Updated 'nim for embedded systems' section to use --os:any and --gc:a…

…rc (#13237)

* Updated 'nim for embedded systems' section to use --os:any and --gc:arc

* Added section about size optimization to embedded systems

doc/nimc.rst

@@ -542,26 +542,44 @@ for further information.
 Nim for embedded systems
 ========================
 
-The standard library can be avoided to a point where C code generation
-for 16bit micro controllers is feasible. Use the `standalone`:idx: target
-(``--os:standalone``) for a bare bones standard library that lacks any
-OS features.
+While the default Nim configuration is targeted for optimal performance on
+modern PC hardware and operating systems with ample memory, it is very well
+possible to run Nim code and a good part of the Nim standard libraries on small
+embedded microprocessors with only a few kilobytes of memory.
 
-To make the compiler output code for a 16bit target use the ``--cpu:avr``
-target.
+A good start is to use the ``any`` operating target together with the
+``malloc`` memory allocator and the ``arc`` garbage collector. For example:
 
-For example, to generate code for an `AVR`:idx: processor use this command::
+``nim c --os:any --gc:arc -d:useMalloc [...] x.nim``
 
-  nim c --cpu:avr --os:standalone --genScript x.nim
+- ``--gc:arc`` will enable the reference counting memory management instead
+  of the default garbage collector. This enables Nim to use heap memory which
+  is required for strings and seqs, for example.
 
-For the ``standalone`` target one needs to provide
-a file ``panicoverride.nim``.
-See ``tests/manyloc/standalone/panicoverride.nim`` for an example
-implementation. Additionally, users should specify the
-amount of heap space to use with the ``-d:StandaloneHeapSize=<size>``
-command line switch. Note that the total heap size will be
-``<size> * sizeof(float64)``.
+- The ``--os:any`` target makes sure Nim does not depend on any specific
+  operating system primitives. Your platform should support only some basic
+  ANSI C library ``stdlib`` and ``stdio`` functions which should be available
+  on almost any platform.
 
+- The ``-d:useMalloc`` option configures Nim to use only the standard C memory
+  manage primitives ``malloc()``, ``free()``, ``realloc()``.
+
+If your platform does not provide these functions it should be trivial to
+provide an implementation for them and link these to your program.
+
+For targets with very restricted memory, it might be beneficial to pass some
+additional flags to both the Nim compiler and the C compiler and/or linker
+to optimize the build for size. For example, the following flags can be used
+when targeting a gcc compiler:
+
+``--opt:size --passC:-flto --passL:-flto``
+
+The ``--opt:size`` flag instructs Nim to optimize code generation for small
+size (with the help of the C compiler), the ``flto`` flags enable link-time
+optimization in the compiler and linker.
+
+Check the `Cross compilation` section for instructions how to compile the
+program for your target.
 
 Nim for realtime systems
 ========================