[ruby-core:109983] [Ruby master Bug#18886] Struct aref and aset don't trigger any tracepoints.

["ioquatix (Samuel Williams)" <noreply@ruby-lang.org>]

Issue #18886 has been updated by ioquatix (Samuel Williams).


Rather than using branch for tracepoints, using a `nop/jmp` instruction which is set or cleared might be a lower impact option.

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Bug #18886: Struct aref and aset don't trigger any tracepoints.
https://bugs.ruby-lang.org/issues/18886#change-99232

* Author: ioquatix (Samuel Williams)
* Status: Open
* Priority: Normal
* Assignee: ko1 (Koichi Sasada)
* Backport: 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN
----------------------------------------
Given the following program, `thing.name` and `thing.shape` don't trigger `c_call` trace points (or any trace points actually).

```ruby
pp RUBY_VERSION

trace_point = TracePoint.new(:line, :call, :c_call, :a_call) do |trace|
  puts trace.event

  if trace.event == :call
    # Ruby doesn't always mark call-sites in sub-expressions, so we use this approach to compute a call site and mark it:
    if location = caller_locations(2, 1).first and path = location.path
      puts "> #{path}:#{location.lineno}:#{trace.event}"
    end
  end
  
  if path = trace.path
    puts "= #{path}:#{trace.lineno}:#{trace.event}"
  end
end

trace_point.enable

# This will trigger call trace points
class Thing
  def name
    :cat
  end
  
  def shape
    :square
  end
end
thing = Thing.new

# Thing = Struct.new(:name, :shape)
# thing = Thing.new(:cat, :rectangle)

[
  name: thing.name,
  shape: thing.shape,
]
```

## Current HEAD

```
= ../test.rb:30:line:
= ../test.rb:30:c_call:new
= ../test.rb:30:c_call:inherited
= ../test.rb:30:c_call:singleton_method_added
= ../test.rb:30:c_call:singleton_method_added
= ../test.rb:30:c_call:singleton_method_added
= ../test.rb:30:c_call:singleton_method_added
= ../test.rb:30:c_call:singleton_method_added
= ../test.rb:30:c_call:method_added
= ../test.rb:30:c_call:method_added
= ../test.rb:30:c_call:method_added
= ../test.rb:30:c_call:method_added
= ../test.rb:30:c_call:const_added
= ../test.rb:31:line:
= ../test.rb:31:c_call:new
= ../test.rb:31:c_call:initialize
= ../test.rb:34:line:
```

## Proposed PR

```
= ../test.rb:30:line:
= ../test.rb:30:c_call:new
= ../test.rb:30:c_call:inherited
= ../test.rb:30:c_call:singleton_method_added
= ../test.rb:30:c_call:singleton_method_added
= ../test.rb:30:c_call:singleton_method_added
= ../test.rb:30:c_call:singleton_method_added
= ../test.rb:30:c_call:singleton_method_added
= ../test.rb:30:c_call:method_added
= ../test.rb:30:c_call:method_added
= ../test.rb:30:c_call:method_added
= ../test.rb:30:c_call:method_added
= ../test.rb:30:c_call:const_added
= ../test.rb:31:line:
= ../test.rb:31:c_call:new
= ../test.rb:31:c_call:initialize
= ../test.rb:34:line:
= ../test.rb:34:c_call:name
= ../test.rb:35:c_call:shape
```

The reason is the internal implementation of struct doesn't have trace point instrumentation in `vm_call_opt_struct_aset` or `vm_call_opt_struct_aref`.

Proposed fix: https://github.com/ruby/ruby/pull/6071 but this would need a review, maybe @jeremyevans0 and @ko1 can help.



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