A silly optimization attempt

* Hard-coding some methods specific to `nil`, to see how far we can push performance

* Add a benchmark yml file to test against, and a new spec verifying `==` is correctly returning true for `nil.respond_to?(:==)`

Author: jpcamara

benchmark/object_respond_to.yml

@@ -0,0 +1,12 @@
+prelude: |
+  class Base; def foo; end end
+  class OneTwentyEight < Base
+    128.times { include(Module.new) }
+  end
+  obj = OneTwentyEight.new
+benchmark:
+  respond_to_false: obj.respond_to?(:bar)
+  respond_to_true: obj.respond_to?(:foo)
+  respond_to_nil_false: nil.respond_to?(:bar)
+loop_count: 1_000_000
+

insns.def

@@ -956,7 +956,9 @@ opt_respond_to
 (VALUE val)
 {
     val = vm_opt_respond_to(recv, mid);
-    CALL_SIMPLE_METHOD();
+    if (UNDEF_P(val)) {
+        CALL_SIMPLE_METHOD();
+    }
 }
 
 DEFINE_INSN

spec/ruby/core/kernel/respond_to_spec.rb

@@ -6,6 +6,10 @@
     @a = KernelSpecs::A.new
   end
 
+  it "returns true for checking for `==` on nil" do
+    nil.respond_to?(:==).should == true
+  end
+
   it "is a public method" do
     Kernel.should have_public_instance_method(:respond_to?, false)
   end

vm_insnhelper.c

@@ -6133,18 +6133,59 @@ vm_opt_ary_freeze(VALUE ary, int bop, ID id)
     }
 }
 
+static struct rb_id_table *method_id_table = NULL;
+
 static VALUE
 vm_opt_respond_to(VALUE recv, VALUE mid)
 {
-    if (SYMBOL_P(mid)) {
-        printf("symbol:");
-    } else if (STRING_P(mid)) {
-        printf("string:");
+    if (method_id_table == NULL) {
+        const char *method_names[] = {
+            "rationalize", "&", "===", "inspect", "=~", "to_a", "to_s", "to_i", "to_f", "to_r",
+            "to_c", "nil?", "pretty_print_cycle", "|", "to_h", "^", "to_json", "to_yaml",
+            "pretty_print", "pretty_print_instance_variables", "pretty_print_inspect", "singleton_class",
+            "dup", "itself", "methods", "singleton_methods", "protected_methods", "private_methods",
+            "public_methods", "instance_variables", "instance_variable_get", "instance_variable_set",
+            "instance_variable_defined?", "remove_instance_variable", "instance_of?", "kind_of?",
+            "is_a?", "display", "frozen?", "class", "then", "yield_self", "tap", "TypeName",
+            "public_send", "extend", "clone", "<=>", "pretty_inspect", "!~", "method", "eql?",
+            "respond_to?", "public_method", "singleton_method", "define_singleton_method", "hash",
+            "freeze", "object_id", "Namespace", "send", "to_enum", "enum_for", "equal?", "!",
+            "__send__", "==", "!=", "__id__", "instance_eval", "instance_exec"
+        };
+
+        size_t method_names_size = sizeof(method_names) / sizeof(method_names[0]);
+        method_id_table = rb_id_table_create(method_names_size);
+
+        for (size_t i = 0; i < method_names_size; i++) {
+            ID id = rb_intern(method_names[i]);
+            rb_id_table_insert(method_id_table, id, Qtrue);
+        }
+    }
+    if (NIL_P(recv)) {
+        ID id = rb_check_id(&mid);
+        if (!id) return Qfalse;
+
+        VALUE val;
+        if (rb_id_table_lookup(method_id_table, id, &val)) {
+            return Qtrue;
+        } else {
+            return Qfalse;
+        }
     }
-    printf("%s\n", rb_builtin_type_name(TYPE(recv)));
+
     return Qundef;
 }
 
+// static VALUE
+// vm_opt_respond_to(VALUE recv, VALUE mid)
+// {
+//     if (NIL_P(recv)) {
+//         return Qfalse;
+//     }
+
+//     return Qundef;
+// }
+
 static VALUE
 vm_opt_hash_freeze(VALUE hash, int bop, ID id)
 {