[ruby-core:96090] [Ruby master Bug#15980] Coverage shows while/until after raise if/unless as uncovered line

Issue #15980 has been updated by mame (Yusuke Endoh).


Oops, I overlooked this issue.  Sorry.


This is a tough problem.  The compiler produces the following code:

```
0006 branchunless 13
...
0013 jump 17         (Line 2)
...
0017 putnil          (Line 3)
```

The peephole optimizer changes it to:

```
0006 branchunless 17
...
0013 jump 17         (Line 2)
...
0017 putnil          (Line 3)
```

Now "jump 17" instruction is unreachable, so Line 2 event is never fired.  But coverage measurement doesn't know it, so it initializes zero entry for Line 2 in coverage.


I think of some options to fix:

1. Stop the peephole optimization (in some cases); it reduces performance even when coverage is not used, so I don't want to do this.
2. Remove unreachable instructions; it requires revamp of the compiler.
3. Analyze unreachable instructions and suppress coverage entry; it is relatively easy, but still requires unreachable code analysis.


@ko1 Do you have any idea?

----------------------------------------
Bug #15980: Coverage shows while/until after raise if/unless as uncovered line
https://bugs.ruby-lang.org/issues/15980#change-82946

* Author: jeremyevans0 (Jeremy Evans)
* Status: Assigned
* Priority: Normal
* Assignee: mame (Yusuke Endoh)
* Target version: 
* ruby -v: ruby 2.7.0dev (2019-07-03) [x86_64-openbsd6.5]
* Backport: 2.4: UNKNOWN, 2.5: UNKNOWN, 2.6: REQUIRED
----------------------------------------
The following code shows line 2 (`while true`) as uncovered:

```ruby
raise if 1 == 2
while true
  break
end
```

Coverage reports the following for this file: `[1, 0, 1, nil]`.  Note that `true` isn't important, any `while` condition will work.  However, if you change line 1 to `raise if false`, line 1 shows `nil` coverage, and line 2 shows as covered (`[nil, 1, 1, nil]`).  That leads me to believe this issue is related to the optimizer.

I bisected this to commit:100bf2757468439106775a7d95a791a8c10b874a, which certainly appears related.

This is not a theoretical case, it affected three lines in Sequel.  While not a major problem, I do think a fix should be backported to 2.6.

Note that this only affects line coverage.  Branch coverage shows:

```ruby
{"file.rb"=>
  {:branches=>
    {[:if, 0, 1, 0, 1, 15]=>
      {[:then, 1, 1, 0, 1, 5]=>0, [:else, 2, 1, 0, 1, 15]=>1},
     [:while, 3, 2, 0, 4, 3]=>{[:body, 4, 3, 2, 3, 7]=>1}}}}
```

If you run with both branch and line coverage, line coverage shows correctly.

This affects `while`/`until` after a line with `raise ... if ...` or `raise ... unless ...`.  If you switch to `if ...; raise ...; end`, then line coverage shows correctly.



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