Constant time class to class ancestor lookup

Previously when checking ancestors, we would walk all the way up the
ancestry chain checking each parent for a matching class or module.

I believe this was especially unfriendly to CPU cache since for each
step we need to check two cache lines (the class and class ext).

This check is used quite often in:
* case statements
* rescue statements
* Calling protected methods
* Class#is_a?
* Module#===
* Module#<=>

I believe it's most common to check a class against a parent class, to
this commit aims to improve that (unfortunately does not help checking
for an included Module).

This is done by storing on each class the number and an array of all
parent classes, in order (BasicObject is at index 0). Using this we can
check whether a class is a subclass of another in constant time since we
know the location to expect it in the hierarchy.

benchmark/module_eqq.yml

@@ -0,0 +1,27 @@
+prelude: |
+  class SimpleClass; end
+  class MediumClass
+    10.times { include Module.new }
+  end
+  class LargeClass
+    100.times { include Module.new }
+  end
+  class HugeClass
+    300.times { include Module.new }
+  end
+  SimpleObj = SimpleClass.new
+  MediumObj = MediumClass.new
+  LargeObj = LargeClass.new
+  HugeObj = HugeClass.new
+benchmark:
+  simple_class_eqq_simple_obj: |
+    SimpleClass === SimpleObj
+  medium_class_eqq_simple_obj: |
+    MediumClass === SimpleObj
+  simple_class_eqq_medium_obj: |
+    SimpleClass === MediumObj
+  simple_class_eqq_large_obj: |
+    SimpleClass === LargeObj
+  simple_class_eqq_huge_obj: |
+    SimpleClass === HugeObj
+loop_count: 20000000

class.c

@@ -259,6 +259,63 @@ rb_class_boot(VALUE super)
     return (VALUE)klass;
 }
 
+void
+rb_class_remove_superclasses(VALUE klass)
+{
+    if (!RB_TYPE_P(klass, T_CLASS))
+        return;
+
+    if (RCLASS_SUPERCLASSES(klass))
+        xfree(RCLASS_SUPERCLASSES(klass));
+
+    RCLASS_SUPERCLASSES(klass) = NULL;
+    RCLASS_SUPERCLASS_DEPTH(klass) = 0;
+}
+
+void
+rb_class_update_superclasses(VALUE klass)
+{
+    VALUE super = RCLASS_SUPER(klass);
+
+    if (!RB_TYPE_P(klass, T_CLASS)) return;
+    if (super == Qundef) return;
+
+    // If the superclass array is already built
+    if (RCLASS_SUPERCLASSES(klass))
+        return;
+
+    // find the proper superclass
+    while (super != Qfalse && !RB_TYPE_P(super, T_CLASS)) {
+        super = RCLASS_SUPER(super);
+    }
+
+    // For BasicObject and uninitialized classes, depth=0 and ary=NULL
+    if (super == Qfalse)
+        return;
+
+    // Sometimes superclasses are set before the full ancestry tree is built
+    // This happens during metaclass construction
+    if (super != rb_cBasicObject && !RCLASS_SUPERCLASS_DEPTH(super)) {
+        rb_class_update_superclasses(super);
+
+        // If it is still unset we need to try later
+        if (!RCLASS_SUPERCLASS_DEPTH(super))
+            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;
+}
+
 void
 rb_check_inheritable(VALUE super)
 {
@@ -667,6 +724,9 @@ make_metaclass(VALUE klass)
     while (RB_TYPE_P(super, T_ICLASS)) super = RCLASS_SUPER(super);
     RCLASS_SET_SUPER(metaclass, super ? ENSURE_EIGENCLASS(super) : rb_cClass);
 
+    // Full class ancestry may not have been filled until we reach here.
+    rb_class_update_superclasses(METACLASS_OF(metaclass));
+
     return metaclass;
 }
 

gc.c

@@ -3187,6 +3187,7 @@ obj_free(rb_objspace_t *objspace, VALUE obj)
         rb_class_remove_subclass_head(obj);
 	rb_class_remove_from_module_subclasses(obj);
 	rb_class_remove_from_super_subclasses(obj);
+	rb_class_remove_superclasses(obj);
 #if SIZEOF_SERIAL_T != SIZEOF_VALUE && USE_RVARGC
         xfree(RCLASS(obj)->class_serial_ptr);
 #endif
@@ -4619,6 +4620,7 @@ obj_memsize_of(VALUE obj, int use_all_types)
             if (RCLASS_CC_TBL(obj)) {
                 size += cc_table_memsize(RCLASS_CC_TBL(obj));
             }
+            size += RCLASS_SUPERCLASS_DEPTH(obj) * sizeof(VALUE);
 #if !USE_RVARGC
 	    size += sizeof(rb_classext_t);
 #endif
@@ -10032,6 +10034,14 @@ update_class_ext(rb_objspace_t *objspace, rb_classext_t *ext)
     }
 }
 
