
get picked from the argument tuple.

Otherwise, the length of the argument tuple should be checked against its
position and if possible, values should be taken from there. If it's already set
(from the keyword dictionary), raise an error instead.

Code Generation towards C
-------------------------

Currently, Nuitka use C++ as a glorified C, it will tend to use less and less
actual C++ patterns. To control the order to object deletion, this is vital.

Exceptions
++++++++++

To handle and work with exceptions, every construct that can raise has to have a
``bool`` return code or ``PyObject *`` with ``NULL`` return value. This is very
much in line with that the Python C-API does.

Every helper function that contains code that might raise needs these
variables. After a failed call, ``PyErr_Fetch`` must be used to catch the
defined error, unless some quick exception cases apply. Sometimes e.g. ``NULL``
return from C-API means ``StopIteration``.

The difficulty here, is only to discover the need for these variables, because
we would like to not have unused variables declared. In principle, the
``mayRaiseException`` should be used to discover this.

As an optimization, functions that raise exceptions, but are known not to do so,
for whatever reason, may only be asserted.

Statement Temporary Variables
+++++++++++++++++++++++++++++

For statements and larger constructs the context object track temporary values,
that represent references. For some, these should be released at the end of the
statement, or they represent a leak.

The larger scope temporary variables, are tracked in the function or module
context, where they are supposed to have explicit "del" to release their
references.

Exit Targets
++++++++++++

Each error or other exit releases statement temporary values and then executes a
``goto`` to the exit target. These targets need to be setup. The
``try``/``except`` will e.g. catch error exits.

Other exits are ``continue``, ``break``, and ``return`` exits. They all work
alike.

For frames, later, local variables will need to be freed on the way out. The way
out for a frame, should either be a function return, or another frame exit. We
will later have a ``try``/``finally`` with

Generally, the exits stack of with constructs that need to register themselves
for some exit types. A loop e.g. registers the ``continue exit, and a contained
``try``/``finally`` too, so it can execute the final code.

Frames
++++++

Frames are containers for variable declarations and cleanups. As such, frames
provide error exits and success exits, which remove the frame from the frame
stack, and then proceed to the parent exit.

Once local variables are to be released, the frames should establish that this
is happening.

Abortive Statements
+++++++++++++++++++

The ``return``, ``continue``, and ``break`` of Python must be treated like an
exception to ``try``/``finally``. So for success exit paths, a "return value
variable" is maintained as well. It being set, causes ``finally`` exit to
``return`` it again.

Similarly their are flags that make the ``continue`` or ``break`` happen at the
end the end of a ``finally`` handler, but these are mere ``bool`` indicator
flags.

Constant Preparation
--------------------

Early versions of Nuitka, created all constants for the whole program for ready
access to generated code, before the program launches. It did so in a single
file, but that approach didn't scale well.

Problems were

* Even unused code contributed to start-up time.

* The massive amount of constant creation codes gave C++ compilers a harder time
  than necessary.

.. note::

   This is so far only a plan.

The new approach is as follows. Code generation uses the same identifiers for
constants as before, but these will be declared module local ("static"), if the
module is the only user, or "extern" if it is not.

The "extern" values will be globally created pre-main. Some values, that are
e.g. used in pre-main code, references to "None" module should enforce this
behavior.

Code for all modules will be created with a delay. The final association with
the module body template must wait until all are ready, because only then the
scope of the constants will be known.

The most important goal is to avoid globally initializing constants that are
used only in one module.

We need to trace used constants per module, and for nested ones, we also need to
associate them. The global constants code is special in that it can only use
"static" for nested values it exclusively uses, and has to export values that
others use.


Language Conversions to make things simpler
-------------------------------------------

There are some cases, where the Python language has things that can in fact be
expressed in a simpler or more general way, and where we choose to do that at
either tree building or optimization time.


The ``assert`` statement
++++++++++++++++++++++++

The ``assert`` statement is a special statement in Python, allowed by the
syntax. It has two forms, with and without a second argument. The later is
probably less known, as is the fact that raise statements can have multiple
arguments too.

The handling in Nuitka is:

.. code-block:: python

   assert value
   # Absolutely the same as:
   if not value:
       raise AssertionError

.. code-block:: python

   assert value, raise_arg
   # Absolutely the same as:
   if not value:
       raise AssertionError, raise_arg

This makes assertions absolutely the same as a raise exception in a conditional
statement.

This transformation is performed at tree building already, so Nuitka never knows
about ``assert`` as an element and standard optimizations apply. If e.g. the
truth value of the assertion can be predicted, the conditional statement will
have the branch statically executed or removed.


The "comparison chain" expressions
++++++++++++++++++++++++++++++++++

.. code-block:: python

   a < b > c < d
   # With "temp variables" and "assignment expressions", absolutely
   # the same as:
   a < ( tmp_b = b ) and tmp_b > ( tmp_c = c ) and ( tmp_c < d )

This transformation is performed at tree building already. The temporary
variables keep the value for the potential read in the same expression. The
syntax is not Python, and only pseudo language to expression the internal
structure of the node tree after the transformation.

This useful "keeper" variables that enable this transformation and allow to
express the short circuit nature of comparison chains by using ``and``
operations.


The ``execfile`` built-in
+++++++++++++++++++++++++

Handling is:

.. code-block:: python

   execfile( filename )
   # Basically the same as:
   exec( compile( open( filename ).read() ), filename, "exec" )

.. note::

   This allows optimizations to discover the file opening nature easily and
   apply file embedding or whatever we will have there one day.

This transformation is performed when the ``execfile`` builtin is detected as
such during optimization.


Generator expressions with ``yield``
++++++++++++++++++++++++++++++++++++

These are converted at tree building time into a generator function body that
yields the iterator given, which is the put into a for loop to iterate, created
a lambda function of and then called with the first iterator.

That eliminates the generator expression for this case. It's a bizarre construct
and with this trick needs no special code generation.


Function Decorators
+++++++++++++++++++

When one learns about decorators, you see that:

.. code-block:: python

   @decorator
   def function():
      pass
   # Is basically the same as:
   def function():
      pass
   function = decorator( function )

The only difference is the assignment to function. In the ``@decorator`` case,
if the decorator fails with an exception, the name ``function`` is not assigned.

Therefore in Nuitka this assignment is from a "function body expression" and
only the last decorator returned value is assigned to the function name.

This removes the need for optimization and code generation to support decorators
at all. And it should make the two variants optimize equally well.


In-place Assignments
++++++++++++++++++++

In-place assignments are re-formulated to an expression using temporary
variables.

These are not as much a reformulation of ``+=`` to ``+``, but instead one which
makes it explicit that the assign target may change its value.

.. code-block:: python

   a += b

.. code-block:: python

   _tmp = a.__iadd__( b )

   if a is not _tmp:
       a = _tmp

Using ``__iadd__`` here to express that not the ``+``, but the in-place variant
``iadd`` is used instead. The ``is`` check may be optimized away depending on
type and value knowledge later on.


Complex Assignments
+++++++++++++++++++

Complex assignments are defined as those with multiple targets to assign from a
single source and are re-formulated to such using a temporary variable and
multiple simple assignments instead.

.. code-block:: python

   a = b = c

.. code-block:: python

   _tmp = c
   b = _tmp
   a = _tmp
   del _tmp


This is possible, because in Python, if one assignment fails, it can just be
interrupted, so in fact, they are sequential, and all that is required is to not
calculate ``c`` twice, which the temporary variable takes care of.


Unpacking Assignments
+++++++++++++++++++++

Unpacking assignments are re-formulated to use temporary variables as well.

