[ruby-core:90437] [Ruby trunk Bug#15303] Return tracepoint doesn't fire when tailcall optimization is applied

[ko1@atdot.net]

Issue #15303 has been updated by ko1 (Koichi Sasada).


Matz decided to remove tailcall opt from core libraries.
r66349 was committed by nobu.


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Bug #15303: Return tracepoint doesn't fire when tailcall optimization is applied
https://bugs.ruby-lang.org/issues/15303#change-75594

* Author: alanwu (Alan Wu)
* Status: Closed
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: ruby 2.6.0dev (2018-11-14 trunk 65727) [x86_64-darwin17]
* Backport: 2.3: UNKNOWN, 2.4: UNKNOWN, 2.5: UNKNOWN
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After a tailcall, the "return" tracepoint event is only fired once. Normally, after a call at the end of a method, the return event is fired twice, once for the callee returning and once for the caller returning.
The following script outputs 
~~~
:call
:call
:call
:return
~~~

~~~ ruby
method_source = <<-RB
def toy(n)
   return if n == 2
   toy(n+1)
end
RB

iseq = RubyVM::InstructionSequence.compile(method_source, tailcall_optimization: true)
#puts iseq.disasm
iseq.eval

trace = TracePoint.new(:call, :return) do |tp|
    p tp.event
end

trace.enable
toy(0)
~~~

The "return" event behaves more like a "stack frame pop" event currently. I don't think it's feasible/desirable to have the same behavior when TCO is applied, but it would be nice if there was some way for the tracepoint to know a tail call is going to happen.
I'm raising this issue because the popular debugger "byebug" relies on these events to track execution in various stack frames. https://github.com/deivid-rodriguez/byebug/issues/481
Forwardable explicitly uses TCO which triggers this issue.


---Files--------------------------------
no-tco-no-problem.patch (520 Bytes)


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