Feature #18349

Updated by k0kubun (Takashi Kokubun) 6 months ago

## Proposal 
 * Rename the current `--jit` to `--mjit`, as well as reverting [Feature #17490] `--mjit` 
 * Rename the current `--yjit` to `--jit` 
   * Alternatively: Leave `--yjit` as well, making it aliased from `--jit`. But I assume it only makes implementation and tests complicated. 

 ## Use case 
 YJIT currently achieves better performance than MJIT in many benchmarks, which means users should choose YJIT over MJIT with Ruby 3.1 in many cases. Even in benchmarks where MJIT could perform well, you need to spend a lot of time to finish warmup and [tune MJIT carefully]( to see the peak performance. 

 However, it's hard for many people, not including heavy users like you reading this, to understand which JIT variant they should try and/or use MJIT properly. Assuming x86 is prevalent enough, I want to make YJIT the default JIT so that non-heavy users will be able to see the benefit of JIT earlier.