Nim/doc/intern.rst

=========================================
    Internals of the Nim Compiler
=========================================


:Author: Andreas Rumpf
:Version: |nimversion|

.. contents::

  "Abstraction is layering ignorance on top of reality." -- Richard Gabriel


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

The Nim project's directory structure is:

============   ==============================================
Path           Purpose
============   ==============================================
``bin``        generated binary files
``build``      generated C code for the installation
``compiler``   the Nim compiler itself; note that this
               code has been translated from a bootstrapping
               version written in Pascal, so the code is **not**
               a poster child of good Nim code
``config``     configuration files for Nim
``dist``       additional packages for the distribution
``doc``        the documentation; it is a bunch of
               reStructuredText files
``lib``        the Nim library
``web``        website of Nim; generated by ``nimweb``
               from the ``*.txt`` and ``*.tmpl`` files
============   ==============================================


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

Compiling the compiler is a simple matter of running::

  nim c koch.nim
  ./koch boot

For a release version use::

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

And for a debug version compatible with GDB::

  nim c koch.nim
  ./koch boot --debuginfo --linedir:on

The ``koch`` program is Nim's maintenance script. It is a replacement for
make and shell scripting with the advantage that it is much more portable.
More information about its options can be found in the `koch <koch.html>`_
documentation.


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/references which
  start with ``P``.

See also the `API naming design <apis.html>`_ document.


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

Porting Nim to a new architecture is pretty easy, since C is the most
portable programming language (within certain limits) and Nim 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 Nim.

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 Nim
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 Nim 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.


Debugging the compiler
======================

You can of course use GDB or Visual Studio to debug the
compiler (via ``--debuginfo --lineDir:on``). However, there
are also lots of procs that aid in debugging:


.. code-block:: nim
  # pretty prints the Nim AST
  echo renderTree(someNode)
  # outputs some JSON representation
  debug(someNode)
  # pretty prints some type
  echo typeToString(someType)
  debug(someType)
  echo symbol.name.s
  debug(symbol)
  # pretty prints the Nim ast, but annotates symbol IDs:
  echo renderTree(someNode, {renderIds})
  if n.info ?? "temp.nim":
    # only output when it comes from "temp.nim"
    echo renderTree(n)
  if n.info ?? "temp.nim":
    # why does it process temp.nim here?
    writeStackTrace()

To create a new compiler for each run, use ``koch temp``::

  ./koch temp c /tmp/test.nim

``koch temp`` creates a debug build of the compiler, which is useful
to create stacktraces for compiler debugging.

``koch temp`` returns 125 as the exit code in case the compiler
compilation fails. This exit code tells ``git bisect`` to skip the
current commit.::

  git bisect start bad-commit good-commit
  git bisect run ./koch temp -r c test-source.nim

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

Nim uses the classic compiler architecture: A lexer/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 Nim correctly, type-checking has to be separated from
parsing. Otherwise generics cannot work.

.. include:: filelist.txt


The syntax tree
---------------
The syntax 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.


Compilation cache
=================

The implementation of the compilation cache is tricky: There are lots
of issues to be solved for the front- and backend.


General approach: AST replay
----------------------------

We store a module's AST of a successful semantic check in a SQLite
database. There are plenty of features that require a sub sequence
to be re-applied, for example:

.. code-block:: nim
  {.compile: "foo.c".} # even if the module is loaded from the DB,
                       # "foo.c" needs to be compiled/linked.

The solution is to **re-play** the module's top level statements.
This solves the problem without having to special case the logic
that fills the internal seqs which are affected by the pragmas.

In fact, this decribes how the AST should be stored in the database,
as a "shallow" tree. Let's assume we compile module ``m`` with the
following contents:

.. code-block:: nim
  import strutils

  var x*: int = 90
  {.compile: "foo.c".}
  proc p = echo "p"
  proc q = echo "q"
  static:
    echo "static"

Conceptually this is the AST we store for the module:

.. code-block:: nim
  import strutils

  var x*
  {.compile: "foo.c".}
  proc p
  proc q
  static:
    echo "static"

The symbol's ``ast`` field is loaded lazily, on demand. This is where most
savings come from, only the shallow outer AST is reconstructed immediately.

It is also important that the replay involves the ``import`` statement so
that the dependencies are resolved properly.


