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Feature #19780

Updated by k0kubun (Takashi Kokubun) over 1 year ago

## Proposal 
 Remove `tailcall_optimization` support from `RubyVM::InstructionSequence` (make it no-op) no-op and print a warning) 

 ## Motivation 
 The consensus at [Feature #6602] seems to be that we're not going to enable tailcall optimization by default until we introduce syntax for it. Until this syntax is introduced, this feature is useless. Can we remove this flag until we do support that future syntax? 

 ## Background 
 This script crashes with `--yjit-call-threshold=1` on any YJIT-enabled build. 

 ```rb 
 src = <<-EOS 
   def apply_one_and_two(&block) 
     [1, p(1)] 
   end 
 ​ 
   def add_one_and_two 
     apply_one_and_two(&:+) 
   end 
 EOS 
 ​ 
 RubyVM::InstructionSequence.new( 
   "proc {|_|_.class_eval {#{src}}}", 
   __FILE__, __FILE__, 1, 
   tailcall_optimization: true, 
   trace_instruction: false, 
 ).eval[self.singleton_class] 
 ​ 
 def entry 
   add_one_and_two 
 end 
 ​ 
 entry 
 ``` 

 I already wasted a lot of time just for noticing it's already broken. I don't want to waste time for users that do not exist yet. 

 It's possible to fix, but you need to add some code that needs to be executed even if you don't enable it unless you add another flag to iseqs (which will consume more memory). The complexity just doesn't seem worth it. Removing tailcall support would simplify the implementation and problems we need to deal with.

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