Use a functional red-black tree for indexing the shapes

This is an experimental commit that uses a functional red-black tree to
create an index of the ancestor shapes.  It uses an Okasaki style
functional red black tree:

  https://www.cs.tufts.edu/comp/150FP/archive/chris-okasaki/redblack99.pdf

This tree is advantageous because:

* It offers O(n log n) insertions and O(n log n) lookups.
* It shares memory with previous "versions" of the tree

When we insert a node in the tree, only the parts of the tree that need
to be rebalanced are newly allocated.  Parts of the tree that don't need
to be rebalanced are not reallocated, so "new trees" are able to share
memory with old trees.  This is in contrast to a sorted set where we
would have to duplicate the set, and also resort the set on each
insertion.

I've added a new stat to RubyVM.stat so we can understand how the red
black tree increases.

1 files changed, 7 insertions, 1 deletions

diff --git a/vm.c b/vm.c
index ebaee54953..7f43484905 100644
--- a/vm.c
+++ b/vm.c
@@ -633,6 +633,8 @@ rb_dtrace_setup(rb_execution_context_t *ec, VALUE klass, ID id,
     return FALSE;
 }
 
+extern unsigned int redblack_buffer_size;
+
 /*
  *  call-seq:
  *    RubyVM.stat -> Hash
@@ -660,6 +662,7 @@ static VALUE
 vm_stat(int argc, VALUE *argv, VALUE self)
 {
     static VALUE sym_constant_cache_invalidations, sym_constant_cache_misses, sym_global_cvar_state, sym_next_shape_id;
+    static VALUE sym_shape_cache_size;
     VALUE arg = Qnil;
     VALUE hash = Qnil, key = Qnil;
 
@@ -681,6 +684,7 @@ vm_stat(int argc, VALUE *argv, VALUE self)
     S(constant_cache_misses);
         S(global_cvar_state);
     S(next_shape_id);
+    S(shape_cache_size);
 #undef S
 
 #define SET(name, attr) \
@@ -693,6 +697,7 @@ vm_stat(int argc, VALUE *argv, VALUE self)
     SET(constant_cache_misses, ruby_vm_constant_cache_misses);
     SET(global_cvar_state, ruby_vm_global_cvar_state);
     SET(next_shape_id, (rb_serial_t)GET_SHAPE_TREE()->next_shape_id);
+    SET(shape_cache_size, (rb_serial_t)GET_SHAPE_TREE()->cache_size);
 #undef SET
 
 #if USE_DEBUG_COUNTER
@@ -3069,7 +3074,8 @@ vm_memsize(const void *ptr)
         vm_memsize_builtin_function_table(vm->builtin_function_table) +
         rb_id_table_memsize(vm->negative_cme_table) +
         rb_st_memsize(vm->overloaded_cme_table) +
-        vm_memsize_constant_cache()
+        vm_memsize_constant_cache() +
+        GET_SHAPE_TREE()->cache_size * sizeof(redblack_node_t)
     );
 
     // TODO