+static void
+update_superclasses(rb_objspace_t *objspace, VALUE obj)
+{
+    for (size_t i = 0; i < RCLASS_SUPERCLASS_DEPTH(obj); i++) {
+        UPDATE_IF_MOVED(objspace, RCLASS_SUPERCLASSES(obj)[i]);
+    }
+}
+
 static void
 gc_update_object_references(rb_objspace_t *objspace, VALUE obj)
 {
@@ -10049,6 +10059,7 @@ gc_update_object_references(rb_objspace_t *objspace, VALUE obj)
         update_m_tbl(objspace, RCLASS_M_TBL(obj));
         update_cc_tbl(objspace, obj);
         update_cvc_tbl(objspace, obj);
+        update_superclasses(objspace, obj);
 
         gc_update_tbl_refs(objspace, RCLASS_IV_TBL(obj));
 

internal/class.h

@@ -47,6 +47,8 @@ struct rb_classext_struct {
     struct rb_id_table *callable_m_tbl;
     struct rb_id_table *cc_tbl; /* ID -> [[ci, cc1], cc2, ...] */
     struct rb_id_table *cvc_tbl;
+    size_t superclass_depth;
+    VALUE *superclasses;
     struct rb_subclass_entry *subclasses;
     struct rb_subclass_entry *subclass_entry;
     /**
@@ -117,6 +119,8 @@ typedef struct rb_classext_struct rb_classext_t;
 #define RCLASS_MODULE_SUBCLASS_ENTRY(c) (RCLASS_EXT(c)->module_subclass_entry)
 #define RCLASS_ALLOCATOR(c) (RCLASS_EXT(c)->allocator)
 #define RCLASS_SUBCLASSES(c) (RCLASS_EXT(c)->subclasses)
+#define RCLASS_SUPERCLASS_DEPTH(c) (RCLASS_EXT(c)->superclass_depth)
+#define RCLASS_SUPERCLASSES(c) (RCLASS_EXT(c)->superclasses)
 
 #define RICLASS_IS_ORIGIN FL_USER5
 #define RCLASS_CLONED     FL_USER6
@@ -125,6 +129,8 @@ typedef struct rb_classext_struct rb_classext_t;
 /* class.c */
 void rb_class_subclass_add(VALUE super, VALUE klass);
 void rb_class_remove_from_super_subclasses(VALUE);
+void rb_class_update_superclasses(VALUE);
+void rb_class_remove_superclasses(VALUE);
 void rb_class_remove_subclass_head(VALUE);
 int rb_singleton_class_internal_p(VALUE sklass);
 VALUE rb_class_boot(VALUE);
@@ -197,6 +203,7 @@ RCLASS_SET_SUPER(VALUE klass, VALUE super)
         rb_class_subclass_add(super, klass);
     }
     RB_OBJ_WRITE(klass, &RCLASS(klass)->super, super);
+    rb_class_update_superclasses(klass);
     return super;
 }
 

object.c

@@ -757,6 +757,26 @@ rb_obj_is_instance_of(VALUE obj, VALUE c)
     return RBOOL(rb_obj_class(obj) == c);
 }
 
+// Returns whether c is a proper (c != cl) subclass of cl
+// Both c and cl must be T_CLASS
+static VALUE
+class_search_class_ancestor(VALUE cl, VALUE c)
+{
+    RUBY_ASSERT(RB_TYPE_P(c, T_CLASS));
+    RUBY_ASSERT(RB_TYPE_P(cl, T_CLASS));
+
+    size_t c_depth = RCLASS_SUPERCLASS_DEPTH(c);
+    size_t cl_depth = RCLASS_SUPERCLASS_DEPTH(cl);
+    VALUE *classes = RCLASS_SUPERCLASSES(cl);
+
+    // If c's inheritance chain is longer, it cannot be an ancestor
+    // We are checking for a proper subclass so don't check if they are equal
+    if (cl_depth <= c_depth)
+        return Qfalse;
+
+    // Otherwise check that c is in cl's inheritance chain
+    return RBOOL(classes[c_depth] == c);
+}
 
 /*
  *  call-seq:
@@ -791,21 +811,34 @@ rb_obj_is_kind_of(VALUE obj, VALUE c)
 {
     VALUE cl = CLASS_OF(obj);
 
-    RUBY_ASSERT(cl);
+    RUBY_ASSERT(RB_TYPE_P(cl, T_CLASS));
+
+    // Fastest path: If the object's class is an exact match we know `c` is a
+    // class without checking type and can return immediately.
+    if (cl == c) return Qtrue;
+
+    // Fast path: Both are T_CLASS
+    if (LIKELY(RB_TYPE_P(c, T_CLASS))) {
+        return class_search_class_ancestor(cl, c);
+    }
 
     // Note: YJIT needs this function to never allocate and never raise when
     // `c` is a class or a module.
     c = class_or_module_required(c);
-    return RBOOL(class_search_ancestor(cl, RCLASS_ORIGIN(c)));
+    c = RCLASS_ORIGIN(c);
+
+    // Slow path: check each ancestor in the linked list and its method table
+    return RBOOL(class_search_ancestor(cl, c));
 }
 
+
 static VALUE
 class_search_ancestor(VALUE cl, VALUE c)
 {
     while (cl) {
-	if (cl == c || RCLASS_M_TBL(cl) == RCLASS_M_TBL(c))
-	    return cl;
-	cl = RCLASS_SUPER(cl);
+        if (cl == c || RCLASS_M_TBL(cl) == RCLASS_M_TBL(c))
+            return cl;
+        cl = RCLASS_SUPER(cl);
     }
     return 0;
 }