`mrb_class_find_path` resolves a `char*` pointer to a class name string
by calling `mrb_class_name`. It then allocates a new string with
capacity 40 to copy that `char*` into.
https://github.com/mruby/mruby/blob/e04184185ab43b94980550e850d8813a415fa438/src/variable.c#L1111-L1112
`mrb_class_name` resolves the class name via `class_name_str`, which
returns an `mrb_value` with type tag `MRB_TT_STRING` and backed by an
`RString*`. Then `mrb_class_name` extracts the `RSTRING_PTR`:
https://github.com/mruby/mruby/blob/e04184185ab43b94980550e850d8813a415fa438/src/class.c#L2133-L2134
That `RString*`-backed `mrb_value` ultimately comes from `mrb_class_path`
which resolves the string from the symbol table:
https://github.com/mruby/mruby/blob/e04184185ab43b94980550e850d8813a415fa438/src/class.c#L2111
The allocation of the target `str` after resolving the class name
`mrb_value` and extracting its pointer is fragile and assumes the
`RString*` is "static". If the `RString*` is not static, the
interleaving of extracting the `RSTRING_PTR` followed by a subsequent
allocation might result in the class name `mrb_value` being garbage
collected, which will leave the extracted pointer invalid.
Fix this bad interleaving by allocating the destination string first
before taking a raw pointer to an `RString*`.
Instead of splitting mp_limb by bit operations (using HIGH/LOW macros),
now we use mp_limb2 (which is bigger integer size, e.g uint64_t). It
makes operations (especially mulitiplication) a lot faster. As a side
effect, it also reduces memory consumption (16,452,033 -> 15,545,853 on
my Linux machine).
Other changes:
- trailing zeros are removed after operations.
- division algorithm is simplified.
`OP_SEND R4 :allocate 0` requires an an invisible `nil` block to `R5`.
If `R5` is not allocated, this can lead to unexpected results due to buffer overflow.
This problem is caused by #5565.
ref. commit 33792c2a02
If the `Errno::EXXX` class is defined minimally when needed, it will save about 17 KiB of RAM in a 32-bit environment.
Define the singular methods `.const_defined?`, `.const_missing` and `.constants` in the `Errno` module and act as if there are classes that have not yet been defined.
However, if real classes are enumerated, for example by the following code, the effect is lost.
```ruby
Errno.constants.map { |id| Errno.const_get(id) }
```
But I believe that such codes are rarely written with the intention of being written.
In this patch, the `Errno::EXXX#initialize` method is no longer defined for further memory reduction.
Instead, it has been merged into the `SystemCallError#initialize` method.
Yukihiro "Matz" Matsumoto