=========================================
    Internals of the Nimrod Compiler
=========================================


:Author: Andreas Rumpf
:Version: |nimrodversion|

.. contents::

  "Abstraction is layering ignorance on top of reality." -- unknown


Directory structure
===================

The Nimrod project's directory structure is:

============   ==============================================
Path           Purpose
============   ==============================================
``bin``        generated binary files
``build``      generated C code for the installation
``nim``        Pascal sources of the Nimrod compiler; this
               has been used for bootstrapping, but new
               development is done with the Nimrod version.
``rod``        Nimrod sources of the Nimrod compiler;
               automatically generated from the Pascal
               version.
``data``       data files that are used for generating source
               code; not used anymore
``doc``        the documentation; it is a bunch of
               reStructuredText files
``dist``       additional packages for the distribution
``config``     configuration files for Nimrod
``lib``        the Nimrod library; ``rod`` depends on it!
``web``        website of Nimrod; generated by ``koch.py``
               from the ``*.txt`` and ``*.tmpl`` files
``obj``        generated ``*.obj`` files
============   ==============================================


Bootstrapping the compiler
==========================

As of version 0.8.5 the compiler is maintained in Nimrod. (The first versions 
have been implemented in Object Pascal.) The Python-based build system has 
been rewritten in Nimrod too.

Compiling the compiler is a simple matter of running::

  nimrod c koch.nim
  ./koch boot

For a release version use::

  nimrod c koch.nim
  ./koch boot -d:release

The ``koch`` program is Nimrod's maintainance script. It is a replacement for
make and shell scripting with the advantage that it is much more portable.


Coding Guidelines
=================

* Use CamelCase, not underscored_identifiers.
* Indent with two spaces.
* Max line length is 80 characters.
* Provide spaces around binary operators if that enhances readability.
* Use a space after a colon, but not before it.
* Start types with a capital ``T``, unless they are pointers which start
  with ``P``. 


Porting to new platforms
========================

Porting Nimrod to a new architecture is pretty easy, since C is the most
portable programming language (within certain limits) and Nimrod generates
C code, porting the code generator is not necessary.

POSIX-compliant systems on conventional hardware are usually pretty easy to
port: Add the platform to ``platform`` (if it is not already listed there),
check that the OS, System modules work and recompile Nimrod.

The only case where things aren't as easy is when the garbage
collector needs some assembler tweaking to work. The standard
version of the GC uses C's ``setjmp`` function to store all registers
on the hardware stack. It may be necessary that the new platform needs to
replace this generic code by some assembler code.


Runtime type information
========================

*Runtime type information* (RTTI) is needed for several aspects of the Nimrod
programming language:

Garbage collection
  The most important reason for RTTI. Generating
  traversal procedures produces bigger code and is likely to be slower on
  modern hardware as dynamic procedure binding is hard to predict.

Complex assignments
  Sequences and strings are implemented as
  pointers to resizeable buffers, but Nimrod requires copying for
  assignments. Apart from RTTI the compiler could generate copy procedures
  for any type that needs one. However, this would make the code bigger and
  the RTTI is likely already there for the GC.

We already know the type information as a graph in the compiler.
Thus we need to serialize this graph as RTTI for C code generation.
Look at the file ``lib/system/hti.nim`` for more information.


The compiler's architecture
===========================

Nimrod uses the classic compiler architecture: A scanner feds tokens to a
parser. The parser builds a syntax tree that is used by the code generator.
This syntax tree is the interface between the parser and the code generator.
It is essential to understand most of the compiler's code.

In order to compile Nimrod correctly, type-checking has to be seperated from
parsing. Otherwise generics cannot work.

.. include:: filelist.txt


The syntax tree
---------------
The synax tree consists of nodes which may have an arbitrary number of
children. Types and symbols are represented by other nodes, because they
may contain cycles. The AST changes its shape after semantic checking. This
is needed to make life easier for the code generators. See the "ast" module
for the type definitions. The `macros <macros.html>`_ module contains many
examples how the AST represents each syntactic structure. 


How the RTL is compiled
=======================

The ``system`` module contains the part of the RTL which needs support by
compiler magic (and the stuff that needs to be in it because the spec
says so). The C code generator generates the C code for it just like any other
module. However, calls to some procedures like ``addInt`` are inserted by
the CCG. Therefore the module ``magicsys`` contains a table (``compilerprocs``)
with all symbols that are marked as ``compilerproc``. ``compilerprocs`` are
needed by the code generator. A ``magic`` proc is not the same as a
``compilerproc``: A ``magic`` is a proc that needs compiler magic for its
semantic checking, a ``compilerproc`` is a proc that is used by the code
generator.


Debugging Nimrod's memory management
====================================

The following paragraphs are mostly a reminder for myself. Things to keep
in mind:

* Segmentation faults can have multiple reasons: One that is frequently
  forgotten is that *stack overflow* can trigger one!
* If an assertion in Nimrod's memory manager or GC fails, the stack trace
  keeps allocating memory! Thus a stack overflow may happen, hiding the
  real issue. 
* What seem to be C code generation problems is often a bug resulting from
  not producing prototypes, so that some types default to ``cint``. Testing
  without the ``-w`` option helps!


