Use 1 byte hint for ar_table [Feature #15602]

On ar_table, Do not keep a full-length hash value (FLHV, 8 bytes)
but keep a 1 byte hint from a FLHV (lowest byte of FLHV).
An ar_table only contains at least 8 entries, so hints consumes
8 bytes at most. We can store hints in RHash::ar_hint.

On 32bit CPU, we use 4 entries ar_table.

The advantages:
* We don't need to keep FLHV so ar_table only consumes
  16 bytes (VALUEs of key and value) * 8 entries = 128 bytes.
* We don't need to scan ar_table, but only need to check hints
  in many cases. Especially we don't need to access ar_table
  if there is no match entries (in many cases).
  It will increase memory cache locality.

The disadvantages:
* This technique can increase `#eql?` time because hints can
  conflicts (in theory, it conflicts once in 256 times).
  It can introduce incompatibility if there is a object x where
  x.eql? returns true even if hash values are different.
  I believe we don't need to care such irregular case.
* We need to re-calculate FLHV if we need to switch from ar_table
  to st_table (e.g. exceeds 8 entries).
  It also can introduce incompatibility, on mutating key objects.
  I believe we don't need to care such irregular case too.

Add new debug counters to measure the performance:
* artable_hint_hit - hint is matched and eql?#=>true
* artable_hint_miss - hint is not matched but eql?#=>false
* artable_hint_notfound - lookup counts

1 files changed, 1 insertions, 1 deletions

diff --git a/marshal.c b/marshal.c
index 7cd4756fd0..db62d3bda2 100644
--- a/marshal.c
+++ b/marshal.c
@@ -917,7 +917,7 @@ w_object(VALUE obj, struct dump_arg *arg, int limit)
 	    if (NIL_P(RHASH_IFNONE(obj))) {
 		w_byte(TYPE_HASH, arg);
 	    }
-	    else if (FL_TEST(obj, HASH_PROC_DEFAULT)) {
+	    else if (FL_TEST(obj, RHASH_PROC_DEFAULT)) {
 		rb_raise(rb_eTypeError, "can't dump hash with default proc");
 	    }
 	    else {