Reduce memory usage of Hash object

## Implementation Summary

* Change entry list from segmented list to flat array.
* Change value of hash bucket from pointer to entry to index of entry list,
  and represent it by variable length bits according to capacity of hash
  buckets.
* Store management information about entry list and hash table to `struct
  RHash` as much as possible.

## Benchmark Summary

Only the results of typical situations on 64-bit Word-boxing are present
here. For more detailed information, including consideration, see below
(although most of the body is written in Japanese).

* https://shuujii.github.io/mruby-hash-benchmark

### Memory Usage

Lower value is better.

| Hash Size |   Baseline    |      New      |   Factor   |
|----------:|--------------:|--------------:|-----------:|
|        16 |          344B |          256B |   0.74419x |
|        40 |        1,464B |          840B |   0.57377x |
|       200 |        8,056B |        3,784B |   0.46971x |
|       500 |       17,169B |        9,944B |   0.57949x |

### Performance

Higher value is better.

#### `mrb_hash_set`

| Hash Size |   Baseline    |      New      |   Factor   |
|----------:|--------------:|--------------:|-----------:|
|        16 |  1.41847M i/s |  1.36004M i/s |   0.95881x |
|        40 |  0.39224M i/s |  0.31888M i/s |   0.81296x |
|       200 |  0.03780M i/s |  0.04290M i/s |   1.13494x |
|       500 |  0.01225M i/s |  0.01314M i/s |   1.07275x |

#### `mrb_hash_get`

| Hash Size |   Baseline    |      New      |   Factor   |
|----------:|--------------:|--------------:|-----------:|
|        16 | 26.05920M i/s | 30.19543M i/s |   1.15872x |
|        40 | 44.26420M i/s | 32.75781M i/s |   0.74005x |
|       200 | 44.55171M i/s | 31.56926M i/s |   0.70860x |
|       500 | 39.19250M i/s | 29.73806M i/s |   0.75877x |

#### `mrb_hash_each`

| Hash Size |   Baseline    |      New      |   Factor   |
|----------:|--------------:|--------------:|-----------:|
|        16 | 25.11964M i/s | 30.34167M i/s |   1.20789x |
|        40 | 11.74253M i/s | 13.25539M i/s |   1.12884x |
|       200 |  2.01133M i/s |  2.97214M i/s |   1.47770x |
|       500 |  0.87411M i/s |  1.21178M i/s |   1.38631x |

#### `Hash#[]=`

| Hash Size |   Baseline    |      New      |   Factor   |
|----------:|--------------:|--------------:|-----------:|
|        16 |  0.50095M i/s |  0.56490M i/s |   1.12764x |
|        40 |  0.19132M i/s |  0.18392M i/s |   0.96129x |
|       200 |  0.03624M i/s |  0.03256M i/s |   0.89860x |
|       500 |  0.01527M i/s |  0.01236M i/s |   0.80935x |
#### `Hash#[]`

| Hash Size |   Baseline    |      New      |   Factor   |
|----------:|--------------:|--------------:|-----------:|
|        16 | 11.53211M i/s | 12.78806M i/s |   1.10891x |
|        40 | 15.26920M i/s | 13.37529M i/s |   0.87596x |
|       200 | 15.28550M i/s | 13.36410M i/s |   0.87430x |
|       500 | 14.57695M i/s | 12.75388M i/s |   0.87494x |

#### `Hash#each`

| Hash Size |   Baseline    |      New      |   Factor   |
|----------:|--------------:|--------------:|-----------:|
|        16 |  0.30462M i/s |  0.27080M i/s |   0.88898x |
|        40 |  0.12912M i/s |  0.11704M i/s |   0.90642x |
|       200 |  0.02638M i/s |  0.02402M i/s |   0.91071x |
|       500 |  0.01066M i/s |  0.00959M i/s |   0.89953x |

