Dedup superclass array in leaf sibling classes

Previously, we would build a new `superclasses` array for each class,
even though for all immediate subclasses of a class, the array is
identical.

This avoids duplicating the arrays on leaf classes (those without
subclasses) by calculating and storing a "superclasses including self"
array on a class when it's first inherited and sharing that among all
superclasses.

An additional trick used is that the "superclass array including self"
is valid as "self"'s superclass array. It just has it's own class at the
end. We can use this to avoid an extra pointer of storage and can use
one bit of a flag to track that we've "upgraded" the array.

1 files changed, 14 insertions, 19 deletions

diff --git a/class.c b/class.c
index ef3f8aac2e..0acac96022 100644
--- a/class.c
+++ b/class.c
@@ -259,17 +259,21 @@ rb_class_boot(VALUE super)
     return (VALUE)klass;
 }
 
-void
-rb_class_remove_superclasses(VALUE klass)
+static VALUE *
+class_superclasses_including_self(VALUE klass)
 {
-    if (!RB_TYPE_P(klass, T_CLASS))
-        return;
+    if (FL_TEST_RAW(klass, RCLASS_SUPERCLASSES_INCLUDE_SELF))
+        return RCLASS_SUPERCLASSES(klass);
 
-    if (RCLASS_SUPERCLASSES(klass))
-        xfree(RCLASS_SUPERCLASSES(klass));
+    size_t depth = RCLASS_SUPERCLASS_DEPTH(klass);
+    VALUE *superclasses = xmalloc(sizeof(VALUE) * (depth + 1));
+    if (depth > 0)
+        memcpy(superclasses, RCLASS_SUPERCLASSES(klass), sizeof(VALUE) * depth);
+    superclasses[depth] = klass;
 
-    RCLASS_SUPERCLASSES(klass) = NULL;
-    RCLASS_SUPERCLASS_DEPTH(klass) = 0;
+    RCLASS_SUPERCLASSES(klass) = superclasses;
+    FL_SET_RAW(klass, RCLASS_SUPERCLASSES_INCLUDE_SELF);
+    return superclasses;
 }
 
 void
@@ -303,17 +307,8 @@ rb_class_update_superclasses(VALUE klass)
             return;
     }
 
-    size_t parent_num = RCLASS_SUPERCLASS_DEPTH(super);
-    size_t num = parent_num + 1;
-
-    VALUE *superclasses = xmalloc(sizeof(VALUE) * num);
-    superclasses[parent_num] = super;
-    if (parent_num > 0) {
-        memcpy(superclasses, RCLASS_SUPERCLASSES(super), sizeof(VALUE) * parent_num);
-    }
-
-    RCLASS_SUPERCLASSES(klass) = superclasses;
-    RCLASS_SUPERCLASS_DEPTH(klass) = num;
+    RCLASS_SUPERCLASSES(klass) = class_superclasses_including_self(super);
+    RCLASS_SUPERCLASS_DEPTH(klass) = RCLASS_SUPERCLASS_DEPTH(super) + 1;
 }
 
 void