renamed 'gc' switch to 'mm'; [backport:1.6] (#19187)

* renamed 'gc' switch to 'mm'; [backport:1.6]
* better docs

1 files changed, 95 insertions, 0 deletions

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+=======================
+Nim's Memory Management
+=======================
+
+.. default-role:: code
+.. include:: rstcommon.rst
+
+:Author: Andreas Rumpf
+:Version: |nimversion|
+
+..
+
+
+  "The road to hell is paved with good intentions."
+
+
+Multi-paradigm Memory Management Strategies
+===========================================
+
+.. default-role:: option
+
+Nim offers multiple different memory management strategies.
+To choose the memory management strategy use the `--mm:` switch.
+
+**The recommended switch for newly written Nim code is `--mm:orc`.**
+
+
+ARC/ORC
+-------
+
+`--mm:orc` is a memory management mode primarily based on reference counting. Cycles
+in the object graph are handled by a "cycle collector" which is based on "trial deletion".
+Since algorithms based on "tracing" are not used, the runtime behavior is oblivious to
+the involved heap sizes.
+
+The reference counting operations (= "RC ops") do not use atomic instructions and do not have to --
+instead entire subgraphs are *moved* between threads. The Nim compiler also aggressively
+optimizes away RC ops and exploits `move semantics <destructors.html#move-semantics>`_.
+
+Nim performs a fair share of optimizations for ARC/ORC; you can inspect what it did
+to your time critical function via `--expandArc:functionName`.
+
+`--mm:arc` uses the same mechanism as `--mm:orc`, but it leaves out the cycle collector.
+Both ARC and ORC offer deterministic performance for `hard realtime`:idx: systems, but
+ARC can be easier to reason about for people coming from Ada/C++/C -- roughly speaking
+the memory for a variable is freed when it goes "out of scope".
+
+We generally advise you to use the `acyclic` annotation in order to optimize away the
+cycle collector's overhead
+but `--mm:orc` also produces more machine code than `--mm:arc`, so if you're on a target
+where code size matters and you know that your code does not produce cycles, you can
+use `--mm:arc`. Notice that the default `async`:idx: implementation produces cycles
+and leaks memory with `--mm:arc`, in other words, for `async` you need to use `--mm:orc`.
+
+
+
+Other MM modes
+--------------
+
+.. note:: The default `refc` GC is incremental, thread-local and not "stop-the-world".
+
+--mm:refc    This is the default memory management strategy. It's a
+  deferred reference counting based garbage collector
+  with a simple Mark&Sweep backup GC in order to collect cycles. Heaps are thread-local.
+  `This document <refc.html>`_ contains further information.
+--mm:markAndSweep  Simple Mark-And-Sweep based garbage collector.
+  Heaps are thread-local.
+--mm:boehm    Boehm based garbage collector, it offers a shared heap.
+--mm:go    Go's garbage collector, useful for interoperability with Go.
+  Offers a shared heap.
+
+--mm:none    No memory management strategy nor a garbage collector. Allocated memory is
+  simply never freed. You should use `--mm:arc` instead.
+
+Here is a comparison of the different memory management modes:
+
+================== ======== ================= ============== ===================
+Memory Management  Heap     Reference Cycles  Stop-The-World Command line switch
+================== ======== ================= ============== ===================
+ORC                Shared   Cycle Collector   No             `--mm:orc`
+ARC                Shared   Leak              No             `--mm:arc`
+RefC               Local    Cycle Collector   No             `--mm:refc`
+Mark & Sweep       Local    Cycle Collector   No             `--mm:markAndSweep`
+Boehm              Shared   Cycle Collector   Yes            `--mm:boehm`
+Go                 Shared   Cycle Collector   Yes            `--mm:go`
+None               Manual   Manual            Manual         `--mm:none`
+================== ======== ================= ============== ===================
+
+.. default-role:: code
+.. include:: rstcommon.rst
+
+JavaScript's garbage collector is used for the `JavaScript and NodeJS
+<backends.html#backends-the-javascript-target>`_ compilation targets.
+The `NimScript <nims.html>`_ target uses the memory management strategy built into
+the Nim compiler.