.. code-block:: python

   a, b.attr, c[ind] = d = e, f, g = h()

Becomes this:

.. code-block:: python

   _tmp = h()

   _iter1 = iter( _tmp )
   _tmp1 = unpack( _iter1, 3 )
   _tmp2 = unpack( _iter1, 3 )
   _tmp3 = unpack( _iter1, 3 )
   unpack_check( _iter1 )
   a = _tmp1
   b.attr = _tmp2
   c[ind] = _tmp3
   d = _tmp
   _iter2 = iter( _tmp )
   _tmp4 = unpack( _iter2, 3 )
   _tmp5 = unpack( _iter2, 3 )
   _tmp6 = unpack( _iter2, 3 )
   unpack_check( _iter1 )
   e = _tmp4
   f = _tmp5
   g = _tmp6

That way, the unpacking is decomposed into multiple simple statementy. It will
be the job of optimizations to try and remove unnecessary unpacking, in case
e.g. the source is a known tuple or list creation.

.. note::

   The ``unpack`` is a special node which is a form of ``next`` that will raise
   a ``ValueError`` when it cannot get the next value, rather than a
   ``StopIteration``. The message text contains the number of values to unpack,
   therefore the integer argument.

.. note::

   The ``unpack_check`` is a special node that raises a ``ValueError`` exception
   if the iterator is not finished, i.e. there are more values to unpack.

With Statements
+++++++++++++++

The ``with`` statements are re-formulated to use temporary variables as
well. The taking and calling of ``__enter__`` and ``__exit__`` with arguments,
is presented with standard operations instead. The promise to call ``__exit__``
is fulfilled by ``try``/``except`` clause instead.

.. code-block:: python

    with some_context as x:
        something( x )

.. code-block:: python

    tmp_source = some_context

    # Actually it needs to be "special look-up" for Python2.7, so attribute
    # look-up won't be exactly what is there.
    tmp_exit = tmp_source.__exit__

    # This one must be held for the whole with statement, it may be assigned
    # or not, in our example it is. If an exception occurs when calling
    # ``__enter__``, the ``__exit__`` should not be called.
    tmp_enter_result = tmp_source.__enter__()

    # Indicator variable to know if "tmp_exit" has been called.
    tmp_indicator = False

    try:
        # Now the assignment is to be done, if there is any name for the
        # manager given, this may become multiple assignment statements and
        # even unpacking ones.
        x = tmp_enter_result

        # Then the code of the "with" block.
        something( x )
    except Exception:
        # Note: This part of the code must not set line numbers, which we
        # indicate with special source code references, which we call "internal".
        # Otherwise the line of the frame would get corrupted.

        tmp_indicator = True

        if not tmp_exit( *sys.exc_info() ):
            raise
    finally:
        if not tmp_indicator
            # Call the exit if no exception occurred with all arguments
            # as "None".
            tmp_exit( None, None, None )

.. note::

   We don't refer really to ``sys.exc_info()`` at all, instead, we have
   references to the current exception type, value and trace, taken directory
   from the caught exception object on the C++ level.

   If we had the ability to optimize ``sys.exc_info()`` to do that, we could use
   the same transformation, but right now we don't have it.


For Loops
+++++++++

The for loops use normal assignments and handle the iterator that is implicit in
the code explicitly.

.. code-block:: python

    for x,y in iterable:
        if something( x ):
            break
    else:
        otherwise()

This is roughly equivalent to the following code:

.. code-block:: python

    _iter = iter( iterable )
    _no_break_indicator = False

    while True:
        try:
            _tmp_value = next( _iter )
        except StopIteration:
            # Set the indicator that the else branch may be executed.
            _no_break_indicator = True

            # Optimization should be able to tell that the else branch is run
            # only once.
            break

         # Normal assignment re-formulation applies to this assignment of course.
         x, y = _tmp_value
         del _tmp_value

         if something( x ):
             break

    if _no_break_indicator:
        otherwise()

.. note::

   The ``_iter`` temporary variable is of course in a temp block and the ``x,
   y`` assignment is the normal is of course re-formulation of an assignment
   that cannot fail.

   The ``try``/``except`` is detected to allow to use a variant of ``next`` that
   throws no C++ exception, but instead to use ``ITERATOR_NEXT`` and which
   returns NULL in that case, so that the code doesn't really have any Python
   level exception handling going on.


While Loops
+++++++++++

Loops in Nuitka have no condition attached anymore, so while loops are
re-formulated like this:

.. code-block:: python

    while condition:
        something()

.. code-block:: python

    while True:
        if not condition:
            break

        something()


This is to totally remove the specialization of loops, with the condition moved
to the loop body in a conditional statement, which contains a break statement.

That makes it clear, that only break statements exit the loop, and allow for
optimization to remove always true loop conditions, without concerning code
generation about it, and to detect such a situation, consider e.g. endless
loops.

.. note::

   Loop analysis can therefore work on a reduced problem (which ``break``
   statements are executed under which conditions) and be very general, but it
   cannot take advantage of the knowledge encoded directly anymore. The fact
   that the loop body may not be entered at all, if the condition is not met, is
   something harder to discover.


Exception Handlers
++++++++++++++++++

Exception handlers in Python may assign the caught exception value to a variable
in the handler definition. And the different handlers are represented as
conditional checks on the result of comparison operations.

.. code-block:: python


   try:
       block()
   except A as e:
       handlerA(e)
   except B as e:
       handlerB(e)
   else:
      handlerElse()

.. code-block:: python

    try:
       block()
    except:
       # These are special nodes that access the exception, and don't really
       # use the "sys" module.
       tmp_exc_type = sys.exc_info()[0]
       tmp_exc_value = sys.exc_info()[1]

       # exception_matches is a comparison operation, also a special node.
       if exception_matches(tmp_exc_type, (A,)):
          e = tmp_exc_value
          handlerA(e)
       elif exception_matches(tmp_exc_type, (B,)):
          e = tmp_exc_value
          handlerB(e)
       else:
          handlerElse()

For Python3, the assigned ``e`` variables get deleted at the end of the handler
block. Should that value be already deleted, that ``del`` does not raise,
therefore it's tolerant. This has to be done in any case, so for Python3 it is
even more complex.

.. code-block:: python

    try:
        block()
    except:
        # These are special nodes that access the exception, and don't really
        # use the "sys" module.
        tmp_exc_type = sys.exc_info()[0]
        tmp_exc_value = sys.exc_info()[1]

        # exception_matches is a comparison operation, also a special node.
        if exception_matches(tmp_exc_type, (A,)):
            try:
                e = tmp_exc_value
                handlerA(e)
            finally:
                del e
        elif exception_matches(tmp_exc_type, (B,)):
            try:
                e = tmp_exc_value
                handlerB(e)
            finally:
                del e
        else:
            handlerElse()

Should there be no ``else:`` branch, a default re-raise statement is used
instead.

And of course, the values of the current exception type and value, both use
special references, that access the C++ and don't go via ``sys.exc_info`` at
all, nodes called ``CaughtExceptionTypeRef`` and ``CaughtExceptionValueRef``.

This means, that the different handlers and their catching run time behavior are
all explicit and reduced the branches.

Statement ``try``/``except`` with ``else``
++++++++++++++++++++++++++++++++++++++++++

Much like ``else`` branches of loops, an indicator variable is used to indicate
the entry into any of the exception handlers.

Therefore, the ``else`` becomes a real conditional statement in the node tree,
checking the indicator variable and guarding the execution of the ``else``
branch.


Class Creation (Python2)
++++++++++++++++++++++++

Classes in Python2 have a body that only serves to build the class dictionary
and is a normal function otherwise. This is expressed with the following
re-formulation:

.. code-block:: python

   # in module "SomeModule"
   # ...

   class SomeClass(SomeBase, AnotherBase)
       """ This is the class documentation. """

       some_member = 3

.. code-block:: python

   def _makeSomeClass:
       # The module name becomes a normal local variable too.
       __module__ = "SomeModule"

       # The doc string becomes a normal local variable.
       __doc__ = """ This is the class documentation. """

       some_member = 3

       return locals()

       # force locals to be a writable dictionary, will be optimized away, but
       # that property will stick. This is only to express, that locals(), where
       # used will be writable to.
       exec ""

   SomeClass = make_class("SomeClass", (SomeBase, AnotherBase), _makeSomeClass())

That is roughly the same, except that ``_makeSomeClass`` is *not* visible to its
child functions when it comes to closure taking, which we cannot express in
Python language at all.

