[Bug tree-optimization/19789] New: tree optimizers do not know that constant global variables do not change

["kazu at cs dot umass dot edu" <gcc-bugzilla at gcc dot gnu dot org>]

Consider:

void abort (void) __attribute__ ((noreturn));

union node
{
  int dummy;
  union node *ptr;
};

extern void bar (union node *tree);

extern const int global_constant;

void
foo (union node *p)
{
  if (global_constant == 1)
    abort ();

  bar (p->ptr);

  if (global_constant == 1)
    abort ();
}

Note that if we get to the second "if", we know that global_constant != 1,
so we could remove the second "if".

The tree optimizers do not take this opportunity, but the RTL optimizers do.

-- 
           Summary: tree optimizers do not know that constant global
                    variables do not change
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Keywords: missed-optimization, TREE
          Severity: enhancement
          Priority: P2
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: kazu at cs dot umass dot edu
                CC: gcc-bugs at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19789