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Internals of the Nim Compiler

Author: Andreas Rumpf
Version: |nimversion|

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

Bootstrapping the compiler

Note: Add . to your PATH so that `koch`:cmd: can be used without the ./.

Compiling the compiler is a simple matter of running:

nim c koch.nim
koch boot -d:release

For a debug version use:

nim c koch.nim
koch boot

And for a debug version compatible with GDB:

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

The `koch`:cmd: 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 documentation.

Developing the compiler

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

koch temp c test.nim

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

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

# dealing with PNode:
echo renderTree(someNode)
debug(someNode) # some JSON representation

# dealing with PType:
echo typeToString(someType)
debug(someType)

# dealing with PSym:
echo symbol.name.s
debug(symbol)

# pretty prints the Nim ast, but annotates symbol IDs:
echo renderTree(someNode, {renderIds})
if `??`(conf, n.info, "temp.nim"):
  # only output when it comes from "temp.nim"
  echo renderTree(n)
if `??`(conf, n.info, "temp.nim"):
  # why does it process temp.nim here?
  writeStackTrace()

These procs may not already be imported by the module you're editing. You can import them directly for debugging:

The compiler's architecture

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

Semantic analysis is separated from parsing.

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 module contains many examples how the AST represents each syntactic structure.

Bisecting for regressions

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

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

You can also bisect using custom options to build the compiler, for example if you don't need a debug version of the compiler (which runs slower), you can replace `./koch temp`:cmd: by explicit compilation command, see `Rebuilding the compiler`_.

Runtimes

Nim has two different runtimes, the "old runtime" and the "new runtime". The old runtime supports the old GCs (markAndSweep, refc, Boehm), the new runtime supports ARC/ORC. The new runtime is active when defined(nimV2).

Coding Guidelines

  • We follow Nim's official style guide, see nep1.html.
  • Max line length is 100 characters.
  • Provide spaces around binary operators if that enhances readability.
  • Use a space after a colon, but not before it.
  • [deprecated] Start types with a capital T, unless they are pointers/references which start with P.

See also the API naming design 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 old runtime's garbage collectors need some assembler tweaking to work. The default implementation uses C's `setjmp`:c: 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

Note: This section describes the "old runtime".

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

Garbage collection
The old GCs use the RTTI for traversing abitrary Nim types, but usually only the marker field which contains a proc that does the traversal.
Complex assignments
Sequences and strings are implemented as pointers to resizeable buffers, but Nim requires copying for assignments. Apart from RTTI the compiler also generates copy procedures as a specialization.

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.

Magics and compilerProcs

The system module contains the part of the RTL which needs support by compiler magic. 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 generator. Therefore there is 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.

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 beneficial 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:

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:

This should produce roughly this code:

Beware of nesting:

This should produce roughly this code:

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.

Accumulator

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*`:c: 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!

Notes on type and AST representation

To be expanded.

Integer literals

In Nim, there is a redundant way to specify the type of an integer literal. First of all, it should be unsurprising that every node has a node kind. The node of an integer literal can be any of the following values:

nkIntLit, nkInt8Lit, nkInt16Lit, nkInt32Lit, nkInt64Lit,
nkUIntLit, nkUInt8Lit, nkUInt16Lit, nkUInt32Lit, nkUInt64Lit

On top of that, there is also the typ field for the type. It the kind of the typ field can be one of the following ones, and it should be matching the literal kind:

tyInt, tyInt8, tyInt16, tyInt32, tyInt64, tyUInt, tyUInt8,
tyUInt16, tyUInt32, tyUInt64

Then there is also the integer literal type. This is a specific type that is implicitly convertible into the requested type if the requested type can hold the value. For this to work, the type needs to know the concrete value of the literal. For example an expression 321 will be of type int literal(321). This type is implicitly convertible to all integer types and ranges that contain the value 321. That would be all builtin integer types except uint8 and int8 where 321 would be out of range. When this literal type is assigned to a new var or let variable, it's type will be resolved to just int, not int literal(321) unlike constants. A constant keeps the full int literal(321) type. Here is an example where that difference matters.

proc foo(arg: int8) =
  echo "def"

const tmp1 = 123
foo(tmp1)  # OK

let tmp2 = 123
foo(tmp2) # Error

In a context with multiple overloads, the integer literal kind will always prefer the int type over all other types. If none of the overloads is of type int, then there will be an error because of ambiguity.

proc foo(arg: int) =
  echo "abc"
proc foo(arg: int8) =
  echo "def"
foo(123) # output: abc

proc bar(arg: int16) =
  echo "abc"
proc bar(arg: int8) =
  echo "def"

bar(123) # Error ambiguous call

In the compiler these integer literal types are represented with the node kind nkIntLit, type kind tyInt and the member n of the type pointing back to the integer literal node in the ast containing the integer value. These are the properties that hold true for integer literal types.

::
n.kind == nkIntLit n.typ.kind == tyInt n.typ.n == n

Other literal types, such as uint literal(123) that would automatically convert to other integer types, but prefers to become a uint are not part of the Nim language.

In an unchecked AST, the typ field is nil. The type checker will set the typ field accordingly to the node kind. Nodes of kind nkIntLit will get the integer literal type (e.g. int literal(123)). Nodes of kind nkUIntLit will get type uint (kind tyUint), etc.

This also means that it is not possible to write a literal in an unchecked AST that will after sem checking just be of type int and not implicitly convertible to other integer types. This only works for all integer types that are not int.