path: root/doc/nimrodc.txt

===================================
   Nimrod Compiler User Guide
===================================

:Author: Andreas Rumpf
:Version: |nimrodversion|

.. contents::


Introduction
============

This document describes the usage of the *Nimrod compiler*
on the different supported platforms. It is not a definition of the Nimrod
programming language (therefore is the `manual <manual>`_).

Nimrod is free software; it is licensed under the
`GNU General Public License <gpl.html>`_.


Compiler Usage
==============

Command line switches
---------------------
Basis command line switches are:

.. include:: ../data/basicopt.txt

Advanced command line switches are:

.. include:: ../data/advopt.txt


Configuration file
------------------
The default configuration file is ``nimrod.cfg``. The ``nimrod`` executable
looks for it in the following directories (in this order):

1. ``/home/$user/.config/nimrod.cfg`` (UNIX) or ``$APPDATA/nimrod.cfg`` (Windows)
2. ``$nimrod/config/nimrod.cfg`` (UNIX, Windows)
3. ``/etc/nimrod.cfg`` (UNIX)

The search stops as soon as a configuration file has been found. The reading
of ``nimrod.cfg`` can be suppressed by the ``--skipCfg`` command line option.
Configuration settings can be overwritten in a project specific
configuration file that is read automatically. This specific file has to
be in the same directory as the project and be of the same name, except
that its extension should be ``.cfg``.

Command line settings have priority over configuration file settings.


Generated C code directory
--------------------------
The generated files that Nimrod produces all go into a subdirectory called
``nimcache`` in your project directory. This makes it easy to delete all
generated files.

However, the generated C code is not platform independant. C code generated for
Linux does not compile on Windows, for instance. The comment on top of the
C file lists the OS, CPU and CC the file has been compiled for.


Additional Features
===================

This section describes Nimrod's additional features that are not listed in the
Nimrod manual.

New Pragmas and Options
-----------------------

Because Nimrod generates C code it needs some "red tape" to work properly.
Lots of options and pragmas for tweaking the generated C code are available.

Importc Pragma
~~~~~~~~~~~~~~
The `importc`:idx: pragma provides a means to import a type, a variable, or a
procedure from C. The optional argument is a string containing the C
identifier. If the argument is missing, the C name is the Nimrod
identifier *exactly as spelled*:

.. code-block::
  proc printf(formatstr: cstring) {.importc: "printf", varargs.}


Exportc Pragma
~~~~~~~~~~~~~~
The `exportc`:idx: pragma provides a means to export a type, a variable, or a
procedure to C. The optional argument is a string containing the C
identifier. If the argument is missing, the C name is the Nimrod
identifier *exactly as spelled*:

.. code-block:: Nimrod
  proc callme(formatstr: cstring) {.exportc: "callMe", varargs.}


Dynlib Pragma
~~~~~~~~~~~~~
With the `dynlib`:idx: pragma a procedure or a variable can be imported from
a dynamic library (``.dll`` files for Windows, ``lib*.so`` files for UNIX). The
non-optional argument has to be the name of the dynamic library:

.. code-block:: Nimrod
  proc gtk_image_new(): PGtkWidget {.cdecl, dynlib: "libgtk-x11-2.0.so", importc.}

In general, importing a dynamic library does not require any special linker
options or linking with import libraries. This also implies that no *devel*
packages need to be installed.

The ``dynlib`` import mechanism supports a versioning scheme: 

.. code-block:: nimrod 
  proc Tcl_Eval(interp: pTcl_Interp, script: cstring): int {.cdecl, 
    importc, dynlib: "libtcl(8.5|8.4|8.3).so.(1|0)".}

At runtime the dynamic library is searched for (in this order)::
  
  libtcl8.5.so.1
  libtcl8.4.so.1
  libtcl8.3.so.1
  libtcl8.5.so.0
  libtcl8.4.so.0
  libtcl8.3.so.0


NoDecl Pragma
~~~~~~~~~~~~~
The `noDecl`:idx: pragma can be applied to almost any symbol (variable, proc,
type, etc.) and is sometimes useful for interoperability with C:
It tells Nimrod that it should not generate a declaration for the symbol in
the C code. For example:

.. code-block:: Nimrod
  var
    EACCES {.importc, noDecl.}: cint # pretend EACCES was a variable, as
                                     # Nimrod does not know its value

However, the ``header`` pragma is often the better alternative.


Header Pragma
~~~~~~~~~~~~~
The `header`:idx: pragma is very similar to the ``noDecl`` pragma: It can be
applied to almost any symbol and specifies that it should not be declared 
and instead the generated code should contain an ``#include``:

.. code-block:: Nimrod
  type
    PFile {.importc: "FILE*", header: "<stdio.h>".} = distinct pointer
      # import C's FILE* type; Nimrod will treat it as a new pointer type

The ``header`` pragma always expects a string constant. The string contant
contains the header file: As usual for C, a system header file is enclosed
in angle brackets: ``<>``. If no angle brackets are given, Nimrod
encloses the header file in ``""`` in the generated C code.


Varargs Pragma
~~~~~~~~~~~~~~
The `varargs`:idx: pragma can be applied to procedures only (and procedure 
types). It tells Nimrod that the proc can take a variable number of parameters 
after the last specified parameter. Nimrod string values will be converted to C
strings automatically:

.. code-block:: Nimrod
  proc printf(formatstr: cstring) {.nodecl, varargs.}

  printf("hallo %s", "world") # "world" will be passed as C string


LineDir Option
~~~~~~~~~~~~~~
The `lineDir`:idx: option can be turned on or off. If turned on the 
generated C code contains ``#line`` directives. This may be helpful for 
debugging with GDB.


StackTrace Option
~~~~~~~~~~~~~~~~~
If the `stackTrace`:idx: option is turned on, the generated C contains code to
ensure that proper stack traces are given if the program crashes or an
uncaught exception is raised.


