Destroyed RGenGC (markdown)

RGenGC.md

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-# RGenGC
-
-RGenGC is generational garbage collection algorithm arranged for MRI/CRuby introduced from Ruby 2.1.
-RGenGC (mostly) keeps compatibility.
-
-# For extension writes
-
-Generally, the use of the technique called write barriers is required in
-extension libraries for generational GC
-(http://en.wikipedia.org/wiki/Garbage_collection_%28computer_science%29).
-RGenGC works fine without write barriers in extension libraries.
-
-If your library adheres to the following tips, performance can
-be further improved. Especially, the "Don't touch pointers directly" section is
-important.
-
-## Incompatibility
-
-You can't write RBASIC(obj)->klass field directly because it is const
-value now.
-
-Basically you should not write this field because MRI expects it to be
-an immutable field, but if you want to do it in your extension you can
-use the following functions:
-
-```
-VALUE rb_obj_hide(VALUE obj) ::
-
-  Clear RBasic::klass field. The object will be an internal object.
-  ObjectSpace::each_object can't find this object.
-
-VALUE rb_obj_reveal(VALUE obj, VALUE klass) ::
-
-  Reset RBasic::klass to be klass.
-  We expect the `klass' is hidden class by rb_obj_hide().
-```
-
-## Write barriers
-
-RGenGC doesn't require write barriers to support generational GC.
-However, caring about write barrier can improve the performance of
-RGenGC. Please check the following tips.
-
-### Don't touch pointers directly
-
-In MRI (include/ruby/ruby.h), some macros to acquire pointers to the
-internal data structures are supported such as RARRAY_PTR(),
-RSTRUCT_PTR() and so on.
-
-DO NOT USE THESE MACROS and instead use the corresponding C-APIs such as
-rb_ary_aref(), rb_ary_store() and so on.
-
-### Consider whether to insert write barriers
-
-You don't need to care about write barriers if you only use built-in
-types.
-
-If you support T_DATA objects, you may consider using write barriers.
-
-Inserting write barriers into T_DATA objects only works with the
-following type objects: (a) long-lived objects, (b) when a huge number
-of objects are generated and (c) container-type objects that have
-references to other objects. If your extension provides such a type of
-T_DATA objects, consider inserting write barriers.
-
-* (a): short-lived objects don't become old generation objects.
-* (b): only a few oldgen objects don't have performance impact.
-* (c): only a few references don't have performance impact.
-
-Inserting write barriers is a very difficult hack, it is easy to
-introduce critical bugs. And inserting write barriers has several areas
-of overhead. Basically we don't recommend you insert write barriers.
-Please carefully consider the risks.
-
-### Combine with built-in types
-
-Please consider utilizing built-in types. Most built-in types support
-write barrier, so you can use them to avoid manually inserting write
-barriers.
-
-For example, if your T_DATA has references to other objects, then you
-can move these references to Array. A T_DATA object only has a reference
-to an array object. Or you can also use a Struct object to gather a
-T_DATA object (without any references) and an that Array contains
-references.
-
-With use of such techniques, you don't need to insert write barriers
-anymore.
-
-### Insert write barriers
-
-*AGAIN* Inserting write barriers is a very difficult hack, and it is
-easy to introduce critical bugs. And inserting write barriers has
-several areas of overhead. Basically we don't recommend you insert write
-barriers. Please carefully consider the risks.
-
-Before inserting write barriers, you need to know about RGenGC algorithm
-(gc.c will help you). Macros and functions to insert write barriers are
-available in in include/ruby/ruby.h. An example is available in iseq.c.
-
-NOTE: The following core interfaces can be changed in the future. Please catch up if you want to insert WB into C-extensions correctly.
-
-Write barrier macros:
-
-```
-RB_OBJ_WRITE(a, slot, b) ::
-
-  WB for new reference from `a' to `b'.
-  Write `b' into `*slot'. `slot' is a pointer in `a'.
-
-RB_OBJ_WRITTEN(a, oldv, b)::
-
-  WB for new reference from `a' to `b'.
-  This doesn't write any values, but only a WB declaration.
-  `oldv' is replaced value with `b' (not used in current Ruby).
-```
-
-See source code carefully to insert write barriers.
-