Issue #16161 has been updated by wanabe (_ wanabe).


I created Pull-Request https://github.com/ruby/ruby/pull/2528

----------------------------------------
Bug #16161: tailcall_optimization may be disabled after r67315
https://bugs.ruby-lang.org/issues/16161#change-81916

* Author: wanabe (_ wanabe)
* Status: Open
* Priority: Normal
* Assignee: ko1 (Koichi Sasada)
* Target version: 
* ruby -v: ruby 2.7.0dev (2019-09-09T23:18:03Z master 3678c37119) [x86_64-linux]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN
----------------------------------------
Before r67315:

```
$ ./miniruby -v -e 'iseq = RubyVM::InstructionSequence.compile("def foo(n, s = 0);return s if n < 1;foo(n - 1, n + s); end", tailcall_optimization: true); iseq.eval; p foo(900_000)'
ruby 2.7.0dev (2019-03-20 trunk 67314) [x86_64-linux]
405000450000
```

After r67315:

```
$ ./miniruby -v -e 'iseq = RubyVM::InstructionSequence.compile("def foo(n, s = 0);return s if n < 1;foo(n - 1, n + s); end", tailcall_optimization: true); iseq.eval; p foo(900_000)'
ruby 2.7.0dev (2019-03-20 trunk 67315) [x86_64-linux]
Traceback (most recent call last):
	10080: from -e:1:in `<main>'
	10079: from <compiled>:1:in `foo'
	10078: from <compiled>:1:in `foo'
	10077: from <compiled>:1:in `foo'
	10076: from <compiled>:1:in `foo'
	10075: from <compiled>:1:in `foo'
	10074: from <compiled>:1:in `foo'
	10073: from <compiled>:1:in `foo'
	 ... 10068 levels...
	    4: from <compiled>:1:in `foo'
	    3: from <compiled>:1:in `foo'
	    2: from <compiled>:1:in `foo'
	    1: from <compiled>:1:in `foo'
<compiled>:1:in `foo': stack level too deep (SystemStackError)
```

master:

```
$ ./miniruby -v -e 'iseq = RubyVM::InstructionSequence.compile("def foo(n, s = 0);return s if n < 1;foo(n - 1, n + s); end", tailcall_optimization: true); iseq.eval; p foo(900_000)'
ruby 2.7.0dev (2019-09-09T23:18:03Z master 3678c37119) [x86_64-linux]
Traceback (most recent call last):
	10080: from -e:1:in `<main>'
	10079: from <compiled>:1:in `foo'
	10078: from <compiled>:1:in `foo'
	10077: from <compiled>:1:in `foo'
	10076: from <compiled>:1:in `foo'
	10075: from <compiled>:1:in `foo'
	10074: from <compiled>:1:in `foo'
	10073: from <compiled>:1:in `foo'
	 ... 10068 levels...
	    4: from <compiled>:1:in `foo'
	    3: from <compiled>:1:in `foo'
	    2: from <compiled>:1:in `foo'
	    1: from <compiled>:1:in `foo'
<compiled>:1:in `foo': stack level too deep (SystemStackError)
```

I think ruby should not raise SystemStackError with tailcall_optimization, should it?



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