[PATCH] PR19864: Revert r209764 in favor of the fix in r215766 (loop backedge jump attributed to confusing location when the last source line of a loop is inside a conditional)

[David Blaikie]

On Mon, Aug 18, 2014 at 2:33 PM, Adrian Prantl <aprantl at apple.com> wrote:
> IMO, the best we can do is to use the end of the CompoundStmt as the
cleanup location if there is one, and otherwise give up and use line 0.

OK. The "let's do better when there's braces" isn't something I'm worrying
about for now - it's a possible incremental improvement after we figure out
the "worst case" behavior.

>
> ```
> if (c)
>   foo(); // cleanups = line 0
>
> if (c) {
> }         // cleanups
>
> if (c)
>   foo();
> else {
>   bar();
> }       // cleanups

In this case, I'm not sure the close } to the else is any better than the
last line of the else, it still feels like part of the else, not the end of
the if scope.

Anyway, using line zero (did this by hand and used no-integrated-as because
Clang doesn't cope well with emitting line table entries for line 0) here's
GDB's behavior in some examples:

GCC is GCC 4.8
ToT is Clang ToT
End is this patch applied to Clang ToT to use the end of the if as the
location
Zer is the end patch applied, and then manually editing the assembly (and
assembling with the system assembler, not the integrated assembler) to have
the non-exceptional dtor call be on line 0.

So we get the totally right behavior from GDB the same as we would by
assigning it to the end. In the cases where we get different behavior, it's
better - we don't step to lines that don't execute, but we still get a bad
location walking up from the dtor call (end up wherever the preceeding code
was - which means sometimes we end up on the last line of the function
where the EH cleanup is ascribed to). Seems better, but not perfect. (but
we have no way to get perfect)

1:   for
2:     if
3:       func();
4: }

GCC: 1 2 3 1 2 1 2 3 1 4
ToT: 1 2 3 2 1 2 2 1 2 3 2 1 4
End: 1 2 3 3 1 2 3 1 2 3 3 1 4
Zer: 1 2 3 1 2 1 2 3 1 4, dtor skipped and ascribed to 4

1:   for
2:     if
3:       func();
4:     else
5:       func();
6: }

GCC: 1 2 3 1 2 5 1 2 3 1 6
ToT: 1 2 3 2 1 2 5 2 2 3 2 1 6
End: 1 2 3 1 2 5 1 2 3 1 6
Zer: 1 2 3 1 2 5 1 2 3 1 6, dtor skipped and ascribed to 5

1:   for
2:     if
3:       ;
4:     else
5:       ;
6: }

GCC: 1 2 5 1 2 5 1 2 5 1 6
ToT: 1 2 2 1 2 2 1 2 2 1 6
End: 1 2 5 1 2 5 1 2 5 1 6
Zer: 1 2 1 2 1 2 1 6, dtor skipped and ascribed to 6

1:   for
2:     if
3:       ;
4: }

GCC: 1 2 3 1 2 3 1 2 3 1 4
ToT: 1 2 2 1 2 2 1 2 2 1 4
End: 1 2 3 1 2 3 1 2 3 1 4
Zer: 1 2 1 2 1 2 1 4, dtor skipped and ascribed to 4

> ```
>
> http://reviews.llvm.org/D4956
>
>
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