Compiler design
===============

This chapter describes the design of the compiler.
The compiler consists a frontend, mid-end and back-end. The frontend deals with
source file parsing and semantics checking. The mid-end performs optimizations.
This is optional. The back-end generates machine code. The front-end produces
intermediate code. This is a simple representation of the source. The back-end
can accept this kind of representation.

The compiler is greatly influenced by the `LLVM`_ design.

.. _llvm: http://www.llvm.org

.. graphviz::
 
   digraph x {
   rankdir="LR"
   1 [label="c3 source file"]
   10 [label="c3 front end" ]
   11 [label="language X front end" ]
   20 [label="mid end" ]
   30 [label="back end for X86" ]
   31 [label="back end for ARM" ]
   40 [label="object file"]
   1 -> 10
   10 -> 20 [label="IR-code"]
   11 -> 20 [label="IR-code"]
   20 -> 30 [label="IR-code"]
   20 -> 31 [label="IR-code"]
   30 -> 40
   }


C3 Front-end
------------

For the front-end a recursive descent parser is created for the c3 language.
This is a subset of the C language with some additional features.

.. graphviz::
  
   digraph c3 {
   rankdir="LR"
   1 [label="source text"]
   10 [label="lexer" ]
   20 [label="parser" ]
   40 [label="code generation"]
   99 [label="IR-code object"]
   1 -> 10
   10 -> 20
   20 -> 40
   40 -> 99
   }

.. autoclass:: ppci.c3.Lexer

.. autoclass:: ppci.c3.Parser

.. autoclass:: ppci.c3.CodeGenerator

.. autoclass:: ppci.c3.Builder


Brainfuck frontend
------------------

The compiler has a front-end for the brainfuck language.

.. autoclass:: ppci.bf.BrainFuckGenerator

IR-code
-------

The intermediate representation (IR) of a program de-couples the front end
from the backend of the compiler.

See :doc:`ir` for details about all the available instructions.


Optimalization
--------------

The IR-code generated by the front-end can be optimized in many ways. The
compiler does not have the best way to optimize code, but instead has a
bag of tricks it can use.

.. autoclass:: ppci.opt.transform.ModulePass

.. autoclass:: ppci.opt.mem2reg.Mem2RegPromotor

.. autoclass:: ppci.opt.transform.LoadAfterStorePass

.. autoclass:: ppci.opt.transform.DeleteUnusedInstructionsPass

.. autoclass:: ppci.opt.transform.RemoveAddZeroPass

.. autoclass:: ppci.opt.transform.CommonSubexpressionEliminationPass


Back-end
--------

The back-end is more complicated. There are several steps to be taken here.

#. Canonicalization
#. Tree creation
#. Instruction selection
#. register allocation
#. Instruction emission
#. TODO: Peep hole optimization?

.. toctree::
    specificationlang

Code generator
~~~~~~~~~~~~~~

.. automodule:: ppci.codegen.codegen

Canonicalize
~~~~~~~~~~~~

During this phase, the IR-code is made simpler. Also unsupported
operations are rewritten into function calls. For example soft floating
point is introduced here.


Tree building
~~~~~~~~~~~~~

From IR-code a tree is generated which can be used to select instructions.

.. automodule:: ppci.codegen.irdag


Instruction selection
~~~~~~~~~~~~~~~~~~~~~

The instruction selection phase takes care of scheduling and instruction
selection.  The output of this phase is a one frame per function with a flat
list of abstract machine instructions.


To select instruction, a tree rewrite system is used. This is also called
bottom up rewrite generator (BURG). See pyburg.


Register allocation
~~~~~~~~~~~~~~~~~~~

The selected instructions are used to select correct registers.

.. autoclass:: ppci.codegen.registerallocator.RegisterAllocator


code emission
~~~~~~~~~~~~~

Code is emitted using the outputstream class. The assembler and compiler use
this class to emit instructions to. The stream can output to object file
or to a logger.

.. autoclass:: ppci.binutils.outstream.OutputStream