Fix for #1026: Enabling LifetimeEnds CFGElement crashes the compiler during CFG construction

[sulekhark]

The crash happens in some scenarios where a goto statement has to be backpatched with CFGLifetimeEnds elements. During CFG construction, the statements enclosed within a compound statement are iterated through in reverse (i.e., last statement is processed first). In the case where the target of the goto statement occurs earlier than the goto, the goto statement is processed before its target labeled statement is processed. Now, the variables that go out of scope when the goto is executed depend on the position of the target of the goto statement. Therefore, the LifetimeEnds CFGElements for these variables, in the case mentioned above, have to be backpatched into the basic block containing the goto only after the target of the goto has been processed.

The bug is in determining the set of variables that go out of scope, while performing the backpatching. Consider the following block structure:

 {
      <variable A declared>
      {
            <variable B declared>
            label:
               ......
      }
      <variable C declared>
      {
             <variable D declared>
             goto label
      }
      <variable E declared>
 }     

The variables that go out of scope when the goto is executed are: D and C. The above block structure represents the general case that is not handled during backpatching. The cases that are handled are when the scope of the goto target is an ancestor of the scope that contains the goto statement in the tree of scopes within the function.

[mgrang]

Can the distance be less than 0?

[sulekhark]

While Dist cannot be less than 0 if the distance function is called as intended, it is possible to provide arguments to it such that the distance between two LocalScope iterators is less than zero. For example, this can happen if the control comes here for a "forward" jumping goto statement (which should not ideally happen). Moreover, this kind of check is there in two other places too and they have used a <= check (see lines 1770 - 1777).

[mgrang]

LGTM.

[dtarditi]

@sulekhark and I discussed this change. Overall, looks good. I suggested using full variables names instead of abbreviations like PofB. Also, this is not a parent, but an ancestor of the scope.

I thought the comment was somewhat confusing because it doesn't explain is actually happening. We find a common shared scope and end the lifetime of any variables defined in the scopes nested within the shared scope that enclose the goto statement. If you goto a scope A from a scope B that lexically scope A, the shared scope is scope B, so no variable definitions end. I understand the comment is taken from another place in the file; the original comment is confusing.

[dtarditi]

Looks good, thanks.