Store object age in a bitmap

Closes [Feature #19729]

Previously 2 bits of the flags on each RVALUE are reserved to store the
number of GC cycles that each object has survived. This commit
introduces a new bit array on the heap page, called age_bits, to store
that information instead.

This patch still reserves one of the age bits in the flags (the old
FL_PROMOTED0 bit, now renamed FL_PROMOTED).

This is set to 0 for young objects and 1 for old objects, and is used as
a performance optimisation for the write barrier. Fetching the age_bits
from the heap page and doing the required math to calculate if the
object was old or not would slow down the write barrier. So we keep this
bit synced in the flags for fast access.

2 files changed, 15 insertions, 41 deletions

diff --git a/include/ruby/internal/core/rtypeddata.h b/include/ruby/internal/core/rtypeddata.h
index 1abf965305..c7904746fd 100644
--- a/include/ruby/internal/core/rtypeddata.h
+++ b/include/ruby/internal/core/rtypeddata.h
@@ -173,10 +173,9 @@ rbimpl_typeddata_flags {
     RUBY_TYPED_WB_PROTECTED     = RUBY_FL_WB_PROTECTED, /* THIS FLAG DEPENDS ON Ruby version */
 
     /**
-     * This flag  is mysterious.  It seems  nobody is currently using  it.  The
-     * intention of this flag is also unclear.  We need further investigations.
+     * This flag no longer in use
      */
-    RUBY_TYPED_PROMOTED1        = RUBY_FL_PROMOTED1,     /* THIS FLAG DEPENDS ON Ruby version */
+    RUBY_TYPED_UNUSED           = RUBY_FL_UNUSED6,
 
     /**
      * This flag determines whether marking and compaction should be carried out
diff --git a/include/ruby/internal/fl_type.h b/include/ruby/internal/fl_type.h
index d59c258a3f..f7f5abdd9b 100644
--- a/include/ruby/internal/fl_type.h
+++ b/include/ruby/internal/fl_type.h
@@ -57,8 +57,7 @@
 
 #define FL_SINGLETON    RBIMPL_CAST((VALUE)RUBY_FL_SINGLETON)            /**< @old{RUBY_FL_SINGLETON} */
 #define FL_WB_PROTECTED RBIMPL_CAST((VALUE)RUBY_FL_WB_PROTECTED)         /**< @old{RUBY_FL_WB_PROTECTED} */
-#define FL_PROMOTED0    RBIMPL_CAST((VALUE)RUBY_FL_PROMOTED0)            /**< @old{RUBY_FL_PROMOTED0} */
-#define FL_PROMOTED1    RBIMPL_CAST((VALUE)RUBY_FL_PROMOTED1)            /**< @old{RUBY_FL_PROMOTED1} */
+#define FL_PROMOTED     RBIMPL_CAST((VALUE)RUBY_FL_PROMOTED)             /**< @old{RUBY_FL_PROMOTED} */
 #define FL_FINALIZE     RBIMPL_CAST((VALUE)RUBY_FL_FINALIZE)             /**< @old{RUBY_FL_FINALIZE} */
 #define FL_TAINT        RBIMPL_CAST((VALUE)RUBY_FL_TAINT)                /**< @old{RUBY_FL_TAINT} */
 #define FL_SHAREABLE    RBIMPL_CAST((VALUE)RUBY_FL_SHAREABLE)            /**< @old{RUBY_FL_SHAREABLE} */
@@ -200,12 +199,15 @@ ruby_fl_type {
     RUBY_FL_WB_PROTECTED = (1<<5),
 
     /**
-     * This flag  has something to do  with our garbage collector.   These days
-     * ruby  objects are  "generational".  There  are those  who are  young and
-     * those who are old.  Young objects are prone to die; monitored relatively
-     * extensively by  the garbage  collector.  OTOH old  objects tend  to live
-     * longer.  They are relatively rarely considered.  This flag is set when a
-     * object experienced promotion i.e. survived a garbage collection.
+     * Ruby objects are "generational".  There are young objects & old objects.
+     * Young objects are prone to die & monitored relatively extensively by the
+     * garbage collector.  Old objects tend to live longer & are monitored less
+     * frequently.  When an object survives a GC, its age is incremented.  When
+     * age is equal to RVALUE_OLD_AGE, the object becomes Old. This flag is set
+     * when an object becomes old, and is used by the write barrier to check if
+     * an old object should be considered for marking more frequently  - as old
+     * objects that have references added between major GCs need to be remarked
+     * to prevent the referred object being mistakenly swept.
      *
      * @internal
      *
@@ -213,41 +215,14 @@ ruby_fl_type {
      * 3rd parties.  It must be an implementation detail that they should never
      * know.  Might better be hidden.
      */
-    RUBY_FL_PROMOTED0    = (1<<5),
+    RUBY_FL_PROMOTED    = (1<<5),
 
     /**
-     * This flag  has something to do  with our garbage collector.   These days
-     * ruby  objects are  "generational".  There  are those  who are  young and
-     * those who are old.  Young objects are prone to die; monitored relatively
-     * extensively by  the garbage  collector.  OTOH old  objects tend  to live
-     * longer.  They are relatively rarely considered.  This flag is set when a
-     * object  experienced two  promotions  i.e.  survived garbage  collections
-     * twice.
+     * This flag is no longer in use
      *
      * @internal
-     *
-     * But honestly, @shyouhei  doesn't think this flag should  be visible from
-     * 3rd parties.  It must be an implementation detail that they should never
-     * know.  Might better be hidden.
-     */
-    RUBY_FL_PROMOTED1    = (1<<6),
-
-    /**
-     * This flag  has something to do  with our garbage collector.   These days
-     * ruby  objects are  "generational".  There  are those  who are  young and
-     * those who are old.  Young objects are prone to die; monitored relatively
-     * extensively by  the garbage  collector.  OTOH old  objects tend  to live
-     * longer.  They are relatively rarely considered.  This flag is set when a
-     * object  experienced  promotions i.e.   survived  more  than one  garbage
-     * collections.
-     *
-     * @internal
-     *
-     * But honestly, @shyouhei  doesn't think this flag should  be visible from
-     * 3rd parties.  It must be an implementation detail that they should never
-     * know.  Might better be hidden.
      */
-    RUBY_FL_PROMOTED     = RUBY_FL_PROMOTED0 | RUBY_FL_PROMOTED1,
+    RUBY_FL_UNUSED6    = (1<<6),
 
     /**
      * This flag has  something to do with finalisers.  A  ruby object can have