Re: activelines and 'end'-only lines

[Rici Lake <lua@...>]

Ivan Kolev wrote:
> Hi,
>
> I'm trying to use a line hook and the "activelines" table returned by 
> "debug.getinfo( f, 'L' )" to calculate coverage of functions in unit 
> tests. This table contains most (or maybe all) of the source lines which 
> contain just the keyword 'end'. This implies that there are some VM 
> instructions associated with the end keyword (I know almost nothing 
> about the VM), which is OK. The problem is that whenever there's a 
> 'return' or 'break' statement before 'end', the 'end'-code (or maybe 
> just the line hook) never gets called, which results in many 'end'-only 
> lines falsely appearing as "non-covered".

The 'end' statement is never executed in this case. You can see the
reason by viewing the compiled code with luac -l

Lua doesn't do dead code elimination, and there are a few cases
where dead code will be generated by the compiler. For example,
a function is always terminated with an OP_RETURN, even if the
last statement in the function is a return statement. The OP_RETURN
carries the line number of the 'end' statement, since that's where
it's generated.

You'd get similar issues with constructs like this, I believe:

while true do
  -- something
  break
end

And there are probably other cases. Short of implementing a dead-code
optimization in the code generator, I'm not sure how this code be
addressed.

> I can see two workarounds - one is to modify the source code so that any 
> 'end's after return/break are placed on the same line as the 
> return/break; the other is to have the unit testing framework scan the 
> source files and ignore the non-covered lines which contain 'end' only. 
> I don't like both of them (especially the first). Is there something 
> better that can be done to avoid this problem?

Personally, I'd go for the second option :)