diff --git a/ChangeLog b/ChangeLog
index f66e2639b6..afd3d56373 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Sat Jul 26 16:55:18 2014  Eric Wong  <e@80x24.org>
+
+	* iseq.h (struct iseq_compile_data_storage): reduce overhead
+	  to 16 bytes (from 32) on 64-bit
+
 Sat Jul 26 16:28:06 2014  Eric Wong  <e@80x24.org>
 
 	* vm_core.h (struct rb_iseq_struct): reduce to 280 bytes
diff --git a/compile.c b/compile.c
index 7dfaa81f47..f9bd9a531e 100644
--- a/compile.c
+++ b/compile.c
@@ -604,13 +604,11 @@ compile_data_alloc(rb_iseq_t *iseq, size_t size)
 	    goto retry;
 	}
 	storage->next = (void *)ALLOC_N(char, alloc_size +
-					sizeof(struct
-					       iseq_compile_data_storage));
+					SIZEOF_ISEQ_COMPILE_DATA_STORAGE);
 	storage = iseq->compile_data->storage_current = storage->next;
 	storage->next = 0;
 	storage->pos = 0;
 	storage->size = alloc_size;
-	storage->buff = (char *)(&storage->buff + 1);
     }
 
     ptr = (void *)&storage->buff[storage->pos];
diff --git a/iseq.c b/iseq.c
index aadb28b8ee..834f3bbb17 100644
--- a/iseq.c
+++ b/iseq.c
@@ -153,7 +153,7 @@ iseq_memsize(const void *ptr)
 
 		cur = iseq->compile_data->storage_head;
 		while (cur) {
-		    size += cur->size + sizeof(struct iseq_compile_data_storage);
+		    size += cur->size + SIZEOF_ISEQ_COMPILE_DATA_STORAGE;
 		    cur = cur->next;
 		}
 		size += sizeof(struct iseq_compile_data);
@@ -293,15 +293,13 @@ prepare_iseq_build(rb_iseq_t *iseq,
     iseq->compile_data->storage_head = iseq->compile_data->storage_current =
       (struct iseq_compile_data_storage *)
 	ALLOC_N(char, INITIAL_ISEQ_COMPILE_DATA_STORAGE_BUFF_SIZE +
-		sizeof(struct iseq_compile_data_storage));
+		SIZEOF_ISEQ_COMPILE_DATA_STORAGE);
 
     RB_OBJ_WRITE(iseq->self, &iseq->compile_data->catch_table_ary, rb_ary_new());
     iseq->compile_data->storage_head->pos = 0;
     iseq->compile_data->storage_head->next = 0;
     iseq->compile_data->storage_head->size =
       INITIAL_ISEQ_COMPILE_DATA_STORAGE_BUFF_SIZE;
-    iseq->compile_data->storage_head->buff =
-      (char *)(&iseq->compile_data->storage_head->buff + 1);
     iseq->compile_data->option = option;
     iseq->compile_data->last_coverable_line = -1;
 
diff --git a/iseq.h b/iseq.h
index 9a225180df..da2b678c1b 100644
--- a/iseq.h
+++ b/iseq.h
@@ -88,11 +88,15 @@ iseq_catch_table_bytes(int n)
 
 struct iseq_compile_data_storage {
     struct iseq_compile_data_storage *next;
-    unsigned long pos;
-    unsigned long size;
-    char *buff;
+    unsigned int pos;
+    unsigned int size;
+    char buff[1]; /* flexible array */
 };
 
+/* account for flexible array */
+#define SIZEOF_ISEQ_COMPILE_DATA_STORAGE \
+    (sizeof(struct iseq_compile_data_storage) - 1)
+
 struct iseq_compile_data {
     /* GC is needed */
     const VALUE err_info;