LineTrace Option
~~~~~~~~~~~~~~~~
The `lineTrace`:idx: option implies the ``stackTrace`` option. If turned on,
the generated C contains code to ensure that proper stack traces with line
number information are given if the program crashes or an uncaught exception
is raised.

Debugger Option
~~~~~~~~~~~~~~~
The `debugger`:idx: option enables or disables the *Embedded Nimrod Debugger*.
See the documentation of endb_ for further information.


Breakpoint Pragma
~~~~~~~~~~~~~~~~~
The *breakpoint* pragma was specially added for the sake of debugging with
ENDB. See the documentation of `endb <endb.html>`_ for further information.


Volatile Pragma
~~~~~~~~~~~~~~~
The `volatile`:idx: pragma is for variables only. It declares the variable as
``volatile``, whatever that means in C/C++.

Register Pragma
~~~~~~~~~~~~~~~
The `register`:idx: pragma is for variables only. It declares the variable as
``register``, giving the compiler a hint that the variable should be placed
in a hardware register for faster access. C compilers usually ignore this
though and for good reasons: Often they do a better job without it anyway.

In highly specific cases (a dispatch loop of an bytecode interpreter for
example) it may provide benefits, though.


Acyclic Pragma
~~~~~~~~~~~~~~
The `acyclic`:idx: pragma can be used for object types to mark them as acyclic
even though they seem to be cyclic. This is an **optimization** for the garbage
collector to not consider objects of this type as part of a cycle:

.. code-block:: nimrod
  type
    PNode = ref TNode
    TNode {.acyclic, final.} = object
      left, right: PNode
      data: string

In the example a tree structure is declared with the ``TNode`` type. Note that
the type definition is recursive and the GC has to assume that objects of
this type may form a cyclic graph. The ``acyclic`` pragma passes the
information that this cannot happen to the GC. If the programmer uses the
``acyclic`` pragma for data types that are in reality cyclic, the GC may leak
memory, but nothing worse happens.


DeadCodeElim Pragma
~~~~~~~~~~~~~~~~~~~~~
The `deadCodeElim`:idx: pragma only applies to whole modules: It tells the
compiler to activate (or deactivate) dead code elimination for the module the
pragma appers in.

The ``--deadCodeElim:on`` command line switch has the same effect as marking
every module with ``{.deadCodeElim:on}``. However, for some modules such as
the GTK wrapper it makes sense to *always* turn on dead code elimination -
no matter if it is globally active or not.

Example:

.. code-block:: nimrod
  {.deadCodeElim: on.}


Disabling certain messages
--------------------------
Nimrod generates some warnings and hints ("line too long") that may annoy the
user. A mechanism for disabling certain messages is provided: Each hint
and warning message contains a symbol in brackets. This is the message's
identifier that can be used to enable or disable it:

.. code-block:: Nimrod
  {.warning[LineTooLong]: off.} # turn off warning about too long lines

This is often better than disabling all warnings at once.


Debugging with Nimrod
=====================

Nimrod comes with its own *Embedded Nimrod Debugger*. See
the documentation of endb_ for further information.


Optimizing for Nimrod
=====================

Nimrod has no separate optimizer, but the C code that is produced is very
efficient. Most C compilers have excellent optimizers, so usually it is
not needed to optimize one's code. Nimrod has been designed to encourage
efficient code: The most readable code in Nimrod is often the most efficient
too.

However, sometimes one has to optimize. Do it in the following order:

1. switch off the embedded debugger (it is **slow**!)
2. turn on the optimizer and turn off runtime checks
3. profile your code to find where the bottlenecks are
4. try to find a better algorithm
5. do low-level optimizations

This section can only help you with the last item. 


Optimizing string handling
--------------------------

String assignments are sometimes expensive in Nimrod: They are required to
copy the whole string. However, the compiler is often smart enough to not copy
strings. Due to the argument passing semantics, strings are never copied when
passed to subroutines. The compiler does not copy strings that are result from
a procedure call, because the called procedure returns a new string anyway.
Thus it is efficient to do:

.. code-block:: Nimrod
  var s = procA() # assignment will not copy the string; procA allocates a new
                  # string anyway

However it is not efficient to do:

.. code-block:: Nimrod
  var s = varA    # assignment has to copy the whole string into a new buffer!

..
  String case statements are optimized too. A hashing scheme is used for them
  if several different string constants are used. This is likely to be more
  efficient than any hand-coded scheme.


..
  The ECMAScript code generator
  =============================
  
  Note: As of version 0.7.0 the ECMAScript code generator is not maintained any
  longer. Help if you are interested.
  
  Note: I use the term `ECMAScript`:idx: here instead of `JavaScript`:idx:,
  since it is the proper term.
  
  The ECMAScript code generator is experimental!
  
  Nimrod targets ECMAScript 1.5 which is supported by any widely used browser.
  Since ECMAScript does not have a portable means to include another module,
  Nimrod just generates a long ``.js`` file.
  
  Features or modules that the ECMAScript platform does not support are not
  available. This includes:
  
  * manual memory management (``alloc``, etc.)
  * casting and other unsafe operations (``cast`` operator, ``zeroMem``, etc.)
  * file management
  * most modules of the Standard library
  * proper 64 bit integer arithmetic
  * proper unsigned integer arithmetic
  
  However, the modules `strutils`:idx:, `math`:idx:, and `times`:idx: are
  available! To access the DOM, use the `dom`:idx: module that is only
  available for the ECMAScript platform.