Therefore, class bodies are just special function bodies that create a
dictionary for use in class creation. They don't really appear after the tree
building stage anymore. The type inference will of course have to become able to
understand ``make_class`` quite well, so it can recognize the created class
again.

Class Creation (Python3)
++++++++++++++++++++++++

In Python3, classes are a complicated way to write a function call, that can
interact with its body. The body starts with a dictionary provided by the
metaclass, so that is different, because it can ``__prepare__`` a non-empty
locals for it, which is hidden away in "prepare_class_dict" below.

What's noteworthy, is that this dictionary, could e.g. be a ``OrderDict``. I am
not sure, what ``__prepare__`` is allowed to return.

.. code-block:: python

   # in module "SomeModule"
   # ...

   class SomeClass(SomeBase, AnotherBase, metaclass = SomeMetaClass)
       """ This is the class documentation. """

       some_member = 3

.. code-block:: python

   # Non-keyword arguments, need to be evaluated first.
   tmp_bases = ( SomeBase, AnotherBase )

   # Keyword arguments go next, __metaclass__ is just one of them. In principle
   # we need to forward the others as well, but this is ignored for the sake of
   # brevity.
   tmp_metaclass = select_metaclass(tmp_bases, SomeMetaClass )

   tmp_prepared = tmp_metaclass.__prepare__("SomeClass", tmp_bases)

   # The function that creates the class dictionary. Receives temporary variables
   # to work with.
   def _makeSomeClass:
       # This has effect, currently I don't know how to force that in Python3
       # syntax, but we will use something that ensures it.
       locals() = tmp_prepared

       # The module name becomes a normal local variable too.
       __module__ = "SomeModule"

       # The doc string becomes a normal local variable.
       __doc__ = """ This is the class documentation. """

       some_member = 3

       # Create the class, share the potential closure variable "__class__"
       # with others.
       __class__ = tmp_metaclass("SomeClass", tmp_bases, locals())

       return __class__

   # Build and assign the class.
   SomeClass = _makeSomeClass()

Generator Expressions
+++++++++++++++++++++

There are re-formulated as functions.

Generally they are turned into calls of function bodies with (potentially
nested) for loops:

.. code-block:: python

    gen = ( x*2 for x in range(8) if cond() )

.. code-block:: python

    def _gen_helper(__iterator):
       for x in __iterator:
          if cond():
              yield x*2

    gen = _gen_helper( range(8 ) )

List Contractions
+++++++++++++++++

The list contractions of Python2 are different from those of Python3, in that
they don't actually do any closure variable taking, and that no function object
ever exists.

.. code-block:: python

   list_value = [ x*2 for x in range(8) if cond() ]

.. code-block:: python

    def _listcontr_helper(__iterator):
       result = []

       for x in __iterator:
          if cond():
              result.append( x*2 )

       return result

    list_value = listcontr_helper( range(8) )

The difference is that with Python3, the function "_listcontr_helper" is real
and named ``<listcomp>``, whereas with Python2 the function must be considered
in-lined.

This in-inlining in case of Python2 causes difficulties, because it's statements
that occur inside an expression, which means a lot of side effects, that may or
may not be possible to unroll to outside.


Set Contractions
++++++++++++++++

TODO.


Dict Contractions
+++++++++++++++++

TODO.


Boolean expressions ``and`` and ``or``
++++++++++++++++++++++++++++++++++++++

The short circuit operators ``or`` and ``and`` tend to be only less general that
the ``if``/``else`` expressions and are therefore re-formulated as such:

.. code-block:: python

    expr1() or expr2()

.. code-block:: python

   _tmp if ( _tmp = expr1() ) else expr2()

.. code-block:: python

    expr1() and expr2()

.. code-block:: python

   expr2() if ( _tmp = expr1() ) else _tmp

In this form, the differences between these two operators becomes very apparent,
the operands are simply switching sides.

With this the branch that the "short-circuit" expresses, becomes obvious, at the
expense of having the assignment expression to the temporary variable, that one
needs to create anyway.

.. note::

   The release of "_tmp" should happen as soon as the expression using the value
   of the ``or``/``and`` expression is finished. This is achieving by wrapping
   that one with a ``del`` statement in a ``ExpressionTryFinally``.


Simple Calls
++++++++++++

As seen below, even complex calls are simple calls. In simple calls of Python
there is still some hidden semantic going on, that we expose.

.. code-block:: python

   func(arg1, arg2, named1 = arg3, named2 = arg4)

On the C-API level there is a tuple and dictionary built. This one is exposed:

.. code-block:: python

   func(*(arg1, arg2), **{"named1" : arg3, "named2" : arg4})

A called function will access this tuple and the dictionary to parse the
arguments, once that is also re-formulated (argument parsing), it can then lead
to simple inlining. This way calls only have 2 arguments with constant
semantics, that fits perfectly with the C-API where it is the same, so it is
actually easier for code generation.

Although the above looks like a complex call, it actually is not. No checks are
needed for the types of the star arguments and it's directly translated to
``PyObject_Call``.

Complex Calls
+++++++++++++

The call operator in Python allows to provide arguments in 4 forms.

* Positional (or normal) arguments
* Named (or keyword) arguments
* Star list arguments
* Star dictionary arguments

The evaluation order is precisely that. An example would be:

.. code-block:: python

   something(pos1, pos2, name1 = named1, name2 = named2, *star_list, **star_dict)

The task here is that first all the arguments are evaluated, left to right, and
then they are merged into only two, that is positional and named arguments
only. for this, the star list argument and the star dict arguments, are merged
with the positional and named arguments.

What's peculiar, is that if both the star list and dict arguments are present,
the merging is first done for star dict, and only after that for the star list
argument. This makes a difference, because in case of an error, the star
argument raises first.

.. code-block:: python

   something(*1, **2)

This raises "TypeError: something() argument after ** must be a mapping, not
int" as opposed to a possibly more expected "TypeError: something() argument
after * must be a sequence, not int."

That doesn't matter much though, because the value is to be evaluated first
anyway, and the check is only performed afterwards. If the star list argument
calculation gives an error, this one is raised before checking the star dict
argument.

So, what we do, is we convert complex calls by the way of special functions,
which handle the dirty work for us. The optimization is then tasked to do the
difficult stuff. Our example becomes this:

.. code-block:: python

   def _complex_call(called, pos, kw, star_list_arg, star_dict_arg):
       # Raises errors in case of duplicate arguments or tmp_star_dict not
       # being a mapping.
       tmp_merged_dict = merge_star_dict_arguments( called, tmp_named, mapping_check( called, tmp_star_dict ) )

       # Raises an error if tmp_star_list is not a sequence.
       tmp_pos_merged = merge_pos_arguments( called, tmp_pos, tmp_star_list )

       # On the C-API level, this is what it looks like.
       return called( *tmp_pos_merged, **tmp_merged_dict )

   returned = _complex_call(
       called        = something,
       pos           = (pos1, pos2),
       named         = {
           "name1" : named1,
           "name2" = named2
       },
       star_list_arg = star_list,
       star_list_arg = star_dict
   )


The call to ``_complex_call`` is be a direct function call with no parameter
parsing overhead. And the call in its end, is a special call operation, which
relates to the "PyObject_Call" C-API.

Print statements
++++++++++++++++

The ``print`` statement exists only in Python2. It implicitly coverts its
arguments to strings before printing them. In order to make this accessible and
compile time optimized, this is made visible in the node tree.

