Documentation GC (#13109)

* Update Documentation about GC, add ARC based on code observations and https://forum.nim-lang.org/t/5734#35562

* Rephrase 'Garbage Collector' on documentation, see #13331

* Peer Review Feedbacks

* When the body of a documentation section is all for garbage collector details then add 'for garbage collectors' to the title to identify they may not apply for ARC

* Change 'GC' for 'garbage collector'.

* Move 'Memory Management Strategies' at the top, without changes

* Move the info for RefC GC from 'Introduction' to 'RefC' section without changes, make 'Introduction' about Nims 'Multi-paradigm Memory Management Strategies'.

1 files changed, 98 insertions, 76 deletions

diff --git a/doc/gc.rst b/doc/gc.rst
index a41a30588..b82edd570 100644
--- a/doc/gc.rst
+++ b/doc/gc.rst
@@ -1,6 +1,6 @@
-==========================
-Nim's Garbage Collector
-==========================
+=======================
+Nim's Memory Management
+=======================
 
 :Author: Andreas Rumpf
 :Version: |nimversion|
@@ -14,37 +14,78 @@ Nim's Garbage Collector
 Introduction
 ============
 
-This document describes how the GC works and how to tune it for
-(soft) `realtime systems`:idx:.
-
-The basic algorithm is *Deferred Reference Counting* with cycle detection.
-References on the stack are not counted for better performance (and easier C
-code generation). Cycle detection is currently done by a simple mark&sweep
-GC that has to scan the full (thread local heap). ``--gc:v2`` replaces this
-with an incremental mark and sweep. That it is not production ready yet,
-however.
-
+This document describes how the multi-paradigm memory management strategies work.
+How to tune the garbage collectors for your needs, like (soft) `realtime systems`:idx:,
+and how the memory management strategies that are not garbage collectors work.
+
+
+Multi-paradigm Memory Management Strategies
+===========================================
+
+You can choose the memory management strategy to use when compiling source code,
+you can pass ``--gc:`` on the compile command with the selected memory management strategy.
+
+- ``--gc:refc`` Deferred `reference counting <https://en.wikipedia.org/wiki/Reference_counting>`_ based garbage collector
+  with `cycle detection <https://en.wikipedia.org/wiki/Reference_counting#Dealing_with_reference_cycles>`_
+  by a simple Mark&Sweep that has to scan the full heap,
+  is only triggered in a memory allocation operation and
+  it is not triggered by some timer and does not run in a background thread,
+  `thread local heap <https://en.wikipedia.org/wiki/Heap_(programming)>`_,
+  references on the stack are not counted for better performance (and easier C code generation), default.
+- ``--gc:markAndSweep`` `Mark-And-Sweep <https://en.wikipedia.org/wiki/Tracing_garbage_collection#Copying_vs._mark-and-sweep_vs._mark-and-don't-sweep>`_ based garbage collector,
+  `thread local heap <https://en.wikipedia.org/wiki/Heap_(programming)>`_.
+- ``--gc:boehm`` `Boehm <https://en.wikipedia.org/wiki/Boehm_garbage_collector>`_ based garbage collector,
+  `stop-the-world <https://en.wikipedia.org/wiki/Tracing_garbage_collection#Stop-the-world_vs._incremental_vs._concurrent>`_,
+  `shared heap <https://en.wikipedia.org/wiki/Heap_(programming)>`_.
+- ``--gc:go`` Go lang like garbage collector,
+  `stop-the-world <https://en.wikipedia.org/wiki/Tracing_garbage_collection#Stop-the-world_vs._incremental_vs._concurrent>`_,
+  `shared heap <https://en.wikipedia.org/wiki/Heap_(programming)>`_.
+- ``--gc:regions`` `Stack <https://en.wikipedia.org/wiki/Memory_management#Stack_allocation>`_ based garbage collector.
+- ``--gc:arc`` Not a garbage collector. Plain `reference counting <https://en.wikipedia.org/wiki/Reference_counting>`_ with
+  `move semantic optimizations <destructors.html#move-semantics>`_,
+  `shared heap <https://en.wikipedia.org/wiki/Heap_(programming)>`_,
+  can be optimized with `sink <destructors.html#sink-parameters>`_ and `lent <destructors.html#lent-type>`_ annotations,
+  designed to work well with `WebAssembly <https://webassembly.org>`_, `Emscripten <https://emscripten.org>`_,
+  `hot code reloading <hcr.html>`_ and `address sanitizers <https://en.wikipedia.org/wiki/AddressSanitizer>`_,
+  basically it is like a shared heap with subgraphs with a single owner,
+  this is not the same as Swift and ObjectiveC lang ARC because those can not handle cycles,
+  can use `GOTO based Exception handling <https://nim-lang.org/araq/gotobased_exceptions.html>`_,
+  may become default in future releases.
+- ``--gc:orc`` Not a garbage collector. Similar to ``--gc:arc`` but with improved
+  `cycle detection <https://en.wikipedia.org/wiki/Reference_counting#Dealing_with_reference_cycles>`_.
+  `Cycle detection <https://en.wikipedia.org/wiki/Reference_counting#Dealing_with_reference_cycles>`_
+  will not be the default, because by definition it conflicts with
+  `deterministic memory management <https://en.wikipedia.org/wiki/Deterministic_memory>`_.
+- ``--gc:none`` No memory management strategy nor garbage collector.
+  You should use `Manual memory management <https://en.wikipedia.org/wiki/Manual_memory_management>`_ with it.
+
+The same Nim code can be compiled to use any of the  memory management strategies;
+the Nim syntax generally will not change from one memory management strategy to another.
+
+No garbage collector nor memory management is used for `JavaScript and NodeJS
+<backends.html#backends-the-javascript-target>`_ compilation targets.
+`NimScript <nims.html>`_ target uses Nim VM memory management strategy.
 
