[Bug middle-end/61118] Spurious -Wclobbered warning generated by gcc 4.9.0 for pthread_cleanup_push

[eggert at gnu dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61118

Paul Eggert <eggert at gnu dot org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |eggert at gnu dot org

--- Comment #12 from Paul Eggert <eggert at gnu dot org> ---
(In reply to Yuri Gribov from comment #11)
> (In reply to Tavian Barnes from comment #10)
> > > I think it is - __cancel_arg is assigned inside a while loop
> > 
> > Specifically a do { } while(0); loop, which obviously has only one iteration.
> 
> Actually I was talking about surrounding while
> ((double)future->progress/future->total < progress)...

The variables in question do not survive from one iteration to the next of the
surrounding while loop, so they cannot contribute to a setjmp/longjmp problem.
The code looks like this:

  while ((double)future->progress/future->total < progress) {
    ...
    void (*__cancel_routine) (void *) = (cleanup_fn);
    void *__cancel_arg = (&future->mutex);
    if (__sigsetjmp (((struct __jmp_buf_tag *) (void *)
                      __cancel_buf.__cancel_jmp_buf),
                     0)) {
      __cancel_routine (__cancel_arg);
      ...
    }
    ...
  }

As the addresses of the locals do not escape and they are never assigned to
after initialization and they do not survive until the next call to
__sigsetjmp, the warnings are false alarms.

Possibly GCC is hoisting the locals out of the loop, incorrectly transforming
the above code into this:

  void (*__cancel_routine) (void *);
  void *__cancel_arg;
  while ((double)future->progress/future->total < progress) {
    ...
    __cancel_routine = (cleanup_fn);
    __cancel_arg = (&future->mutex);
    if (__sigsetjmp (((struct __jmp_buf_tag *) (void *)
                      __cancel_buf.__cancel_jmp_buf),
                     0)) {
      __cancel_routine (__cancel_arg);
      ...
    }
    ...
  }

where the warning would be valid.

Also see Bug 48968 which has similar symptoms.