#### `Hash#delete`

| Hash Size |   Baseline    |      New      |   Factor   |
|----------:|--------------:|--------------:|-----------:|
|        16 |  7.84167M i/s |  6.96419M i/s |   0.88810x |
|        40 |  6.91292M i/s |  7.41427M i/s |   1.07252x |
|       200 |  3.75952M i/s |  7.32080M i/s |   1.94727x |
|       500 |  2.10754M i/s |  7.05963M i/s |   3.34970x |

#### `Hash#shift`

| Hash Size |   Baseline    |      New      |   Factor   |
|----------:|--------------:|--------------:|-----------:|
|        16 | 14.66444M i/s | 13.18876M i/s |   0.89937x |
|        40 | 11.95124M i/s | 11.10420M i/s |   0.92913x |
|       200 |  5.53681M i/s |  7.88155M i/s |   1.42348x |
|       500 |  2.96728M i/s |  5.40405M i/s |   1.82121x |

#### `Hash#dup`

| Hash Size |   Baseline    |      New      |   Factor   |
|----------:|--------------:|--------------:|-----------:|
|        16 |  0.15063M i/s |  5.37889M i/s |  35.71024x |
|        40 |  0.06515M i/s |  3.38196M i/s |  51.91279x |
|       200 |  0.01359M i/s |  1.46538M i/s | 107.84056x |
|       500 |  0.00559M i/s |  0.75411M i/s | 134.88057x |

### Binary Size

Lower value is better.

|    File    |   Baseline    |      New      |  Factor   |
|:-----------|--------------:|--------------:|----------:|
| mruby      |      730,408B |      734,176B |  1.00519x |
| libmruby.a |    1,068,134B |    1,072,846B |  1.00441x |

## Other Fixes

The following issues have also been fixed in the parts where there was some
change this time.

* [Heap use-after-free in `Hash#value?`](https://gist.github.com/shuujii/30e4fcd5844a4112a0ecd4a5b3483101#file-heap-use-after-free-in-hash-value-md)
* [Heap use-after-free in `ht_hash_equal`](https://gist.github.com/shuujii/30e4fcd5844a4112a0ecd4a5b3483101#file-heap-use-after-free-in-ht_hash_equal-md)
* [Heap use-after-free in `ht_hash_func`](https://gist.github.com/shuujii/30e4fcd5844a4112a0ecd4a5b3483101#file-heap-use-after-free-in-ht_hash_func-md)
* [Heap use-after-free in `mrb_hash_merge`](https://gist.github.com/shuujii/30e4fcd5844a4112a0ecd4a5b3483101#file-heap-use-after-free-in-mrb_hash_merge-md)
* [Self-replacement does not work for `Hash#replace`](https://gist.github.com/shuujii/30e4fcd5844a4112a0ecd4a5b3483101#file-self-replacement-does-not-work-for-hash-replace-md)
* [Repeated deletes and inserts increase memory usage of `Hash`](https://gist.github.com/shuujii/30e4fcd5844a4112a0ecd4a5b3483101#file-repeated-deletes-and-inserts-increase-memory-usage-of-hash-md)
* [`Hash#rehash` does not reindex completely](https://gist.github.com/shuujii/30e4fcd5844a4112a0ecd4a5b3483101#file-hash-rehash-does-not-reindex-completely-md)
* `mrb_hash_delete_key` does not cause an error for frozen object
* `mrb_hash_new_capa` does not allocate required space first
* [`mrb_os_memsize_of_hash_table` result is incorrect](https://github.com/mruby/mruby/pull/5032#discussion_r457994075)

1 files changed, 34 insertions, 13 deletions

diff --git a/include/mruby/hash.h b/include/mruby/hash.h
index 7dab4a85c..c2af3b976 100644
--- a/include/mruby/hash.h
+++ b/include/mruby/hash.h
@@ -14,10 +14,22 @@
  */
 MRB_BEGIN_DECL
 
+/* offset of `iv` must be 3 words */
 struct RHash {
   MRB_OBJECT_HEADER;
+#ifdef MRB_64BIT
+  uint32_t size;
   struct iv_tbl *iv;
-  struct htable *ht;
+  uint32_t ea_capa;
+  uint32_t ea_n_used;
+#else
+  struct iv_tbl *iv;
+  uint32_t size;
+#endif
+  union {
+    struct hash_entry *ea;
+    struct hash_table *ht;
+  };
 };
 
