Re: PATCH for to use tree clobbers for c++/51060 (temporary re-use)

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Sat, Nov 12, 2011 at 4:42 PM, Jason Merrill <jason@redhat.com> wrote:
> Now that we have a way of explicitly marking a variable as dead, we can use
> that to indicate the end of a temporary's lifetime by adding it as a cleanup
> for that temporary. ?Since gimple_push_cleanup still deals in trees I needed
> to tweak a couple of places to avoid trying to treat a clobber as a real
> CONSTRUCTOR, but the changes were small.
>
> One somewhat surprising thing that showed up as a result of this change were
> some failures in the libstdc++ testsuite. ?When I investigated, I found that
> the tests were relying on temporaries living longer than they should:
>
>> ?const int& x = std::max(1, 2);
>
>
> Since std::max takes its arguments by const reference and returns one of
> them, x ends up as a dangling reference to the temporary containing 2, which
> dies at the end of the declaration-statement. ?This patch breaks the
> testcase because now the temporary stack slots get reused by the next
> statement, so by the time we look at x it no longer points to a 2. ?I've
> fixed the tests by removing the references on the variables.
>
> Tested x86_64-pc-linux-gnu, applying to trunk.

This caused:

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53220

-- 
H.J.