Issue #14830 has been reported by k0kubun (Takashi Kokubun).

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Feature #14830: RubyVM::MJIT.pause / RubyVM::MJIT.resume
https://bugs.ruby-lang.org/issues/14830

* Author: k0kubun (Takashi Kokubun)
* Status: Open
* Priority: Normal
* Assignee: k0kubun (Takashi Kokubun)
* Target version: 2.6
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This ticket is picking only MJIT.pause from https://bugs.ruby-lang.org/issues/14491 and adding MJIT.resume.

# Purpose
* When benchmarking JIT-ed code, I want to get rid of bad impact for performance that comes from ongoing JIT compilation, without waiting for all JIT compilation finish.
* To make it easy to maintain Ruby's test cases that expect there is no child process with --jit. Using --jit makes them fail by existence of gcc or clang process, and I don't want to wait all JIT compilation finish to make it pass.

# Proposals

~~~ ruby
# Pause MJIT compilations but keep JIT-ed code available
RubyVM::MJIT.pause

# Start MJIT compilations again after the MJIT.pause
RubyVM::MJIT.resume
~~~

# Details
## RubyVM::MJIT.pause
* When MJIT is disabled (without --jit), it raises RuntimeError: "MJIT is not enabled".
* When MJIT is enabled,
  * and when MJIT is paused, it does nothing and returns false.
  * and when MJIT is running, it blocks to wait for current JIT finish, pauses MJIT worker and returns true.

## RubyVM::MJIT.resume
* When MJIT is disabled (without --jit), it raises RuntimeError: "MJIT is not enabled".
* When MJIT is enabled,
  * and when MJIT is paused, it starts MJIT worker and returns true. It raises RuntimeError if it fails to resume (very rare).
  * and when MJIT is running, it does nothing and returns false.



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