## Implementation Summary
* Only keys and only values of hash table are contiguous to eliminate
structure padding.
* Change upper limit of `iv_tbl` size to `UINT16_MAX` (it seems to be
acceptable in mruby because the total number of classes/modules
immediately after starting Redmine is 20,000 or less).
* `iv_tbl*` point hash buckets directly.
## Benchmark Summary
Only the results of typical situations on 64-bit Word-boxing are present
here. For more detailed information, including consideration, see below
report (although most of the body is written in Japanese).
* https://shuujii.github.io/mruby-iv-benchmark
### Memory Usage
Lower value is better.
| iv_tbl Size | Baseline | New | Factor |
|------------:|---------------:|---------------:|-----------:|
| 4 | 88B | 52B | 0.59091x |
| 30 | 536B | 388B | 0.72388x |
| 100 | 2072B | 1540B | 0.74324x |
| 200 | 4120B | 3076B | 0.74660x |
Although not mentioned in the above report, the memory usage of `mrbtest`
(full-core gembox) is as follows in the result by Valgrind.
* Baseline: 108,086 allocs, 16,313,122 bytes allocated
* New: 94,273 allocs, 15,875,214 bytes allocated
### Performance
Higher value is better.
#### `mrb_obj_iv_set`
| iv_tbl Size | Baseline | New | Factor |
|------------:|---------------:|---------------:|-----------:|
| 4 | 88.63003M i/s | 92.60611M i/s | 1.04486x |
| 30 | 32.97066M i/s | 25.25095M i/s | 0.76586x |
| 100 | 16.33224M i/s | 22.74998M i/s | 1.39295x |
| 200 | 5.64484M i/s | 6.79949M i/s | 1.20455x |
#### `mrb_obj_iv_get`
| iv_tbl Size | Baseline | New | Factor |
|------------:|---------------:|---------------:|-----------:|
| 4 | 217.58391M i/s | 237.59912M i/s | 1.09199x |
| 30 | 139.56195M i/s | 160.49470M i/s | 1.14999x |
| 100 | 143.09716M i/s | 190.95047M i/s | 1.33441x |
| 200 | 89.75291M i/s | 134.78717M i/s | 1.50176x |
### Binary Size
Lower value is better.
| File | Baseline | New | Factor |
|:------------|---------------:|---------------:|-----------:|
| mruby | 697,520B | 697,520B | 1.00000x |
| libmruby.a | 1,046,570B | 1,046,682B | 0.99989x |
## Note
The address in `struct RObject::iv` may change after initialization because
`iv_tbl*` points directly to hash buckets. Therefore, the address cannot be
copied and shared when include/prepend. So, when sharing `iv_tbl`, refer to
it via the sharing source class. As a result, the following bug have also
been fixed.
* [An `iv_tbl` is not shared when a class includes or prepends an empty module](https://gist.github.com/shuujii/0ac23fa24b0c55b2c602b534d81e4a95)
We have introduced following new instructions.
* `OP_LAMBDA16`
* `OP_BLOCK16`
* `OP_METHOD16`
* `OP_EXEC16`
Each instruction uses 16 bits operand for `reps` index. Since new
instructions are added, `mruby/c` VM should be updated.
Due to new instructions, dump format compatibility is lost, we have
increased `RITE_BINARY_MAJOR_VER`.
In addition, we have decreased the size of `refcnt` in `mrb_irep` from
`uint32_t` to `uint16_t`, which is reasonably big enough.
Instead of including `mruby/presym.h` everywhere, we provided the
fallback `mruby/presym.inc` under `include/mruby` directory, and specify
`-I<build-dir>/include` before `-I<top-dir>/include` in `presym.rake`.
So even when someone drops `-I<build-dir>/include` in compiler options,
it just compiles without failure.
This enhances self-containment.
- Changed the `mrb_callinfo::pc` field to point to itself.
Previously it indicated the return destination of the previous call level.
`mrb_callinfo::pc` will now hold the address to its own `proc->body.irep->iseq`.
- Removed `mrb_callinfo::err` field.
This is because `mrb_callinfo::pc - 1` is semantically the same as the previous `err`.
- The `pc0` and `pc_save` variables in `mrb_vm_exec()` are no longer needed and have been deleted.
- It removes the argument because `cipush()` doesn't need to save the previous `pc`.
This enhances self-containment.
Previously `mrb_context::stack` had the current call level stack, but now it owns it.
The `mrb_context::stack` field, which is no longer needed, will be removed.
KOBAYASHI Shuji