 #define mrb_hash_ptr(v)    ((struct RHash*)(mrb_ptr(v)))
@@ -179,8 +191,9 @@ MRB_API mrb_value mrb_hash_clear(mrb_state *mrb, mrb_value hash);
 MRB_API mrb_int mrb_hash_size(mrb_state *mrb, mrb_value hash);
 
 /*
- * Copies the hash.
- *
+ * Copies the hash. This function does NOT copy the instance variables
+ * (except for the default value). Use mrb_obj_dup() to copy the instance
+ * variables as well.
  *
  * @param mrb The mruby state reference.
  * @param hash The target hash.
@@ -198,16 +211,24 @@ MRB_API mrb_value mrb_hash_dup(mrb_state *mrb, mrb_value hash);
  */
 MRB_API void mrb_hash_merge(mrb_state *mrb, mrb_value hash1, mrb_value hash2);
 
-/* RHASH_TBL allocates st_table if not available. */
-#define RHASH(obj)   ((struct RHash*)(mrb_ptr(obj)))
-#define RHASH_TBL(h)          (RHASH(h)->ht)
-#define RHASH_IFNONE(h)       mrb_iv_get(mrb, (h), MRB_SYM(ifnone))
-#define RHASH_PROCDEFAULT(h)  RHASH_IFNONE(h)
-
-#define MRB_HASH_DEFAULT      1
-#define MRB_HASH_PROC_DEFAULT 2
-#define MRB_RHASH_DEFAULT_P(h) (RHASH(h)->flags & MRB_HASH_DEFAULT)
-#define MRB_RHASH_PROCDEFAULT_P(h) (RHASH(h)->flags & MRB_HASH_PROC_DEFAULT)
+#define RHASH(hash) ((struct RHash*)(mrb_ptr(hash)))
+#define RHASH_IFNONE(hash) mrb_iv_get(mrb, (hash), MRB_SYM(ifnone))
+#define RHASH_PROCDEFAULT(hash) RHASH_IFNONE(hash)
+
+#define MRB_HASH_IB_BIT_BIT         5
+#define MRB_HASH_AR_EA_CAPA_BIT     5
+#define MRB_HASH_IB_BIT_SHIFT       0
+#define MRB_HASH_AR_EA_CAPA_SHIFT   0
+#define MRB_HASH_AR_EA_N_USED_SHIFT MRB_HASH_AR_EA_CAPA_BIT
+#define MRB_HASH_SIZE_FLAGS_SHIFT   (MRB_HASH_AR_EA_CAPA_BIT * 2)
+#define MRB_HASH_IB_BIT_MASK        ((1 << MRB_HASH_IB_BIT_BIT) - 1)
+#define MRB_HASH_AR_EA_CAPA_MASK    ((1 << MRB_HASH_AR_EA_CAPA_BIT) - 1)
+#define MRB_HASH_AR_EA_N_USED_MASK  (MRB_HASH_AR_EA_CAPA_MASK << MRB_HASH_AR_EA_N_USED_SHIFT)
+#define MRB_HASH_DEFAULT            (1 << (MRB_HASH_SIZE_FLAGS_SHIFT + 0))
+#define MRB_HASH_PROC_DEFAULT       (1 << (MRB_HASH_SIZE_FLAGS_SHIFT + 1))
+#define MRB_HASH_HT                 (1 << (MRB_HASH_SIZE_FLAGS_SHIFT + 2))
+#define MRB_RHASH_DEFAULT_P(hash) (RHASH(hash)->flags & MRB_HASH_DEFAULT)
+#define MRB_RHASH_PROCDEFAULT_P(hash) (RHASH(hash)->flags & MRB_HASH_PROC_DEFAULT)
 
 /* GC functions */
 void mrb_gc_mark_hash(mrb_state*, struct RHash*);