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cleaned up the internal documentation (#17524)

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Andreas Rumpf
2021-03-26 16:27:55 +01:00
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commit 3e03f67335
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doc/intern.rst
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@@ -32,28 +32,29 @@ Path Purpose
`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 `*.nimf` files
============ ===================================================
Bootstrapping the compiler
==========================
**Note**: Add ``.`` to your PATH so that `koch` can be used without the `./`.
Compiling the compiler is a simple matter of running::
nim c koch.nim
./koch boot
koch boot -d:release
For a release version use::
For a debug version use::
nim c koch.nim
./koch boot -d:release
koch boot
And for a debug version compatible with GDB::
nim c koch.nim
./koch boot --debuginfo --linedir:on
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.
@@ -61,12 +62,105 @@ More information about its options can be found in the `koch <koch.html>`_
documentation.
Developing the compiler
=======================
To create a new compiler for each run, use `koch temp`::
koch temp c test.nim
`koch temp` 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`). However, there
are also lots of procs that aid in debugging:
.. code-block:: nim
# 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:
.. code-block:: nim
from astalgo import debug
from types import typeToString
from renderer import renderTree
from msgs import `??`
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 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.
.. 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.
Bisecting for regressions
=========================
`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
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` 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
=================
* Use CamelCase, not underscored_identifiers.
* Indent with two spaces.
* Max line length is 80 characters.
* 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
@@ -86,9 +180,9 @@ 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
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` 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.
@@ -96,136 +190,33 @@ 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 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.
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 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.
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.
Rebuilding the compiler
Magics and compilerProcs
========================
After an initial build via `sh build_all.sh` on posix or `build_all.bat` on windows,
you can rebuild the compiler as follows:
* `nim c koch` if you need to rebuild koch
* `./koch boot -d:release` this ensures the compiler can rebuild itself
(use `koch` instead of `./koch` on windows), which builds the compiler 3 times.
A faster approach if you don't need to run the full bootstrapping implied by `koch boot`,
is the following:
* `pathto/nim c --lib:lib -d:release -o:bin/nim_temp compiler/nim.nim`
Where `pathto/nim` is any nim binary sufficiently recent (e.g. `bin/nim_cources`
built during bootstrap or `$HOME/.nimble/bin/nim` installed by `choosenim 1.2.0`)
You can pass any additional options such as `-d:leanCompiler` if you don't need
certain features or `-d:debug --stacktrace:on --excessiveStackTrace --stackTraceMsgs`
for debugging the compiler. See also `Debugging the compiler`_.
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 `??`(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 be imported by a module. You can import them directly for debugging:
.. code-block:: nim
from astalgo import debug
from types import typeToString
from renderer import renderTree
from msgs import `??`
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. See also
`Rebuilding the compiler`_ if you need more control.
Bisecting for regressions
=========================
`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
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` by explicit compilation command, see `Rebuilding the compiler`_.
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
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 CCG. Therefore the module `magicsys` contains a table (`compilerprocs`)
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
@@ -233,325 +224,6 @@ 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 describes 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 std/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 std/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 dependencies are resolved properly.
Shared global compiletime state
-------------------------------
Nim allows `.global, compiletime` variables that can be filled by macro
invocations 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 would also 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 traversal 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
============================
@@ -598,14 +270,14 @@ This should produce roughly this code:
.. code-block:: nim
type
PEnv = ref object
Env = ref object
x: int # data
proc anon(y: int, c: PEnv): int =
proc anon(y: int, c: Env): int =
return y + c.x
proc add(x: int): tuple[prc, data] =
var env: PEnv
var env: Env
new env
env.x = x
result = (anon, env)
@@ -630,25 +302,25 @@ This should produce roughly this code:
.. code-block:: nim
type
PEnvX = ref object
EnvX = ref object
x: int # data
PEnvY = ref object
EnvY = ref object
y: int
ex: PEnvX
ex: EnvX
proc lambdaZ(z: int, ey: PEnvY): int =
proc lambdaZ(z: int, ey: EnvY): int =
return ey.ex.x + ey.y + z
proc lambdaY(y: int, ex: PEnvX): tuple[prc, data: PEnvY] =
var ey: PEnvY
proc lambdaY(y: int, ex: EnvX): tuple[prc, data: EnvY] =
var ey: EnvY
new ey
ey.y = y
ey.ex = ex
result = (lambdaZ, ey)
proc add(x: int): tuple[prc, data: PEnvX] =
var ex: PEnvX
proc add(x: int): tuple[prc, data: EnvX] =
var ex: EnvX
ex.x = x
result = (labmdaY, ex)
@@ -663,17 +335,11 @@ 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 =
@@ -708,20 +374,26 @@ backend somehow. We deal with this by modifying `s.ast[paramPos]` to contain
the formal hidden parameter, but not `s.typ`!
Integer literals:
-----------------
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:
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:
should be matching the literal kind::
tyInt, tyInt8, tyInt16, tyInt32, tyInt64, tyUInt, tyUInt8,
tyUInt16, tyUInt32, tyUInt64
@@ -777,6 +449,7 @@ 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

35
lib/system/cellsets.nim
View File

@@ -7,7 +7,40 @@
# distribution, for details about the copyright.
#
# Efficient set of pointers for the GC (and repr)
#[
Efficient set of pointers for the GC (and repr)
-----------------------------------------------
The GC depends on an extremely efficient datastructure for storing a
set of pointers - this is called a `CellSet` in the source code.
Inserting, deleting and searching are done in constant time. However,
modifying a `CellSet` during traversal leads to undefined behaviour.
All operations on a CellSet 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
]#
when defined(gcOrc) or defined(gcArc):
type

47
lib/system/gc.nim
View File

@@ -12,6 +12,53 @@
# Refcounting + Mark&Sweep. Complex algorithms avoided.
# Been there, done that, didn't work.
#[
A *cell* is anything that is traced by the GC
(sequences, refs, strings, closures).
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`.
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).
]#
{.push profiler:off.}
const