The Garbage Collector
=====================

Introduction
------------

I use the term *cell* here to refer to everything that is traced
(sequences, refs, strings).
This section describes how the new GC works.

The basic algorithm is *Deferrent Reference Counting* with cycle detection.
References on the stack are not counted for better performance and easier C
code generation.

Each cell has a header consisting of a RC and a pointer to its type
descriptor. However the program does not know about these, so they are placed at
negative offsets. In the GC code the type ``PCell`` denotes a pointer
decremented by the right offset, so that the header can be accessed easily. It
is extremely important that ``pointer`` is not confused with a ``PCell``
as this would lead to a memory corruption.


The CellSet data structure
--------------------------

The GC depends on an extremely efficient datastructure for storing a
set of pointers - this is called a ``TCellSet`` in the source code.
Inserting, deleting and searching are done in constant time. However,
modifying a ``TCellSet`` during traversation leads to undefined behaviour.

.. code-block:: Nimrod
  type
    TCellSet # hidden

  proc CellSetInit(s: var TCellSet) # initialize a new set
  proc CellSetDeinit(s: var TCellSet) # empty the set and free its memory
  proc incl(s: var TCellSet, elem: PCell) # include an element
  proc excl(s: var TCellSet, elem: PCell) # exclude an element

  proc `in`(elem: PCell, s: TCellSet): bool # tests membership

  iterator elements(s: TCellSet): (elem: PCell)


All the operations have to perform efficiently. Because a Cellset can
become huge a hash table alone is not suitable for this.

We use a mixture of bitset and hash table for this. The hash table maps *pages*
to a page descriptor. The page descriptor contains a bit for any possible cell
address within this page. So including a cell is done as follows:

- Find the page descriptor for the page the cell belongs to.
- Set the appropriate bit in the page descriptor indicating that the
  cell points to the start of a memory block.

Removing a cell is analogous - the bit has to be set to zero.
Single page descriptors are never deleted from the hash table. This is not
needed as the data structures needs to be rebuilt periodically anyway.

Complete traversal is done in this way::

  for each page decriptor d:
    for each bit in d:
      if bit == 1:
        traverse the pointer belonging to this bit


Further complications
---------------------

In Nimrod the compiler cannot always know if a reference
is stored on the stack or not. This is caused by var parameters.
Consider this example:

.. code-block:: Nimrod
  proc setRef(r: var ref TNode) =
    new(r)

  proc usage =
    var
      r: ref TNode
    setRef(r) # here we should not update the reference counts, because
              # r is on the stack
    setRef(r.left) # here we should update the refcounts!

We have to decide at runtime whether the reference is on the stack or not. 
The generated code looks roughly like this:

.. code-block:: C
  void setref(TNode** ref) {
    unsureAsgnRef(ref, newObj(TNode_TI, sizeof(TNode)))
  }
  void usage(void) {
    setRef(&r)
    setRef(&r->left)
  }

Note that for systems with a continous stack (which most systems have)
the check whether the ref is on the stack is very cheap (only two
comparisons).


Code generation for closures
============================

Example code:

.. code-block:: nimrod
  proc add(x: int): proc (y: int): int {.closure.} = 
    return lambda (y: int): int = 
      return x + y

  var add2 = add(2)
  echo add2(5) #OUT 7

This should produce roughly this code:

.. code-block:: nimrod
  type
    PClosure = ref object 
      fn: proc (x: int, c: PClosure): int
      x: int # data
  
  proc wasLambda(y: int, c: PClosure): int = 
    return y + c.x
  
  proc add(x: int): PClosure = 
    var c: PClosure
    new(c)
    c.x = x
    c.fn = wasLambda
  
  var add2 = add(2)
  echo add2.fn(5, add2)


Beware of nesting:

.. code-block:: nimrod
  proc add(x: int): proc (y: int): proc (z: int): int {.closure.} {.closure.} = 
    return lamba (y: int): proc (z: int): int {.closure.} = 
      return lambda (z: int): int = 
        return x + y + z

  var add24 = add(2)(4)
  echo add24(5) #OUT 11

This should produce roughly this code:

.. code-block:: nimrod
  type
    PClosure1 = ref object 
      fn: proc (x: int, c: PClosure1): int
      x: int # data
      
    PClosure2 = ref object 
      fn: proc (x: int, c: PClosure2): int
      y: int
      c1: PClosure1
      
  
  proc innerLambda(z: int, c2: PClosure2): int = 
    return c2.c1.x + c2.y + z
  
  proc outerLambda1(y: int, c1: PClosure1): PClosure2 = 
    new(result)
    result.c1 = c1
    result.y = y
    result.fn = innerLambda
  
  proc add(x: int): PClosure1 = 
    new(result)
    result.x = x
    result.fn = outerLambda
  
  var tmp = add(2)
  var tmp2 = tmp.fn(4, tmp)
  var add24 = tmp2.fn(4, tmp2)
  echo add24(5)


Accumulator
-----------

.. code-block:: nimrod
  proc GetAccumulator(start: int): proc (): int {.closure} = 
    var i = start
    return lambda: int = 
      inc i
      return i