.. code-block:: python

    print arg1, "1", 1

.. code-block:: python

    print str(arg1), "1", str(1)

Only string objects are spared from the ``str`` built-in wrapper, because that
would only cause noise in optimization stage.

Additionally, each ``print`` may have a target, and multiple arguments, which we
break down as well for dumber code generation. The target is evaluated first and
should be a file, kept referenced throughout the whole print statement.

.. code-block:: python

    print >>target_file, str(arg1), "1", str(1)

This is being reformulated to:

    try:
        tmp_target = target_file

        print >>tmp_target, str(arg1),
        print >>tmp_target, "1",
        print >>tmp_target, str(1),
        print >>tmp_target
    finally:
        del tmp_target

This allows code generation to not deal with arbitrary amount of arguments to
``print``. It also separates the newline indicator from the rest of things,
which makes sense too, having it as a special node, as it's behaviour with
regards to soft-space is different of course.

And finally, for ``print`` without a target, we still assume that a target was
given, which would be ``sys.stdout`` in a rather hard-coded way (no variable
look-ups involved).


Nodes that serve special purposes
---------------------------------

Side Effects
++++++++++++

When an exception is bound to occur, and this can be determined at compile time,
Nuitka will not generate the code the leads to the exception, but directly just
raise it. But not in all cases, this is the full thing.

Consider this code:

.. code-block:: python

   f(a(), 1 / 0)

The second argument will create a ``ZeroDivisionError`` exception, but before
that ``a()`` must be executed, but the call to ``f`` will never happen and no
code is needed for that, but the name look-up must still succeed. This then
leads to code that is internally like this:

.. code-block:: python

   f(a(), raise ZeroDivisionError)

which is then modeled as:

.. code-block:: python

   side_effect(a(), f, raise ZeroDivisionError)

where we can consider "side_effect" to be a function that returns the last
expression. Of course, if this is not part of another expression, but close to
statement level, side effects, can be converted to multiple statements simply.

Another use case, is that the value of an expression can be predicted, but that
the language still requires things to happen, consider this:

.. code-block:: python

   a = len(
      ( f(), g() )
   )

We can tell that ``a`` will be 2, but the call to ``f`` and ``g`` must still be
performed, so it becomes:

.. code-block:: python

   a = side_effects(f(), g(), 2)

Modelling side effects explicitely has the advantage of recognizing them easily
and allowing to drop the call to the tuple building and checking its length,
only to release it.

Caught Exception Type/Value References
++++++++++++++++++++++++++++++++++++++

When catching an exception, in C++, an exception object is used. Exception
handler code is being re-formulated to assign the caught exception to a name, to
check its type for values, etc.

For these, not ``sys.exc_info()`` is used, instead there are special nodes
dedicated to these values: ``CaughtExceptionTypeRef`` and
``CaughtExceptionValueRef``.

Call to ``dir`` without arguments
---------------------------------

This expression is reformulated to ``locals().keys()`` for Python2, and
``list(locals.keys())``.


Hard Module Imports
-------------------

These are module look-ups that don't depend on any local variable for the module
to be looked up, but with hard-coded names. These may be the result of
optimization gaining such level of certainty.

Currently they are used to represent ``sys.stdout`` usage for ``print``
statements, but other usages will follow.


Plan to replace "python-qt" for the GUI
=======================================

Porting the tree inspector available with ``--dump-gui`` to "wxWindows" is very
much welcome as the "python-qt4" bindings are severely under documented.


Plan to add "ctypes" support
============================

Add interfacing to C code, so Nuitka can turn a ``ctypes`` binding into an
efficient binding as if it were written manually with Python C-API or better.

Goals/Allowances to the task
----------------------------

1. Goal: Must not use any pre-existing C/C++ language file headers, only
   generate declarations in generated C++ code ourselves. We would rather write
   a C header to ``ctypes`` declarations convert if it needs to be, but not mix
   and use declarations from existing header code.
2. Allowance: May use ``ctypes`` module at compile time to ask things about
   ``ctypes`` and its types.
3. Goal: Should make use of ``ctypes``, to e.g. not hard code what
   ``ctypes.c_int()`` gives on the current platform, unless there is a specific
   benefit.
4. Allowance: Not all ``ctypes`` usages must be supported immediately.
5. Goal: Try and be as general as possible. For the compiler, ``ctypes`` support
   should be hidden behind a generic interface of some sort. Supporting ``math``
   module should be the same thing.

Type Inference - The Discussion
-------------------------------

Main goal is to forward value knowledge. When you have ``a = b``, that means
that a and b now "alias". And if you know the value of ``b`` you can assume to
know the value of ``a``. This is called "Aliasing".

When assigning ``a`` to something new, that won't change ``b`` at all. But when
an attribute is set, a method called of it, that impacts both, or actually the
value. We need to understand mutable vs. immutable though.

.. code-block:: python

   a = 3
   b = 3

   b += 4 # a is not changed

   a = [ 3 ]
   b = [ 3 ]

   b += [ 4 ] # a is changed

If we cannot tell, we must assume that ``a`` might be changed. It's either ``b``
or what ``a`` was before. If the type is not mutable, we can assume the aliasing
to be broken up, and if it is, we can assume both to be the same value still.

When that value is a compile time constant, we will want to push it forward,
because storing such a constant under a variable name has a cost and loading it
back from the variable as well. So, you want to be able collapse such code:

.. code-block:: python

   a = 3
   b = 7
   c = a / b

to:

.. code-block:: python

   c = 3 / 7

and that obviously to:

.. code-block:: python

   c = 0

This may be called "(Constant) Value Propagation". But we are aiming for even
more. We want to forward propagate abstract properties of the values.

.. note::

   Built-in exceptions, and built-in names are also compile time constants.

In order to fully benefit from type knowledge, the new type system must be able
to be fully friends with existing built-in types.  The behavior of a type
``long``, ``str``, etc. ought to be implemented as far as possible with the
builtin ``long``, ``str`` as well.

.. note::

   This "use the real thing" concept extends beyond builtin types,
   e.g. ``ctypes.c_int()`` should also be used, but we must be aware of platform
   dependencies. The maximum size of ``ctypes.c_int`` values would be an example
   of that. Of course that may not be possible for everything.

   This approach has well proven itself with built-in functions already, where
   we use real built-ins where possible to make computations. We have the
   problem though that built-ins may have problems to execute everything with
   reasonable compile time cost.

Another example, consider the following code:

.. code-block:: python

   len( "a" * 1000000000000 )

To predict this code, calculating it at compile time using constant operations,
while feasible, puts an unacceptable burden on the compilation.

Esp. we wouldn't want to produce such a huge constant and stream it, the C++
code would become too huge. So, we need to stop the ``*`` operator from being
used at compile time and cope with reduced knowledge, already here:

.. code-block:: python

   "a" * 10000000000000

Instead, we would probably say that for this expression:

   - The result is a ``str`` or ``PyStringObject``.
   - We know its length exactly, it's ``10000000000000``.
   - Can predict every of its elements when sub-scripted, sliced, etc., if need
     be, with a function we may create.

Similar is true for this horrible thing:

.. code-block:: python

   range( 10000000000000 )

So it's a rather general problem, this time we know:

   - The result is a ``list`` or ``PyListObject``
   - We know its length exactly, ``10000000000000``
   - Can predict every of its elements when index, sliced, etc., if need be,
     with a function.

Again, we wouldn't want to create the list. Therefore Nuitka avoids executing
these calculation, when they result in constants larger than a threshold of
e.g. 256. This concept has to be also applied to integers and more CPU and
memory traps.

Now lets look at a more common use case:

.. code-block:: python

   for x in range( 10000000000000 ):
       doSomething()

Looking at this example, one traditional way to look at it, would be to turn
``range`` into ``xrange``, and to note that ``x`` is unused. That would already
perform better. But really better is to notice that ``range()`` generated values
are not used at all, but only the length of the expression matters.

