[ruby-core:118171] [Ruby master Bug#20522] YJIT can panic if shape transition in `gen_setinstancevariable` emits a performance warning

4 Jun 2024

      Issue #20522 has been updated by byroot (Jean Boussier).

Patch: https://github.com/ruby/ruby/pull/10910

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Bug #20522: YJIT can panic if shape transition in `gen_setinstancevariable` emits a performance warning
https://bugs.ruby-lang.org/issues/20522#change-108616

* Author: byroot (Jean Boussier)
* Status: Open
* ruby -v: ruby 3.3.2 (2024-05-30 revision e5a195edf6) [arm64-darwin23]
* Backport: 3.1: DONTNEED, 3.2: DONTNEED, 3.3: REQUIRED
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Reproduction script:

```ruby
  Warning[:performance] = true

  module StrictWarnings
    def warn(msg, category: nil, **)
      raise msg
    end
  end
  Warning.singleton_class.prepend(StrictWarnings)

  class A
    def compiled_method(is_private)
      @some_ivar = is_private
    end
  end

  100.times do |i|
    klass = Class.new(A)
    7.times do |j|
      obj = klass.new
      obj.instance_variable_set("@base_#{i}", 42)
      obj.instance_variable_set("@ivar_#{j}", 42)
    end
    obj = klass.new
    obj.instance_variable_set("@base_#{i}", 42)
    begin
      obj.compiled_method(true)
    rescue
    end
  end
```

### Context

`compiled_method` contains a `setinstancevariable` instruction, which when compiled by YJIT eagerly create the next shape if it doesn't already exist.

If `Warning.warn` is redefined, then Ruby code may be invoked while YJIT is compiling, which is unsafe.

### Solution

Performance warnings are best effort, so the simplest solution is to not emit warnings when YJIT is compiling. (patch incoming).

-- 
https://bugs.ruby-lang.org/