ZJIT: Fix forwardable crash

[tekknolagi]

There's some local variable becoming false when it should be nil: ruby/forwardable#39

[k0kubun]

Taking a look at this now. In fluentd, it was caused by require "csv". With --zjit-call-threshold=2, I have reduced it to:

class Test
  def self.baz(obj, accessor, method, ali)
    accessor = "" unless Symbol === accessor

    if false
      accessor = nil
    end

    args = nil
    method_call = nil
    if false
      loc = nil
      pre = nil
    end

    eval(<<~RUBY, nil, __FILE__, __LINE__ + 1)
      proc do
        def #{ali}(#{args})
          #{pre}#{accessor}
        end
      end
    RUBY
  end

  def self.bar(accessor, method, ali = method)
    gen = baz(self, accessor, method, ali)

    # If it's not a class or module, it's an instance
    mod = Module === self ? self : singleton_class
    mod.module_eval(&gen)
  end

  def self.foo(accessor, *methods)
    methods.each do |method|
      next if /\A__(?:send|id)__\z/ =~ method
      bar(accessor, method)
    end
  end

  foo :@x, :a, :b, :c=
end

[k0kubun]

def foo
  Object

  if false
    _a = nil
    _b = nil
    _c = nil
    _d = nil
    _e = nil
    _f = nil
    _g = nil
    var = nil
  end

  eval("#{var}@x")
end

def entry
  foo
  eval("binding")
end

entry
entry
entry

(Note: This repro expects a revision before ruby#15459)

[k0kubun]

I think I understand the situation.

Here's a synthetic repro for --zjit-call-threshold=2 (again, this repro expects a revision before ruby#15459):

def callee(setlocal, escape_ep)
  if setlocal
    foo = setlocal
  end
  if escape_ep
    binding
  end
  foo
end

def entry(setlocal, escape_ep)
  callee(setlocal, escape_ep)
end

p entry(true, false)  # profile
p entry(true, true)   # compile with NoEPEscape PatchPoint, spill foo=true, and then invalidate the PatchPoint
p entry(false, false) # side-exit on NoEPEscape PatchPoint, which spills no locals, and let the interpreter read the un-initialized local

A similar issue is still happening on the benchmark, so I'll craft a repro for the latest Ruby master, which re-compiles PatchPoint.

[tekknolagi]

Adding this to 4.0 because it broke rails benchmarks :(

[rwstauner]

I was able to slim down the repro and propose a fix: ruby#15536

[k0kubun]

confirmed that it fixes psych-load, railsbench, and fluentd too. Thanks 🙏