Shared global compiletime state
-------------------------------

Nim allows ``.global, compiletime`` variables that can be filled by macro
invokations across different modules. This feature breaks modularity in a
severe way. Plenty of different solutions have been proposed:

- Restrict the types of global compiletime variables to ``Set[T]`` or
  similar unordered, only-growable collections so that we can track
  the module's write effects to these variables and reapply the changes
  in a different order.
- In every module compilation, reset the variable to its default value.
- Provide a restrictive API that can load/save the compiletime state to
  a file.

(These solutions are not mutually exclusive.)

Since we adopt the "replay the top level statements" idea, the natural
solution to this problem is to emit pseudo top level statements that
reflect the mutations done to the global variable. However, this is
MUCH harder than it sounds, for example ``squeaknim`` uses this
snippet:

.. code-block:: nim
  apicall.add(") module: '" & dllName & "'>\C" &
              "\t^self externalCallFailed\C!\C\C")
  stCode.add(st & "\C\t\"Generated by NimSqueak\"\C\t" & apicall)

We can "replay" ``stCode.add`` only if the values of ``st``
and ``apicall`` are known. And even then a hash table's ``add`` with its
hashing mechanism is too hard to replay.

In practice, things are worse still, consider ``someGlobal[i][j].add arg``.
We only know the root is ``someGlobal`` but the concrete path to the data
is unknown as is the value that is added. We could compute a "diff" between
the global states and use that to compute a symbol patchset, but this is
quite some work, expensive to do at runtime (it would need to run after
every module has been compiled) and also would break for hash tables.

We need an API that hides the complex aliasing problems by not relying
on Nim's global variables. The obvious solution is to use string keys
instead of global variables:

.. code-block:: nim

  proc cachePut*(key: string; value: string)
  proc cacheGet*(key: string): string

However, the values being strings/json is quite problematic: Many
lookup tables that are built at compiletime embed *proc vars* and
types which have no obvious string representation... Seems like
AST diffing is still the best idea as it will not require to use
an alien API and works with some existing Nimble packages, at least.

On the other hand, in Nim's future I would like to replace the VM
by native code. A diff algorithm wouldn't work for that.
Instead the native code would work with an API like ``put``, ``get``:

.. code-block:: nim

  proc cachePut*(key: string; value: NimNode)
  proc cacheGet*(key: string): NimNode

The API should embrace the AST diffing notion: See the
module ``macrocache`` for the final details.



Methods and type converters
---------------------------

In the following
sections *global* means *shared between modules* or *property of the whole
program*.

Nim contains language features that are *global*. The best example for that
are multi methods: Introducing a new method with the same name and some
compatible object parameter means that the method's dispatcher needs to take
the new method into account. So the dispatching logic is only completely known
after the whole program has been translated!

Other features that are *implicitly* triggered cause problems for modularity
too. Type converters fall into this category:

.. code-block:: nim
  # module A
  converter toBool(x: int): bool =
    result = x != 0

.. code-block:: nim
  # module B
  import A

  if 1:
    echo "ugly, but should work"

If in the above example module ``B`` is re-compiled, but ``A`` is not then
``B`` needs to be aware of ``toBool`` even though  ``toBool`` is not referenced
in ``B`` *explicitly*.

Both the multi method and the type converter problems are solved by the
AST replay implementation.


Generics
~~~~~~~~

We cache generic instantiations and need to ensure this caching works
well with the incremental compilation feature. Since the cache is
attached to the ``PSym`` datastructure, it should work without any
special logic.


Backend issues
--------------

- Init procs must not be "forgotten" to be called.
- Files must not be "forgotten" to be linked.
- Method dispatchers are global.
- DLL loading via ``dlsym`` is global.
- Emulated thread vars are global.

However the biggest problem is that dead code elimination breaks modularity!
To see why, consider this scenario: The module ``G`` (for example the huge
Gtk2 module...) is compiled with dead code elimination turned on. So none
of ``G``'s procs is generated at all.

Then module ``B`` is compiled that requires ``G.P1``. Ok, no problem,
``G.P1`` is loaded from the symbol file and ``G.c`` now contains ``G.P1``.

Then module ``A`` (that depends on ``B`` and ``G``) is compiled and ``B``
and ``G`` are left unchanged. ``A`` requires ``G.P2``.

So now ``G.c`` MUST contain both ``P1`` and ``P2``, but we haven't even
loaded ``P1`` from the symbol file, nor do we want to because we then quickly
would restore large parts of the whole program.

Solution
~~~~~~~~ 

The backend must have some logic so that if the currently processed module
is from the compilation cache, the ``ast`` field is not accessed. Instead
the generated C(++) for the symbol's body needs to be cached too and
inserted back into the produced C file. This approach seems to deal with
all the outlined problems above.


