diff --git a/ChangeLog b/ChangeLog
index d980868eb8f1d8..dd7b9c9c16d11c 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,10 @@
+Sun Sep  6 17:31:28 2009  Koichi Sasada  <ko1@atdot.net>
+
+	* compile.c (iseq_specialized_instruction), insns.def (opt_size):
+	  optimize #size methods (by specialized instruction).
+
+	* id.c, id.h, vm.c, vm_insnhelper.h: ditto.
+
 Sun Sep  6 16:13:06 2009  Koichi Sasada  <ko1@atdot.net>
 
 	* insns.def (setinstancevariable), vm_insnhelper.c (vm_setivar): 
diff --git a/compile.c b/compile.c
index 42828a802b8a20..d3c26671a620be 100644
--- a/compile.c
+++ b/compile.c
@@ -1803,6 +1803,9 @@ iseq_specialized_instruction(rb_iseq_t *iseq, INSN *iobj)
 		if (mid == idLength) {
 		    insn_set_specialized_instruction(iobj, BIN(opt_length));
 		}
+		else if (mid == idSize) {
+		    insn_set_specialized_instruction(iobj, BIN(opt_size));
+		}
 		else if (mid == idSucc) {
 		    insn_set_specialized_instruction(iobj, BIN(opt_succ));
 		}
diff --git a/id.c b/id.c
index 4fd760af0314c6..2111751a37619a 100644
--- a/id.c
+++ b/id.c
@@ -34,6 +34,7 @@ Init_id(void)
 
     REGISTER_SYMID(idEach, "each");
     REGISTER_SYMID(idLength, "length");
+    REGISTER_SYMID(idSize, "size");
     REGISTER_SYMID(idLambda, "lambda");
     REGISTER_SYMID(idIntern, "intern");
     REGISTER_SYMID(idGets, "gets");
diff --git a/id.h b/id.h
index 1f94dbb9963448..8087683cf5c8e6 100644
--- a/id.h
+++ b/id.h
@@ -98,6 +98,7 @@ enum ruby_method_ids {
     tIntern,
     tMethodMissing,
     tLength,
+    tSize,
     tGets,
     tSucc,
     tEach,
@@ -118,6 +119,7 @@ enum ruby_method_ids {
     TOKEN2ID(Intern),
     TOKEN2ID(MethodMissing),
     TOKEN2ID(Length),
+    TOKEN2ID(Size),
     TOKEN2ID(Gets),
     TOKEN2ID(Succ),
     TOKEN2ID(Each),
diff --git a/insns.def b/insns.def
index 98febf89b85030..7de873d4bf155e 100644
--- a/insns.def
+++ b/insns.def
@@ -1921,8 +1921,8 @@ opt_length
 (VALUE recv)
 (VALUE val)
 {
-    if (!SPECIAL_CONST_P(recv) &&
-	BASIC_OP_UNREDEFINED_P(BOP_LENGTH)) {
+    if (LIKELY(!SPECIAL_CONST_P(recv) &&
+	       BASIC_OP_UNREDEFINED_P(BOP_LENGTH))) {
 	if (HEAP_CLASS_OF(recv) == rb_cString) {
 	    val = rb_str_length(recv);
 	}
@@ -1943,6 +1943,39 @@ opt_length
     }
 }
 
+/**
+  @c optimize
+  @e optimized size
+  @j 最適化された recv.size()。
+ */
+DEFINE_INSN
+opt_size
+()
+(VALUE recv)
+(VALUE val)
+{
+    if (LIKELY(BASIC_OP_UNREDEFINED_P(BOP_SIZE) &&
+	       !SPECIAL_CONST_P(recv))) {
+	if (HEAP_CLASS_OF(recv) == rb_cString) {
+	    val = rb_str_length(recv);
+	}
+	else if (HEAP_CLASS_OF(recv) == rb_cArray) {
+	    val = LONG2NUM(RARRAY_LEN(recv));
+	}
+	else if (HEAP_CLASS_OF(recv) == rb_cHash) {
+	    val = INT2FIX(RHASH_SIZE(recv));
+	}
+	else {
+	    goto INSN_LABEL(normal_dispatch);
+	}
+    }
+    else {
+      INSN_LABEL(normal_dispatch):
+	PUSH(recv);
+	CALL_SIMPLE_METHOD(0, idSize, recv);
+    }
+}
+
 /**
   @c optimize
   @e optimized succ
diff --git a/vm.c b/vm.c
index 4af968653df3fd..c794b08dcf102f 100644
--- a/vm.c
+++ b/vm.c
@@ -980,6 +980,7 @@ vm_init_redefined_flag(void)
     OP(AREF, AREF), (C(Array), C(Hash));
     OP(ASET, ASET), (C(Array), C(Hash));
     OP(Length, LENGTH), (C(Array), C(String), C(Hash));
+    OP(Size, SIZE), (C(Array), C(String), C(Hash));
     OP(Succ, SUCC), (C(Fixnum), C(String), C(Time));
     OP(GT, GT), (C(Fixnum));
     OP(GE, GE), (C(Fixnum));
diff --git a/vm_insnhelper.h b/vm_insnhelper.h
index 055c43c16ad99e..bb4cef553b6934 100644
--- a/vm_insnhelper.h
+++ b/vm_insnhelper.h
@@ -48,6 +48,7 @@ enum {
   BOP_AREF,
   BOP_ASET,
   BOP_LENGTH,
+  BOP_SIZE,
   BOP_SUCC,
   BOP_GT,
   BOP_GE,