[Bug c++/59500] New: Bogus maybe-unintialized warning due to optimizations

[luto at mit dot edu]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59500

            Bug ID: 59500
           Summary: Bogus maybe-unintialized warning due to optimizations
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: luto at mit dot edu

I would have sworn that there was a bug open about this, but I can't find it,
and this bug has been annoying me for years.

Compiling this code with g++ -O2 -Wall:

#ifndef __cplusplus
#include <stdbool.h>
#endif

extern int intval();
extern bool cond();

int main()
{
  bool valid = false;
  int value;

  if (cond()) {
    valid = true;
    value = intval();
  }

  while (!valid || intval() < value)
    ;

  return 0;
}

says:

trivial_uninit_warning.cc: In function ‘int main()’:
trivial_uninit_warning.cc:18:17: warning: ‘value’ may be used uninitialized in
this function [-Wmaybe-uninitialized]
   while (!valid || intval() < value)


Oddly, compiling exactly the same code in C (instead of C++) does not warn.

This seems quite sensitive to exactly what the code is doing.  IIRC the issue
is that the "load" of value is moved ahead of the check of valid by the
optimizer (which is entirely valid), but then that load appears to access an
uninitialized value and warns.