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.

1 files changed, 1 insertions, 1 deletions

diff --git a/proc.c b/proc.c
index 17c837d164..08474506ac 100644
--- a/proc.c
+++ b/proc.c
@@ -70,7 +70,7 @@ CLONESETUP(VALUE clone, VALUE obj)
     RBIMPL_ASSERT_OR_ASSUME(! RB_SPECIAL_CONST_P(obj));
     RBIMPL_ASSERT_OR_ASSUME(! RB_SPECIAL_CONST_P(clone));
 
-    const VALUE flags = RUBY_FL_PROMOTED0 | RUBY_FL_PROMOTED1 | RUBY_FL_FINALIZE;
+    const VALUE flags = RUBY_FL_PROMOTED | RUBY_FL_FINALIZE;
     rb_obj_setup(clone, rb_singleton_class_clone(obj),
                  RB_FL_TEST_RAW(obj, ~flags));
     rb_singleton_class_attached(RBASIC_CLASS(clone), clone);