And even if ``x`` were used, only the ability to predict the value from a
function would be interesting, so we would use that computation function instead
of having an iteration source. Being able to predict from a function could mean
to have Python code to do it, as well as C++ code to do it. Then code for the
loop can be generated without any CPython library usage at all.

.. note::

   Of course, it would only make sense where such calculations are "O(1)"
   complexity, i.e. do not require recursion like "n!" does.

The other thing is that CPython appears to at - run time - take length hints
from objects for some operations, and there it would help too, to track length
of objects, and provide it, to outside code.

Back to the original example:

.. code-block:: python

   len( "a" * 1000000000000 )

The theme here, is that when we can't compute all intermediate expressions, and
we sure can't do it in the general case. But we can still, predict some of
properties of an expression result, more or less.

Here we have ``len`` to look at an argument that we know the size of. Great. We
need to ask if there are any side effects, and if there are, we need to maintain
them of course. This is already done by existing optimization if an operation
generates an exception.

.. note::

   The optimization of ``len`` has been implemented and works for all kinds of
   container creation and ranges.

Applying this to "ctypes"
-------------------------

The not so specific problem to be solved to understand ``ctypes`` declarations
is maybe as follows:

.. code-block:: python

   import ctypes

This leads to Nuitka in its tree to have an assignment from a ``__import__``
expression to the variable ``ctypes``. It can be predicted by default to be a
module object, and even better, it can be known as ``ctypes`` from standard
library with more or less certainty. See the section about "Importing".

So that part is "easy", and it's what will happen. During optimization, when the
module ``__import__`` expression is examined, it should say:

   - ``ctypes`` is a module
   - ``ctypes`` is from standard library (if it is, may not be true)
   - ``ctypes`` has a ``ModuleFriend`` that knows things about it attributes,
     that should be asked.

The later is the generic interface, and the optimization should connect the two,
of course via package and module full names. It will need a
``ModuleFriendRegistry``, from which it can be pulled. It would be nice if we
can avoid ``ctypes`` to be loaded into Nuitka unless necessary, so these need to
be more like a plug-in, loaded only if necessary, i.e. the user code actually
uses ``ctypes``.

Coming back to the original expression, it also contains an assignment
expression, because it is more like this:

.. code-block:: python

   ctypes = __import__( "ctypes" )

The assigned to object, simply gets the type inferred propagated as part of an
SSA form. Ideally, we could be sure that nothing in the program changes the
variable, and therefore have only one version of that variable.

For module variables, when the execution leaves the module to unknown code, or
unclear code, it might change the variable. Therefore, likely we will often only
assume that it could still be ctypes, or something else.

Depending on how well we control module variable assignment, we can decide this
more of less quickly. With "compiled modules" types, the expectation is that
it's merely a quick C++ `==` comparison check. The module friend should offer
code to allow a check if it applies, for uncertain cases.

Then when we come to uses of it:

.. code-block:: python

   ctypes.c_int()

At this point, using SSA, we are more of less sure, that ``ctypes`` is at that
point the module, and that we know what it's ``c_int`` attribute is, at comile
time, and what it's call result is. We will use the module friend to help with
that. It will attach knowledge about the result of that expression during the
SSA collection process.

This is more like a value forward propagation than anything else. In fact,
constant propagation should only be the special case of it, and one design goal
of Nuitka was always to cover these two cases with the same code.

Excursion to Functions
----------------------

In order to decide what this means to functions and their call boundaries, if we
propagate forward, how to handle this:

.. code-block:: python

   def my_append(a, b):
      a.append( b )

      return a

We would notate that ``a`` is first a "unknown but defined parameter object",
then later on something that definitely has an ``append`` attribute, when
returned. Otherwise an exception occurs.

The type of ``a`` changes to that after ``a.append`` look-up succeeds. It might
be many kinds of an object, but e.g. it could have a higher probability of being
a ``PyListObject``. And we would know it cannot be a ``PyStringObject``, as that
one has no "append".

.. note::

   If classes, i.e. other types in the program, have an ``append`` attribute, it
   should play a role too, there needs to be a way to plug-in to this decisions.

.. note::

   On the other hand, types without ``append`` attribute could be eliminated.

It would be great, if functions provided some sort of analysis on their return
type, or a quick way to predict return value properties, based on input value
knowledge.

So this could work:

.. code-block:: python

   b = my_append( [], 3 )

   assert b == [3] # Could be decided now

Goal: The structure we use makes it easy to tell what ``my_append`` may be. So,
there should be a means to ask it about call results with given type/value
information. We need to be able to tell, if evaluating ``my_append`` makes sense
with given parameters or not, if it does impact the return value.

We should e.g. be able to make ``my_append`` tell, one or more of these:

  - Returns the first parameter value as return value (unless it raises an
    exception).

  - The return value has the same type as ``a`` (unless it raises an
    exception).

  - The return value has an ``append`` attribute.

  - The return value might be a ``list`` object.

  - The return value may not be a ``str`` object.

  - The function will raise if first argument has no ``append`` attribute.

The exactness of statements may vary. But some things may be more
interesting. If e.g. the aliasing of a parameter value to the return value is
known exactly, then information about it need to all be given up, but some can
survive.

It would be nice, if ``my_append`` had sufficient information, so we could
specialize with ``list`` and ``int`` from the parameters, and then e.g. know at
least some things that it does in that case. Such specialization would have to
be decided if it makes sense. In the alternative, it could be done for each
variant anyway, as there won't be that many of them.

Doing this "forward" analysis appears to be best suited for functions and
therefore long term. We will try it that way.

Excursion to Loops
------------------

.. code-block:: python

   a = 1

   while 1:   # think loop: here
       b = a + 1
       a = b

       if cond():
          break

   print a

The handling of loops (both "for" and "while" are re-formulated to loops with
breaks) has its own problem. The loop start and may have an assumption from
before it started, that "a" is constant, but that is only true for the first
iteration. So, we can't pass knowledge from outside loop forward directly into
the for loop body.

So the collection for loops needs to be two pass. First, to collect assignments,
and merge these into the start state, before entering the loop body. The need to
make two passes is special to loops.

For a start, it could be done like this though: At loop entry, all knowledge is
removed about everything, and so is at loop exit. That way, only the loop inner
working is optimized, and before and after the loop are separate things. The
optimal handling of "a" in the example code will take a while.

For a general solution, it would be sweet to trace different exit paths
differently. One loop exit may be good enough, as it will be the common case.

Excursion to Conditions
-----------------------

.. code-block:: python

   if cond:
      x = 1
   else:
      x = 2

   b = x < 3

The above code contains a condition, and these have the problem, that when
exiting the conditional block, a merge must be done, of the "x" versions. It
could be either one. The merge may trace the condition under which a choice is
taken. That way, we could decide pairs of traces under the same condition.

These merges of SSA variable versions, represent alternatives. They pose
difficulties, and might have to be reduced to commonality. In the above example,
the "<" operator will have to check for each version, and then to decide that
both indeed give the same result.

The constraint collection tracks variable changes in conditional branches, and
then merges the existing state at conditional statement exits.

.. note::

   A branch is considered "exiting" if it is not abortive. Should it end in a
   ``raise``, ``break``, ``continue``, or ``return``, there is no need to merge
   that branch, as execution of that branch is terminated.

   Should both branches be abortive, that makes things really simple, as there
   is no need to even continue.

   Should only one branch exist, but be abortive, then no merge is needed, and
   the collection can assume after the conditional statement, that the branch
   was not taken, and continue.

When exiting both the branches, these branches must both be merged, with their
new information.

In the above case:

   - The "yes" branch knows variable ``x`` is an ``int`` of constant value ``1``
   - The "no" branch knows variable ``x`` is an ``int`` of constant value ``2``

That might be collapsed to:

   - The variable ``x`` is an integer of value in ``(1,2)``

Given this, we then should be able to precompute the value of this:

.. code-block:: python

   b = x < 3

The comparison operator can therefore decide and tell:

    - The variable ``b`` is a boolean of constant value ``True``.

