[ruby-core:106853] [Ruby master Bug#18440] YJIT is enabled if any YJIT tuning options are set

Issue #18440 has been updated by georgeclaghorn (George Claghorn).


From [the code](https://github.com/ruby/ruby/blob/2c7aed46ba886c6fc4fb15e404035755f9884d54/ruby.c#L1545), this appears to be intentional. [`setup_yjit_options`](https://github.com/ruby/ruby/blob/2c7aed46ba886c6fc4fb15e404035755f9884d54/ruby.c#L1086) returns `true` if any of the `--yjit-*` options are provided:

```c
            else if (strcmp("yjit", s) == 0 || setup_yjit_options(s, &opt->yjit)) {
#if USE_MJIT
                FEATURE_SET(opt->features, FEATURE_BIT(yjit));
#else
                rb_warn("Ruby was built without JIT support");
#endif
            }
```

----------------------------------------
Bug #18440: YJIT is enabled if any YJIT tuning options are set
https://bugs.ruby-lang.org/issues/18440#change-95664

* Author: georgeclaghorn (George Claghorn)
* Status: Open
* Priority: Normal
* ruby -v: ruby 3.1.0p0 (2021-12-25 revision fb4df44d16) [arm64-darwin21]
* Backport: 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN
----------------------------------------
I was testing YJIT in a Rails app with `RUBYOPT="--yjit --yjit-exec-mem-size=32"`. I saw some weird issues around Rails view caching, so I attempted to temporarily disable YJIT by removing `--yjit` and leaving the tuning options in place (`RUBYOPT="--yjit-exec-mem-size=32"`). However, YJIT remained enabled until I also removed `--yjit-exec-mem-size=32`.

```
$ ruby -e 'puts RubyVM::YJIT.runtime_stats.inspect'
nil

$ ruby --yjit-exec-mem-size -e 'puts RubyVM::YJIT.runtime_stats.inspect'
{:inline_code_size=>378495, :outlined_code_size=>311079}
```

I expected removing `--yjit` to disable YJIT and was surprised by this.



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