### Example (32-bit Word-boxing)

  ```ruby
  # example.rb
  int_count = ObjectSpace.count_objects[:T_INTEGER]||0
  int = 1<<30
  p (ObjectSpace.count_objects[:T_INTEGER]||0) - int_count
  int = nil
  GC.start
  p (ObjectSpace.count_objects[:T_INTEGER]||0) - int_count
  ```

#### Before this patch:

  ```console
  $ bin/mruby example.rb
  1
  1
  ```

#### After this patch:

  ```console
  $ bin/mruby example.rb
  1
  0
  ```

Fix overflow detection in integer operations with `MRB_WORD_BOXING`.
This bug made `1073741824 == 1073741824+0` to be `false` on 32bit
platforms.

Should raise `RangeError` if the operation overflows.

Remove redundant command execution in `header_search_paths`

There is no need to execute command in duplicate for the same
reason as #5128.

Remove unused constant in `lib/mruby/build/command.rb`

Workaround to build `mirb` with Homebrew's readline on Mac; fix #4537

Simplify `MRuby::Command#_run` to avoid duplicated compilation

Don't use private structs with `mrb_mt_foreach()`

The `MRB_API` function `mrb_mt_foreach()` previously used the private structure `struct mt_elem`.
Therefore, use `mrb_method_t` instead.

ref: https://github.com/mruby/mruby/pull/4959#discussion_r402086196

Compiles twice because it falls back to `build.filename(command)` when
`command` fails. This process was added at 9968af45 to support `ccache gcc`
etc. At that time, it seems that it was necessary because
`build.filename(command)` quoted the whole `command`, but now it does not
quote, so we can just run `build.filename(command)`.

### Example

  ```console
  $ echo 1 > src/a.c
  $ rake -v
  ```

#### Before this patch:

  ```console
  (snip)
  gcc -std=gnu99 -g -O3 -Wall -Wundef -Wdeclaration-after-statement -Werror-implicit-function-declaration -Wwrite-strings -I"/mruby/mruby/include" -MMD -o "/mruby/mruby/build/host/src/a.o" -c "/mruby/mruby/src/a.c"
  /mruby/mruby/src/a.c:1:1: error: expected identifier or '('
  1
  ^
  1 error generated.
  gcc -std=gnu99 -g -O3 -Wall -Wundef -Wdeclaration-after-statement -Werror-implicit-function-declaration -Wwrite-strings -I"/mruby/mruby/include" -MMD -o "/mruby/mruby/build/host/src/a.o" -c "/mruby/mruby/src/a.c"
  /mruby/mruby/src/a.c:1:1: error: expected identifier or '('
  1
  ^
  1 error generated.
  rake aborted!
  (snip)
  ```

#### After this patch:

  ```console
  (snip)
  gcc -std=gnu99 -g -O3 -Wall -Wundef -Wdeclaration-after-statement -Werror-implicit-function-declaration -Wwrite-strings -I"/mruby/mruby/include" -MMD -o "/mruby/mruby/build/host/src/a.o" -c "/mruby/mruby/src/a.c"
  /mruby/mruby/src/a.c:1:1: error: expected identifier or '('
  1
  ^
  1 error generated.
  rake aborted!
  (snip)
  ```

Merge pull request #5127 from shuujii/include-size-of-iv-table-in-ObjectSpace.memsize_of-to-Hash-object

Include size of iv table in `ObjectSpace.memsize_of` to `Hash` object

Use `enable_sanitizer` to simplify `build_config/clang-asan.rb`

Rename `mrb_os_memsize_of_hash_table` to `mrb_hash_memsize`

* The term `hash_table` can be misleading because the return value of this
  function includes memory usage of entire `Hash` object, including not only
  hash table part but also entry list part, etc.
* This function takes a `Hash` object as a receiver and is defined in
  `src/hash.c`, so it is natural to have a `mrb_hash_` prefix.

Change name and usage of presym macros

To be also able to build mruby without presym in the future. However,
`MRB_QSYM` has been removed and changed as follows:

### Example

|       Type                | Symbol |  Previous Style  |   New Style    |
|---------------------------|--------|------------------|----------------|
| Operator                  | &      | MRB_QSYM(and)    | MRB_OPSYM(and) |
| Class Variable            | @@foo  | MRB_QSYM(00_foo) | MRB_CVSYM(foo) |
| Instance Variable         | @foo   | MRB_QSYM(0_foo)  | MRB_IVSYM(foo) |
| Method with Bang          | foo!   | MRB_QSYM(foo_b)  | MRB_SYM_B(foo) |
| Method with Question mark | foo?   | MRB_QSYM(foo_p)  | MRB_SYM_Q(foo) |
| Mmethod with Equal        | foo=   | MRB_QSYM(foo_e)  | MRB_SYM_E(foo) |

This change makes it possible to define, for example, `MRB_IVSYM(foo)` as
`mrb_intern_lit(mrb, "@" "foo")`, which is useful if we support building
without presym in the future.

Remove unused argument of `mrb_obj_iv_tbl_memsize`

Reduce memory usage of Hash object

Fix typo in backward compatible mrb_exc_new_str_lit() macro

## 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)

Define `mrb_value` in struct for Word/NaN boxing

Don't check FD for STDIN/STDOUT/STDERR; #5114

We don't need to require valid STDIN/STDOUT/STDERR. If we require it,
we can't use mruby on an environment that doesn't have valid
STDIN/STDOUT/STDERR such as Windows service process. Windows service
process doesn't have valid STDIN.

Fixed build NaN boxing with 32-bit CPU mode

`SET_CPTR_VALUE()` requires the `p` field on 32-bit CPU mode.

This is to make it possible to distinguish between `mrb_value` and `mrb_sym` in C++ "Function overloading" and C11 "Generic selection".