Debugging Nim's memory management
=================================

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

* If an assertion in Nim'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 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:: Nim
  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 descriptor d:
    for each bit in d:
      if bit == 1:
        traverse the pointer belonging to this bit


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

In Nim 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:: Nim
  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 continuous stack (which most systems have)
the check whether the ref is on the stack is very cheap (only two
comparisons).


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

Code generation for closures is implemented by `lambda lifting`:idx:.

Design
------

A ``closure`` proc var can call ordinary procs of the default Nim calling
convention. But not the other way round! A closure is implemented as a
``tuple[prc, env]``. ``env`` can be nil implying a call without a closure.
This means that a call through a closure generates an ``if`` but the
interoperability is worth the cost of the ``if``. Thunk generation would be
possible too, but it's slightly more effort to implement.

Tests with GCC on Amd64 showed that it's really beneficical if the
'environment' pointer is passed as the last argument, not as the first argument.

Proper thunk generation is harder because the proc that is to wrap
could stem from a complex expression:

.. code-block:: nim
  receivesClosure(returnsDefaultCC[i])

A thunk would need to call 'returnsDefaultCC[i]' somehow and that would require
an *additional* closure generation... Ok, not really, but it requires to pass
the function to call. So we'd end up with 2 indirect calls instead of one.
Another much more severe problem which this solution is that it's not GC-safe
to pass a proc pointer around via a generic ``ref`` type.


Example code:

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

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

This should produce roughly this code:

.. code-block:: nim
  type
    PEnv = ref object
      x: int # data

  proc anon(y: int, c: PEnv): int =
    return y + c.x

  proc add(x: int): tuple[prc, data] =
    var env: PEnv
    new env
    env.x = x
    result = (anon, env)

  var add2 = add(2)
  let tmp = if add2.data == nil: add2.prc(5) else: add2.prc(5, add2.data)
  echo tmp


Beware of nesting:

.. code-block:: nim
  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:: nim
  type
    PEnvX = ref object
      x: int # data

    PEnvY = ref object
      y: int
      ex: PEnvX

  proc lambdaZ(z: int, ey: PEnvY): int =
    return ey.ex.x + ey.y + z

  proc lambdaY(y: int, ex: PEnvX): tuple[prc, data: PEnvY] =
    var ey: PEnvY
    new ey
    ey.y = y
    ey.ex = ex
    result = (lambdaZ, ey)

  proc add(x: int): tuple[prc, data: PEnvX] =
    var ex: PEnvX
    ex.x = x
    result = (labmdaY, ex)

  var tmp = add(2)
  var tmp2 = tmp.fn(4, tmp.data)
  var add24 = tmp2.fn(4, tmp2.data)
  echo add24(5)


We could get rid of nesting environments by always inlining inner anon procs.
More useful is escape analysis and stack allocation of the environment,
however.


Alternative
-----------

Process the closure of all inner procs in one pass and accumulate the
environments. This is however not always possible.


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

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

  proc p =
    var delta = 7
    proc accumulator(start: int): proc(): int =
      var x = start-1
      result = proc (): int =
        x = x + delta
        inc delta
        return x

    var a = accumulator(3)
    var b = accumulator(4)
    echo a() + b()


Internals
---------

Lambda lifting is implemented as part of the ``transf`` pass. The ``transf``
pass generates code to setup the environment and to pass it around. However,
this pass does not change the types! So we have some kind of mismatch here; on
the one hand the proc expression becomes an explicit tuple, on the other hand
the tyProc(ccClosure) type is not changed. For C code generation it's also
important the hidden formal param is ``void*`` and not something more
specialized. However the more specialized env type needs to passed to the
backend somehow. We deal with this by modifying ``s.ast[paramPos]`` to contain
the formal hidden parameter, but not ``s.typ``!