-The GC is only triggered in a memory allocation operation. It is not triggered
-by some timer and does not run in a background thread.
+All memory management strategies are supported equally on Nim when possible and aplicable,
+even if there is a default one, all others should also work as documented.
 
-To force a full collection call ``GC_fullCollect``. Note that it is generally
-better to let the GC do its work and not enforce a full collection.
+If you are new to Nim and just starting, the default memory management strategy is balanced to fit most common use cases.
 
 
-Cycle collector
-===============
+Cycle collector for garbage collectors
+======================================
 
 The cycle collector can be en-/disabled independently from the other parts of
-the GC with ``GC_enableMarkAndSweep`` and ``GC_disableMarkAndSweep``.
+the garbage collector with ``GC_enableMarkAndSweep`` and ``GC_disableMarkAndSweep``.
 
 
-Realtime support
-================
+Realtime support for garbage collectors
+=======================================
 
 To enable realtime support, the symbol `useRealtimeGC`:idx: needs to be
 defined via ``--define:useRealtimeGC`` (you can put this into your config
-file as well). With this switch the GC supports the following operations:
+file as well).
+With this switch the garbage collector supports the following operations:
 
 .. code-block:: nim
   proc GC_setMaxPause*(maxPauseInUs: int)
@@ -52,24 +93,26 @@ file as well). With this switch the GC supports the following operations:
 
 The unit of the parameters ``maxPauseInUs`` and ``us`` is microseconds.
 
-These two procs are the two modus operandi of the realtime GC:
+These two procs are the two modus operandi of the realtime garbage collector:
 
 (1) GC_SetMaxPause Mode
 
     You can call ``GC_SetMaxPause`` at program startup and then each triggered
-    GC run tries to not take longer than ``maxPause`` time. However, it is
+    garbage collector run tries to not take longer than ``maxPause`` time. However, it is
     possible (and common) that the work is nevertheless not evenly distributed
-    as each call to ``new`` can trigger the GC and thus take  ``maxPause``
+    as each call to ``new`` can trigger the garbage collector and thus take  ``maxPause``
     time.
 
 (2) GC_step Mode
 
-    This allows the GC to perform some work for up to ``us`` time. This is
-    useful to call in a main loop to ensure the GC can do its work. To
-    bind all GC activity to a ``GC_step`` call, deactivate the GC with
-    ``GC_disable`` at program startup. If ``strongAdvice`` is set to ``true``,
-    GC will be forced to perform collection cycle. Otherwise, GC may decide not
-    to do anything, if there is not much garbage to collect.
+    This allows the garbage collector to perform some work for up to ``us`` time.
+    This is useful to call in a main loop to ensure the garbage collector can do its work.
+    To bind all garbage collector activity to a ``GC_step`` call,
+    deactivate the garbage collector with ``GC_disable`` at program startup.
+    If ``strongAdvice`` is set to ``true``,
+    then the garbage collector will be forced to perform collection cycle.
+    Otherwise, the garbage collector may decide not to do anything,
+    if there is not much garbage to collect.
     You may also specify the current stack size via ``stackSize`` parameter.
     It can improve performance, when you know that there are no unique Nim
     references below certain point on the stack. Make sure the size you specify
@@ -82,11 +125,11 @@ time will be met in almost all cases on modern CPUs (with the cycle collector
 disabled).
 
 
-Time measurement
-----------------
+Time measurement with garbage collectors
+----------------------------------------
 
-The GC's way of measuring time uses (see ``lib/system/timers.nim`` for the
-implementation):
+The garbage collectors's way of measuring time uses
+(see ``lib/system/timers.nim`` for the implementation):
 
 1) ``QueryPerformanceCounter`` and ``QueryPerformanceFrequency`` on Windows.
 2) ``mach_absolute_time`` on Mac OS X.
@@ -95,14 +138,13 @@ implementation):
 As such it supports a resolution of nanoseconds internally; however the API
 uses microseconds for convenience.
 
+Define the symbol ``reportMissedDeadlines`` to make the
+garbage collector output whenever it missed a deadline.
+The reporting will be enhanced and supported by the API in later versions of the collector.
 