Were it unable to decide, it would still be able to say:

    - The variable ``b`` is a boolean.

For conditional statements optimization, it's also noteworthy, that the
condition is known to pass or not pass the truth check, inside branches, and in
the case of non-exiting single branches, after the statement it's not true.

We may want to take advantage of it. Consider e.g.

.. code-block:: python

   if type( a ) is list:
       a.append( x )
   else:
       a += ( x, )

In this case, the knowledge that ``a`` is a list, could be used to generate
better code and with the definite knowledge that ``a`` is of type list. With
that knowledge the ``append`` attribute call will become the ``list`` built-in
type operation.

Excursion to ``return`` statements
----------------------------------

The ``return`` statement (like ``break``, ``continue``, ``raise``) is "aborting"
to control flow. It is always the last statement of inspected block. Were there
statements to follow it, optimization will remove it as dead code.

If all branches of a conditional statement are "aborting", the statement is
decided "aborting" too. If a loop doesn't break, it should be considered
"aborting" too.

.. note::

   The removal of statements following "aborting" statements is implemented, and
   so is the discovery of abortive conditional statements. It's not yet done for
   loops, temp blocks, etc. though.

So, ``return`` statements are easy for local optimization. In the general
picture, it would be sweet to collect all return statements, and analyze the
commonality of them. The goal to predict function results, might be solvable by
looking at their traces.

Excursion to ``yield`` expressions
----------------------------------

The ``yield`` expression can be treated like a normal function call, and as such
invalidates some known constraints just as much as they do. It executes outside
code for an unknown amount of time, and then returns, with little about the
outside world known anymore.

Mixed Types
-----------

Consider the following inside a function or module:

.. code-block:: python

   if cond is not None:
      a = [ x for x in something() if cond(x) ]
   else:
      a = ()

A programmer will often not make a difference between ``list`` and ``tuple``. In
fact, using a ``tuple`` is a good way to express that something won't be changed
later, as these are mutable.

.. note::

   Better programming style, would be to use this:

   .. code-block:: python

      if cond is not None:
         a = tuple( x for x in something() if cond(x) )
      else:
         a = ()

   People don't do it, because they dislike the performance hit encountered by
   the generator expression being used to initialize the tuple. But it would be
   more consistent, and so Nuitka is using it, and of course one day Nuitka
   ought to be able to make no difference in performance for it.

To Nuitka though this means, that if ``cond`` is not predictable, after the
conditional statement we may either have a ``tuple`` or a ``list`` type object
in ``a``. In order to represent that without resorting to "I know nothing about
it", we need a kind of ``min``/``max`` operating mechanism that is capable of
say what is common with multiple alternative values.

.. note::

   At this time, we don't really have that mechanism to find the commonality
   between values.

Back to "ctypes"
----------------

.. code-block:: python

   v = ctypes.c_int()

Coming back to this example, we needed to propagate ``ctypes``, then we can
propagate "something" from ``ctypes.int`` and then known what this gives with a
call and no arguments, so the walk of the nodes, and diverse operations should
be addressed by a module friend.

In case a module friend doesn't know what to do, it needs to say so by
default. This should be enforced by a base class and give a warning or note.

Now to the interface
--------------------

The following is the intended interface:

- Iteration with node methods ``computeStatement`` and ``computeNode``.

  These traverse modules and functions (i.e. scopes) and visit everything in the
  order that Python executes it. The visiting object is ``ConstraintCollection``
  and pass forward. Some node types, e.g. ``StatementConditional`` new create
  child constraint collections and handle the SSA merging at exit.

- Replacing nodes during the visit.

  Both ``computeStatement`` and ``computeNode`` are tasked to return potential
  replacements of themselves, together with "tags" (meaningless now), and a
  "message", used for verbose tracing.

  The replacement node of "+" operator, may e.g. the pre-computed result,
  wrapped in side effects of the node.

- Assignments and references affect SSA.

  The SSA tree is initialized every time a scope is visited. Then during
  traversal, traces are built up. Every assignment and merge starts a new trace
  for that matter. References to a given variable version are traced that way.

- Value escapes are traced too.

  When an operation hands over a value to outside code, it indicates so to the
  constraint collection. This is for it to know, when e.g. a constant value,
  might be mutated meanwhile.

- Nodes can be queried about their properties.

  The node base classes offers methods that allow to check if certain operations
  are supported or not. These can always return ``True`` (yes), ``False`` (no),
  and ``None`` (cannot decide). In the case of the later, optimizations may not
  be able do much about it. Lets call these values "tri-state".

  The default implementation will be very pessimistic. Specific node types may
  then declare, that they e.g. have no side effects, do no raise, have a know
  truth value, have a known iteration length, can predict their iteration
  values, etc.

- Nodes are linked to certain states.

  During the collect, a variable reference, is linked to a certain trace state,
  and that can be used by parent operations.

  .. code-block:: python

     a = 1
     b = a + a

  In this example, the references to "a", can look-up the "1" in the trace, and
  base their responses to "+" on it. It will ask "isCompileTimeConstant()" and
  both nodes will respond "True", then "getCompileTimeConstant()" will return
  "1", which will be computed. Then "extractSideEffects()" will return "()" and
  therefore, the result "2" will not be wrapped.

- Class for module import expression ``ExpressionImportModule``.

  This one just knows that something is imported, but not how or what it is
  assigned to. It will be able in a recursive compile, to provide the module as
  an assignment source, or the module variables or submodules as an attribute
  source when referenced from a variable trace or in an expression.

- Base class for module friend ``ModuleFriendBase``.

  This is intended to provide something to overload, which e.g. can handle
  ``math`` in a better way.

- Module ``ModuleFriendRegistry``

  Provides a register function with ``name`` and instances of
  ``ValueFriendModuleBase`` to be registered. Recursed to modules should
  integrate with that too. The registry could well be done with a metaclass
  approach.

- The module friends should each live in a module of their own.

  With a naming policy to be determined. These modules should add themselves via
  above mechanism to ``ModuleFriendRegistry`` and all shall be imported and
  register. Importing of e.g. ``ctypes`` should be delayed to when the friend is
  actually used. A meta class should aid this task.

  The delay will avoid unnecessary blot of the compiler at run time, if no such
  module is used. For "qt" and other complex stuff, this will be a must.

- The walk should initially be single pass, and not maintain history.

  Instead optimization that needs to look at multiple things, e.g. "unused
  assignment", will look at the whole SSA collection afterwards.


Discussing with examples
------------------------

The following examples:

.. code-block:: python

   # Assignment, the source decides the type of the assigned expression
   a = b

   # Operator "attribute look-up", the looked up expression "ctypes" decides
   # via its trace.
   ctypes.c_int

   # Call operator, the called expressions decides with help of arguments,
   # which have been walked, before the call itself.
   called_expression_of_any_complexity()

   # import gives a module any case, and the "ModuleRegistry" may say more.
   import ctypes

   # From import need not give module, "x" decides what it is.
   from x import y

   # Operations are decided by arguments, and CPython operator rules between
   # argument states.
   a + b

The optimization is mostly performed by walking of the tree and performing
constraint collection. When it encounters assignments and references to them, it
considers current state of traces and uses it for ``computeExpression``.

.. note::

   Assignments to attributes, indexes, slices, etc. will also need to follow the
   flow of ``append``, so it cannot escape attention that a list may be
   modified. Usages of ``append`` that we cannot be sure about, must be traced
   to exist, and disallow the list to be considered known value again.

Code Generation Impact
----------------------

Right now, code generation assumes that everything is a ``PyObject *``, i.e. a
Python object, and does not take knowledge of ``int`` or other types into
consideration at all, and it should remain like that for some time to come.

