[Truffle] Incorrect frame used for block variable access

[nirvdrum]

This is a complicated example that's been distilled down from a larger problem JRuby+Truffle has running RSpec. What appears to be happening is a variable declared in some ancestor frame is being accessed from the current frame instead. In this case, the frame slot index exceeds the number of locals in the frame and an exception is thrown.

Example:

class X
  def y(new_name)
    yield(:y_name)
  end

  def z(name, opts)
    yield
  end

  def w(category, name, backtrace_line, *args)
    p backtrace_line
  end

  def x(new_name, label = 'default')
    y(new_name) do |name, *args, &block|
      group = z(name, :caller => (the_caller = caller)) do
        w("examples", name, the_caller.first, *args, &block)
      end
    end
  end
end

X.new.x(:x_name)

MRI 2.2.2:

> ruby -v blah.rb 
ruby 2.2.2p95 (2015-04-13 revision 50295) [x86_64-linux]
blah.rb:16: warning: assigned but unused variable - group
"blah.rb:3:in `y'

JRuby+Truffle:

> bin/jruby -X+T -v blah.rb 
jruby 9.0.0.0-SNAPSHOT (2.2.2) 2015-05-28 e0b1489 Java HotSpot(TM) 64-Bit Server VM 25.45-b02 on 1.8.0_45-b14 +jit [linux-amd64]
blah.rb:6:in `z': internal implementation error - IllegalArgumentException The frame slot '[4,the_caller,Object]' is not known by the frame descriptor. com.oracle.truffle.api.impl.DefaultVirtualFrame.getSlotIndexChecked(DefaultVirtualFrame.java:161) (RubyTruffleError)
    from blah.rb:16:in `block in x'
    from blah.rb:3:in `y'
    from blah.rb:15:in `x'
    from blah.rb:23:in `<main>'

[nirvdrum]

This looks like a much simpler case showing the same problem:

def m(arg)
  p arg
  yield
end

def z
  yield
end

z do
  m(something_unique = :arg) do
    p something_unique
  end
end

z {}

In order for this to fail, it needs to be a block-declared variable with a depth > 0 that hasn't been previously seen before. Doing a local variable assignment in a method call seems to be the easiest way to induce this.