-Define the symbol ``reportMissedDeadlines`` to make the GC output whenever it
-missed a deadline. The reporting will be enhanced and supported by the API in
-later versions of the collector.
 
-
-Tweaking the GC
----------------
+Tweaking the garbage collector
+------------------------------
 
 The collector checks whether there is still time left for its work after
 every ``workPackage``'th iteration. This is currently set to 100 which means
@@ -111,25 +153,27 @@ that up to 100 objects are traversed and freed before it checks again. Thus
 highly specialized environments or for older hardware.
 
 
-Keeping track of memory
------------------------
+Keeping track of memory with garbage collectors
+-----------------------------------------------
 
 If you need to pass around memory allocated by Nim to C, you can use the
 procs ``GC_ref`` and ``GC_unref`` to mark objects as referenced to avoid them
-being freed by the GC. Other useful procs from `system <system.html>`_ you can
-use to keep track of memory are:
+being freed by the garbage collector.
+Other useful procs from `system <system.html>`_ you can use to keep track of memory are:
 
-* ``getTotalMem()`` Returns the amount of total memory managed by the GC.
-* ``getOccupiedMem()`` Bytes reserved by the GC and used by objects.
-* ``getFreeMem()`` Bytes reserved by the GC and not in use.
+* ``getTotalMem()`` Returns the amount of total memory managed by the garbage collector.
+* ``getOccupiedMem()`` Bytes reserved by the garbage collector and used by objects.
+* ``getFreeMem()`` Bytes reserved by the garbage collector and not in use.
+* ``GC_getStatistics()`` Garbage collector statistics as a human-readable string.
 
 These numbers are usually only for the running thread, not for the whole heap,
 with the exception of ``--gc:boehm`` and ``--gc:go``.
 
-In addition to ``GC_ref`` and ``GC_unref`` you can avoid the GC by manually
-allocating memory with procs like ``alloc``, ``allocShared``, or
-``allocCStringArray``. The GC won't try to free them, you need to call their
-respective *dealloc* pairs when you are done with them or they will leak.
+In addition to ``GC_ref`` and ``GC_unref`` you can avoid the garbage collector by manually
+allocating memory with procs like ``alloc``, ``alloc0``, ``allocShared``, ``allocShared0`` or ``allocCStringArray``.
+The garbage collector won't try to free them, you need to call their respective *dealloc* pairs
+(``dealloc``, ``deallocShared``, ``deallocCStringArray``, etc)
+when you are done with them or they will leak.
 
 
 Heap dump
@@ -143,29 +187,7 @@ the total amount of object instances for this type as well as the total
 amount of bytes these instances take up. This list is currently unsorted!
 You need to use external shell script hacking to sort it.
 
-The numbers count the number of objects in all GC heaps, they refer to
+The numbers count the number of objects in all garbage collector heaps, they refer to
 all running threads, not only to the current thread. (The current thread
 would be the thread that calls ``dumpNumberOfInstances``.) This might
 change in later versions.
-
-
-Garbage collector options
--------------------------
-
-You can choose which garbage collector to use when compiling source code,
-you can pass ``--gc:`` on the compile command with the choosed garbage collector.
-
-- ``--gc:refc`` Deferred `reference counting <https://en.wikipedia.org/wiki/Reference_counting>`_ with cycle detection, `thread local heap <https://en.wikipedia.org/wiki/Heap_(programming)>`_, default.
-- ``--gc:markAndSweep`` `Mark-And-Sweep <https://en.wikipedia.org/wiki/Tracing_garbage_collection#Copying_vs._mark-and-sweep_vs._mark-and-don't-sweep>`_ based garbage collector, `thread local heap <https://en.wikipedia.org/wiki/Heap_(programming)>`_.
-- ``--gc:boehm`` `Boehm <https://en.wikipedia.org/wiki/Boehm_garbage_collector>`_ based garbage collector, `stop-the-world <https://en.wikipedia.org/wiki/Tracing_garbage_collection#Stop-the-world_vs._incremental_vs._concurrent>`_, `shared heap <https://en.wikipedia.org/wiki/Heap_(programming)>`_.
-- ``--gc:go`` Go lang like garbage collector, `stop-the-world <https://en.wikipedia.org/wiki/Tracing_garbage_collection#Stop-the-world_vs._incremental_vs._concurrent>`_, `shared heap <https://en.wikipedia.org/wiki/Heap_(programming)>`_.
-- ``--gc:regions`` `Stack <https://en.wikipedia.org/wiki/Memory_management#Stack_allocation>`_ based garbage collector.
-- ``--gc:none`` No garbage collector.
-
-The same Nim code can be compiled to use any of the garbage collectors;
-the Nim syntax generally will not change from one garbage collector to another.
-No garbage collector is used for `JavaScript and NodeJS
-<backends.html#backends-the-javascript-target>`_ compilation targets.
-`NimScript <nims.html>`_ target uses Nim VM garbage collector.
-
-If you are new to Nim and just starting, the default garbage collector is balanced to fit most common use cases.