Instead, ``ctypes`` value friend will be asked give ``Identifiers``, like other
codes do too. And these need to be able to convert themselves to objects to work
with the other things.

But Code Generation should no longer require that operations must be performed
on that level. Imagine e.g. the following calls:

.. code-block:: python

   c_call( other_c_call() )

Value returned by "other_c_call()" of say ``c_int`` type, should be possible to
be fed directly into another call. That should be easy by having a ``asIntC()``
in the identifier classes, which the ``ctypes`` Identifiers handle without
conversions.

Code Generation should one day also become able to tell that all uses of a
variable have only ``c_int`` value, and use ``int`` instead of
``PyObjectLocalVariable`` more or less directly. We could consider
``PyIntLocalVariable`` of similar complexity as ``int`` after the C++ compiler
performed its in-lining.

Such decisions would be prepared by finalization, which then would track the
history of values throughout a function or part of it.

Initial Implementation
----------------------

The basic interface will be added to *all* expressions and a node may override
it, potentially using constraint collection state, as attached during
"computeExpression".

Goal 1
++++++

Initially most things will only be able to give up on about anything. And it
will be little more than a tool to do simple look-ups in a general form. It will
then be the first goal to turn the following code into better performing one:

.. code-block:: python

   a = 3
   b = 7
   c = a / b
   return c

to:

.. code-block:: python

   a = 3
   b = 7
   c = 3 / 7
   return c

and then:

.. code-block:: python

   a = 3
   b = 7
   c = 0
   return c

and then:

.. code-block:: python

   a = 3
   b = 7
   c = 0
   return 0

This depends on SSA form to be able to tell us the values of ``a``, ``b``, and
``c`` to be written to by constants, which can be forward propagated at no cost.

Goal 2
++++++

The assignments to ``a``, ``b``, and ``c`` shall all become prey to "unused"
assignment analysis in the next step. They are all only assigned to, and the
assignment source has no effect, so they can be simply dropped.

.. code-block:: python

   return 0

In the SSA form, these are then assignments without references. These
assignments, can be removed if the assignment source has no side effect. Or at
least they could be made "anonymous", i.e. use a temporary variable instead of
the named one. That would have to take into account though, that the old version
still needs a release.

The most general form would first merely remove assignments that have no impact,
and leave the value as a side effect, so we arrive at this first:

.. code-block:: python

   3
   7
   0
   return 0

When applying the removal of expression only statements without effect, this
gives us:

.. code-block:: python

   return 0

which is the perfect result. Doing it in one step would only be an optimization.

In order to be able to manipulate nodes related to a variable trace, we need to
attach the nodes that did it. Consider this:

.. code-block:: python

   if cond():
       x = 1
   elif other():
       x = 3

   # Not using "x".
   return 0

In the above case, the merge of the value friends, should say that ``x`` may be
undefined, or one of ``1`` or ``3``, but since ``x`` is not used, apply the
"dead value" trick to each branch.

The removal of the "merge" of the 3 ``x`` versions, should exhibit that the
other versions are also only assigned to, and can be removed. These merges of
course appear as usages of the ``x`` versions.

Goal 3
++++++

Then third goal is to understand all of this:

.. code-block:: python

   def f():
      a = []

      print a

      for i in range(1000):
          print a

          a.append( i )

      return len( a )

.. note::

   There are many operations in this, and all of them should be properly
   handled, or at least ignored in safe way.

The first goal code gave us that the ``list`` has an annotation from the
assignment of ``[]`` and that it will be copied to ``a`` until the for loop in
encountered. Then it must be removed, because the ``for`` loop somehow says so.

The ``a`` may change its value, due to the unknown attribute look-up of it
already, not even the call. The for loop must be able to say "may change value"
due to that, of course also due to the call of that attribute too.

The code should therefore become equivalent to:

.. code-block:: python

   def f():
      a = []

      print []

      for i in range(1000):
          print a

          a.append( i )

      return len( a )

But no other changes must occur, especially not to the "return" statement, it
must not assume "a" to be constant "[]" but an unknown "a" instead.

With that, we would handle this code correctly and have some form constant value
propagation in place, handle loops at least correctly, and while it is not much,
it is important demonstration of the concept.

Goal 4
++++++

The fourth goal is to understand the following:

.. code-block:: python

   def f(cond):
       y = 3

       if cond:
           x = 1
       else:
           x = 2

   return x < y

In this we have a branch, and we will be required to keep track of both the
branches separately, and then to merge with the original knowledge. After the
conditional statement we will know that "x" is an "int" with possible values in
"(1,2)", which can be used to predict that the return value is always "True".

The forth goal will therefore be that the "ValueFriendConstantList" knows that
append changes "a" value, but it remains a list, and that the size increases by
one. It should provide an other value friend "ValueFriendList" for "a" due to
that.

In order to do that, such code must be considered:

.. code-block:: python

   a = []

   a.append( 1 )
   a.append( 2 )

   print len( a )

It will be good, if "len" still knows that "a" is a list, but not the constant
list anymore.

From here, work should be done to demonstrate the correctness of it with the
basic tests applied to discover undetected issues.

Fifth and optional goal: Extra bonus points for being able to track and predict
"append" to update the constant list in a known way. Using "list.append" that
should be done and lead to a constant result of "len" being used.

The sixth and challenging goal will be to make the code generation be impacted
by the value friends types. It should have a knowledge that "PyList_Append" does
the job of append and use "PyList_Size" for "len". The "ValueFriends" should aid
the code generation too.

Last and right now optional goal will be to make "range" have a value friend,
that can interact with iteration of the for loop, and "append" of the "list"
value friend, so it knows it's possible to iterate 5000 times, and that "a" has
then after the "loop" this size, so "len( a )" could be predicted. For during
the loop, about a the range of its length should be known to be less
than 5000. That would make the code of goal 2 completely analyzed at compile
time.

Limitations for now
-------------------

- Aim only for limited examples. For ``ctypes`` that means to compile time
  evaluate:

  .. code-block:: python

     print ctypes.c_int( 17 ) + ctypes.c_long( 19 )

  Later then call to "libc" or something else universally available,
  e.g. "strlen()" or "strcmp()" from full blown declarations of the callable.

- We won't have the ability to test that optimization are actually performed, we
  will check the generated code by hand.

  With time, we will add XML based checks with "xpath" queries, expressed as
  hints, but that is some work that will be based on this work here. The "hints"
  fits into the "ValueFriends" concept nicely or so the hope is.

- No inter-function optimization functions yet

  Of course, once in place, it will make the ``ctypes`` annotation even more
  usable. Using ``ctypes`` objects inside functions, while creating them on the
  module level, is therefore not immediately going to work.

- No loops yet

  Loops break value propagation. For the ``ctypes`` use case, this won't be much
  of a difficulty. Due to the strangeness of the task, it should be tackled
  later on at a higher priority.

- Not too much.

  Try and get simple things to work now. We shall see, what kinds of constraints
  really make the most sense. Understanding ``list`` subscript/slice values
  e.g. is not strictly useful for much code and should not block us.

.. note::

   This design is not likely to be the final one.

.. raw:: pdf

   PageBreak


Idea Bin
========

This an area where to drop random ideas on our minds, to later sort it out, and
out it into action, which could be code changes, plan changes, issues created,
etc.

* Make "SELECT_METACLASS" meta class selection transparent.

  Looking at the "SELECT_METACLASS" it should become an anonymous helper
  function. In that way, the optimization process can remove choices at compile
  time, and e.g. inline the effect of a meta class, if it is known.

  This of course makes most sense, if we have the optimizations in place that
  will allow this to actually happen.

* Keeping track of iterations

  The constraint collection trace should become the place, where variables or
  values track their use state. The iterator should keep track of the "next()"
  calls made to it, so it can tell which value to given in that case.

  That would solve the "iteration of constants" as a side effect and it would
  allow to tell that they can be removed.

