.. Copyright 2011 David Malcolm <dmalcolm@redhat.com>
   Copyright 2011 Red Hat, Inc.

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   <http://www.gnu.org/licenses/>.

.. For notes on how to document Python in RST form, see e.g.:
.. http://sphinx.pocoo.org/domains.html#the-python-domain

Optimization passes
===================

Working with existing passes
----------------------------
GCC organizes the optimization work it does as "passes", and these form trees:
passes can have both successors and child passes.

There are actually five "roots" to this tree:

   * The gcc.Pass holding :ref:`all "lowering" passes <all_lowering_passes>`,
     invoked per function within the callgraph, to turn high-level GIMPLE into
     lower-level forms (this wraps `all_lowering_passes` within gcc/passes.c).

   * The gcc.Pass holding :ref:`all "small IPA" passes <all_small_ipa_passes>`,
     working on the whole callgraph (IPA is "Interprocedural Analysis";
     `all_small_ipa_passes` within gcc/passes.c)

   * The gcc.Pass holding :ref:`all regular IPA passes <all_regular_ipa_passes>`
     (`all_regular_ipa_passes` within gcc/passes.c)

   * The gcc.Pass holding those :ref:`passes relating to link-time-optimization
     <all_lto_gen_passes>` (`all_lto_gen_passes` within gcc/passes.c)

   * The :ref:`"all other passes" gcc.Pass catchall <all_passes>`, holding the
     majority of the passes.  These are called on each function within the call
     graph (`all_passes`  within gcc/passes.c)

.. classmethod:: gcc.Pass.get_roots()

   Returns a tuple of `gcc.Pass` instances, giving the 5 top-level passes
   within GCC's tree of passes, in the order described above.

.. classmethod:: gcc.Pass.get_by_name(name)

   Get the gcc.Pass instance for the pass with the given name, raising
   ValueError if it isn't found

.. py:class:: gcc.Pass

   This wraps one of GCC's `struct opt_pass *`, but the wrapper class is still
   a work-in-progress.  Hopefully we'll eventually be able to subclass this and
   allow creating custom passes written in Python.

   Beware:  "pass" is a reserved word in Python, so use e.g. `ps` as a variable
   name for an instance of gcc.Pass

   .. py:attribute:: name

      The name of the pass, as a string

   .. py:attribute:: sub

      The first child pass of this pass (if any)

   .. py:attribute:: next

      The next sibling pass of this pass (if any)

   .. py:attribute:: properties_required
   .. py:attribute:: properties_provided
   .. py:attribute:: properties_destroyed

      Currently these are int bitfields, expressing the flow of data betweeen
      the various passes.

      They can be accessed using bitwise arithmetic::

          if ps.properties_provided & gcc.PROP_cfg:
	       print(fn.cfg)

      Here are the bitfield flags:

         =========================   ============================================   =========================   =======================
         Mask                        Meaning                                        Which pass sets this up?    Which pass clears this?
         =========================   ============================================   =========================   =======================
         gcc.PROP_gimple_any         Is the full GIMPLE grammar allowed?            (the frontend)              `"expand"`
         gcc.PROP_gimple_lcf         Has control flow been lowered?                 `"lower"`                   `"expand"`
         gcc.PROP_gimple_leh         Has exception-handling been lowered?           `"eh"`                      `"expand"`
         gcc.PROP_cfg                Does the gcc.Function have a non-None "cfg"?   `"cfg"`                     `"*free_cfg"`
         gcc.PROP_referenced_vars    Do we have data on which functions reference   `"\*referenced_vars"`       (none)
	                             which variables? (Dataflow analysis, aka
				     DFA)
         gcc.PROP_ssa                Is the GIMPLE in SSA form?                     `"ssa"`                     `"expand"`
         gcc.PROP_no_crit_edges      Have all critical edges within the CFG been    `"crited"`                  (none)
                                     split?
         gcc.PROP_rtl                Is the function now in RTL form? (rather       `"expand"`                  `"*clean_state"`
	                             than GIMPLE-SSA)
         gcc.PROP_gimple_lomp        Have OpenMP directives been lowered into       `"omplower"`                `"expand"`
	                             explicit calls to the runtime library
				     (libgomp)
         gcc.PROP_cfglayout          Are we reorganizing the CFG into a more        `"into_cfglayout"`          `"outof_cfglayout"`
	                             efficient order?
         gcc.PROP_gimple_lcx         Have operations on complex numbers been        `"cplxlower"`               `"cplxlower0"`
	                             lowered to scalar operations?
         =========================   ============================================   =========================   =======================


