Misc #20814: iseq optimizations on constant condition

• Status: Feedback
• Assignee: -

[hurricup (Alexandr Evstigneev)]

ruby 3.4-preview2 seems optimizes out some stuff when building iseq and this may lead to user unexpected behavior when debugging, because some lines just optimized out.

Example:

def foo1
  if true
    nil
  else
    1
  end
end

produces:

== disasm: #<ISeq:foo1@/test.rb:1 (1,0)-(7,3)>
0000 putnil                                                           (   3)[LiCa]
0001 leave                                                            (   7)[Re]

Two questions:

• is there way to disable such optimizations?
• are there some specs on things that can be optimized.

[nobu (Nobuyoshi Nakada)]

• Status changed from Open to Feedback

Because of the dynamic feature of Ruby, the optimization on ISeq is very limited.
For instance, elimination of unreachable branch because of a simple always-true/false condition expression, in your example.

• is there way to disable such optimizations?

To disable optimizations, see the document of RubyVM::InstructionSequence.compile_option=.
https://docs.ruby-lang.org/en/master/RubyVM/InstructionSequence.html#method-c-compile_option-3D

• are there some specs on things that can be optimized.

We do not provide explicit documentation or specifications regarding optimizations because they are internal features and may change without notice.

[nobu (Nobuyoshi Nakada)]

• Description updated (diff)

[hurricup (Alexandr Evstigneev)]

nobu (Nobuyoshi Nakada) wrote in #note-1:

To disable optimizations, see the document of RubyVM::InstructionSequence.compile_option=.
https://docs.ruby-lang.org/en/master/RubyVM/InstructionSequence.html#method-c-compile_option-3D

My question is more about the runtime. E.g I want to run a ruby program with optimizations disabled at all. Or load using rb_load/rb_load_protect while ruby process is already alive with default settings.