[Bug tree-optimization/39358] New: [4.4 regression] Wrong aliasing warning with lists

["reichelt at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

The following valid code snippet triggers an invalid warning on trunk
when compiled (on i686 or x64) with "-O2 -Wall":

===================================
#include<list>

struct A
{
  virtual ~A();
};

A* foo();

void bar(std::list<int> x)
{
  std::list<int> y = x;
  if (*y.rbegin())
    delete foo();
}
===================================

bug.cc: In function 'void bar(std::list<int, std::allocator<int> >)':
bug.cc:13: warning: dereferencing pointer '<anonymous>' does break
strict-aliasing rules
.../gcc-4.4-20090226/include/c++/4.4.0/bits/stl_list.h:139: note: initialized
from here

The problem appeared (on i686) between 2008-11-20 and 2008-12-19.

To me it looks like something is wrong with alias computation,
although I cannot rule out that there might be something wrong
with our list implementationd instead.

In either case that's serious because the result could be wrong code.


-- 
           Summary: [4.4 regression] Wrong aliasing warning with lists
           Product: gcc
           Version: 4.4.0
            Status: UNCONFIRMED
          Keywords: diagnostic, monitored
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: reichelt at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39358