There are four subclasses of gcc.Pass:

.. py:class:: gcc.GimplePass

   Subclass of gcc.Pass, signifying a pass called per-function on the GIMPLE
   representation of that function.

.. py:class:: gcc.RtlPass

   Subclass of gcc.Pass, signifying a pass called per-function on the RTL
   representation of that function.

.. py:class:: gcc.SimpleIpaPass

   Subclass of gcc.Pass, signifying a pass called once (not per-function)

.. py:class:: gcc.IpaPass

   Subclass of gcc.Pass, signifying a pass called once (not per-function)

.. _creating-new-passes:

Creating new optimization passes
--------------------------------
You can create new optimization passes.  This involves three steps:

   * subclassing the appropriate gcc.Pass subclass (e.g. gcc.GimplePass)

   * creating an instance of your subclass

   * registering the instance within the pass tree, relative to another pass

Here's an example::

   # Here's the (trivial) implementation of our new pass:
   class MyPass(gcc.GimplePass):
      # This is optional.
      # If present, it should return a bool, specifying whether or not
      # to execute this pass (and any child passes)
      def gate(self, fun):
          print('gate() called for %r' % fun)
          return True

      def execute(self, fun):
          print('execute() called for %r' % fun)

   # We now create an instance of the class:
   my_pass = MyPass(name='my-pass')

   # ...and wire it up, after the "cfg" pass:
   my_pass.register_after('cfg')

For gcc.GimplePass and gcc.IpaPass, the signatures of `gate` and `execute` are:

   .. method:: gate(self, fun)
   .. method:: execute(self, fun)

where fun is a :py:class:`gcc.Function`.

For gcc.SimpleIpaPass and gcc.IpaPass, the signature of `gate` and `execute` are:

   .. method:: gate(self)
   .. method:: execute(self)

If an unhandled exception is raised within `gate` or `execute`, it will lead
to a GCC error:

.. code-block:: pytb

   /home/david/test.c:36:1: error: Unhandled Python exception raised calling 'execute' method
   Traceback (most recent call last):
     File "script.py", line 79, in execute
      dot = gccutils.tree_to_dot(fun)
   NameError: global name 'gccutils' is not defined

.. method:: gcc.Pass.register_after(name [, instance_number=0 ])

   Given the name of another pass, register this gcc.Pass to occur immediately
   after that other pass.

   If the other pass occurs multiple times, the pass will be inserted at the
   specified instance number, or at every instance, if supplied 0.

   .. note::

      The other pass must be of the same kind as this pass.  For example,
      if it is a subclass of gcc.GimplePass, then this pass must also be
      a subclass of gcc.GimplePass.

      If they don't match, GCC won't be able to find the other pass, giving
      an error like this::

         cc1: fatal error: pass 'ssa' not found but is referenced by new pass 'my-ipa-pass'

      where we attempted to register a gcc.IpaPass subclass relative to 'ssa',
      which is a gcc.GimplePass

.. method:: gcc.Pass.register_before(name [, instance_number=0 ])

   As above, but this pass is registered immediately before the referenced
   pass.

.. method:: gcc.Pass.replace(name [, instance_number=0 ])

   As above, but replace the given pass.  This method is included for
   completeness; the result is unlikely to work well.