  That would mean to go back in the tree and modify it long after.

  .. code-block:: python

     a = iter( ( 2, 3 ) )
     b = next( a )
     c = next( a )
     del a

  It would be sweet if we could recognize that:

  .. code-block:: python

     a = iter( ( 2, 3 ) )
     b = side_effect( next( a ), 2 )
     c = side_effect( next( a ), 3 )
     del a

  That trivially becomes:

  .. code-block:: python

     a = iter( ( 2, 3 ) )
     next( a )
     b = 2
     next( a )
     c = 3
     del a

  When the "del a" is examined at the end of scope, or due to another assignment
  to the same variable, ending the trace, we would have to consider of the
  "next" uses, and retrofit the information that they had no effect.

  .. code-block:: python

     a = iter( ( 2, 3 ) )
     b = 2
     b = 3
     del a

* Aliasing

  Each time an assignment is made, an alias is created. A value may have
  different names.

  .. code-block:: python

     a = iter( range(9 ))
     b = a
     c = next(b)
     d = next(a)

  If we fail to detect the aliasing nature, we will calculate "d" wrongly. We
  may incref and decref values to trace it.

  Aliasing is automatically traced already in SSA form. The "b" is assigned to
  version of "a". So, that should allow to replace it with this:

  .. code-block:: python

     a = iter( range(9 ))
     c = next(a)
     d = next(a)

  Which then will be properly handled.

* Shelve for caching

  If we ever came to the conclusion to want and cache complex results of
  analysis, we could do so with the shelve module. We would have to implement
  ``__deepcopy__`` and then could store in there optimized node structures from
  start values after parsing.

* Tail recursion optimization.

  Functions that return the results of calls, can be optimized. The Stackless
  Python does it already.

* Integrate with "upx" compression.

  Calling "upx" on the created binaries, would be easy.

* In-lining constant "exec" and "eval".

  It should be possible to re-formulate at least cases without "locals" or
  "globals" given.

  .. code-block:: python

     def f():
        a = 1
        b = 2

        exec( """a+=b;c=1""" )

        return a, c

  Should become this here:

  .. code-block:: python

     def f():
        a = 1
        b = 2

        a+=b  #
        c=1   # MaybeLocalVariables for everything except known local ones.

        return a, c

  If this holds up, inlining ``exec`` should be relatively easy.

* Original and overloaded built-ins

  This is about making things visible in the node tree. In Nuitka things that
  are not visible in the node tree tend to be wrong. We already pushed around
  information to the node tree a lot.

  Later versions, Nuitka will become able to determine it has to be the original
  built-in at compilt time, then a condition that checks will be optimized away,
  together with the slow path. Or the other path, if it won't be.  Then it will
  be optimized away, or if doubt exists, it will be correct. That is the goal.

  Right now, the change would mean to effectively disable all built-in call
  optimization, which is why we don't immediately do it.

  Making the compatible version, will also require a full listing of all
  built-ins, which is typing work merely, but not needed now. And a way to stop
  built-in optimization from optimizing builtin calls that it used in a
  wrap. Probably just some flag to indicate it when it visits it to skip
  it. That's for later.

  But should we have that both, I figure, we could not raise a ``RuntimeError``
  error, but just do the correct thing, in all cases. An earlier step may raise
  ``RuntimeError`` error, when built-in module values are written to, that we
  don't support.

.. raw:: pdf

   PageBreak

* SSA form for Nuitka nodes

  * Assignments collect a counter from the variable, which becomes the variable
    version. This happens during tree building phase.

  * References need to back track to the last assignment on their path, which
    may be a merge. Constraint collection can do that.

  * Data structures

    Every constraint collection has these:

    * variable_actives

      Dictionary, where per "variable" the currently used version is. Used to
      track situations changes in branches. This is the main input for merge
      process.

    * variable_traces

      Dictionary, where "variable" and "version" form the key. The values are
      objects with or without an assignment, and a list of usages, which starts
      out empty.

      These objects have usages appended to them. In "onVariableSet", a new
      version is allocated, which gives a new object for the dictionary, with an
      empty usages list, because each write starts a new version. In
      "onVariableUsage" the version is detected from the current version. It may
      be not set yet, which means, it's a read of an undefined value (local
      variable, not a parameter name), or unknown in case of global variable.

      These objects may be told that their value has escaped. This should
      influence the value friend they attached to the initial assignment. Each
      usage may have a current value friend state that is different.

  * When merging branches of conditional statements, the merge shall apply as
    follows.

    * Branches have their own collection, with deviating sets of
      "variable_actives". These are children of an outer collections

    * Case a) One branch only.

      For that branch a collection is performed. As usual new assignments
      generate a new version making it "active", references then related to
      these "active" versions.

      Then, when the branch is merged, for all "active" variables, it is
      considered, if that is a change related to before the branch. If it's not
      the same, a merge trace with the branch condition is created with the one
      active in the collection before that statement.

    * Case b) Two branches.

      When there are two branches, they both as are treated as above, except for
      the merge.

      When merging, a difference in active variables between the two branches
      creates the merge trace.

    .. note::

       For conditional expressions, there are always only two branches. Even if
       you think you have more than one branch, you do not. It's always nested
       branches, already when it comes out of the parser.

  * Trace structure

    * Initial write of the version

      There may be a initial write for each version. It can only occur at the
      start of it, but not later, and there is only one. The "value friend" of
      it.

    * Merge of other one or two other versions

      One could be empty, i.e. the variable would not be assigned. This is kind
      of the initial write, and the merge references one or multiple "value
      friends", which are optional.

    * Bunch of read usages. They may allow escape of the value or not. When they
      do, it's a change. The value friend must be informed of it. If it's a real
      escape, usage is not known. If it's merely an alias, e.g. the value is now
      in another variable trace, they could be linked. Otherwise the "value
      friend" must be demoted immediately to one that gives more vague
      information.

    This should be reflected in a class "VariableTrace".

* Recursion checks are expensive.

  If the "caller" or the "called" can declare that it cannot be called by
  itself, we could leave it out.

  TODO: Are they really that expensive? Unnecessary yes, but expensive may not
  be true.

* References

  Currently Nuitka has "Variable" objects. Every variable reference node type
  refers to a "VariableReference" node and there are multiple of them. Every
  variable traces the reference objects created.

  The idea of references started out with closure references and has expanded
  from there. It's now used to decide that a variable is shared. You can ask a
  variable about it, and because it knows all its references, it can tell.

  The thing is, this is not updated, so should a closure variable reference go
  away, it's still shared, as the reference remains. The thing with replaced and
  removed nodes, is that currently they do not remove themselves, there is no
  ``__del__`` being called. I consider this too unreliable.

  That makes the detection of "shared" unreliable and with false positives, that
  so far do not harm much. There is an issue with Python3 not compiling with
  debug mode that is a cause of it.

  Anyway, the problem is increased by the scope of code in use in each
  optimization pass is only ever increasing, but starts out small. That a
  variable is shared or merely used elsewhere, might be discovered late. By
  starting from scratch again, over and over, we might discover this only later.

  That may mean, we should do trace based optimization only after it's all
  complete, and not before. During the collection, information about the sharing
  should be reset at the start, and the built up and judged at the end.

  The task to maintain this would be near ModuleRegistry.

* Statement Sequences with only a frame contained should be optimized

  While it's probably not all that relevant, it appears that the empty module at
  least contains a statement sequence that ends up with only a frame child.

.. raw:: pdf

   PageBreak

Updates for this Manual
=======================

This document is written in REST. That is an ASCII format which is readable as
ASCII, but used to generate PDF or HTML documents.

You will find the current source under:
http://nuitka.net/gitweb/?p=Nuitka.git;a=blob_plain;f=Developer_Manual.rst

And the current PDF under:
http://nuitka.net/doc/Developer_Manual.pdf