rb_io_buffer_t: pack structure

* include/ruby/io.h (rb_io_buffer_t): pack structure
  Reduces rb_io_t from 200 to 192 bytes, allowing rb_io_t to
  occupy one less cache line.
  [Feature #10050]

n.b. this leaves a 4-byte hole after the `cbuf' field which may
be used for future expansion.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46884 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ChangeLog

@@ -1,3 +1,10 @@
+Sun Jul 20 12:44:23 2014  Eric Wong  <e@80x24.org>
+
+	* include/ruby/io.h (rb_io_buffer_t): pack structure
+	  Reduces rb_io_t from 200 to 192 bytes, allowing rb_io_t to
+	  occupy one less cache line.
+	  [Feature #10050]
+
 Sun Jul 20 12:41:53 2014  Eric Wong  <e@80x24.org>
 
 	* include/ruby/io.h (rb_io_t): shrink to 200 bytes from 216 on 64-bit

include/ruby/io.h

@@ -51,12 +51,12 @@ extern "C" {
 
 RUBY_SYMBOL_EXPORT_BEGIN
 
-typedef struct {
+typedef PACKED_STRUCT(struct {
     char *ptr;                  /* off + len <= capa */
     int off;
     int len;
     int capa;
-} rb_io_buffer_t;
+}) rb_io_buffer_t;
 
 typedef struct rb_io_t {
     FILE *stdio_file;		/* stdio ptr for read/write if available */