[Bug tree-optimization/30708] New: [4.1 Regression] Aliasing miscompilation

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

struct obj_symbol
{
 struct obj_symbol *next;
 unsigned int size;
};

struct common_entry
{
 struct common_entry *next;
 struct obj_symbol *sym;
};

extern void do_something(struct obj_symbol *sym);

void obj_allocate_commons(struct obj_symbol *sym)
{
 struct common_entry *common_head = ((void *)0);

 for (; sym ; sym = sym->next) {
  struct common_entry **p, *n;
  for (p = &common_head; *p ; p = &(*p)->next)
   if (sym->size <= (*p)->sym->size)
    break;

  n = __builtin_alloca (sizeof(*n));
  n->next = *p;
  n->sym = sym;
  *p = n;
 }

 {
  struct common_entry *p;
  for (p = common_head; p ; p = p->next)
   do_something(p->sym);
 }
}

is miscompiled on gcc-4_1-branch at least on x84_64-linux, with -O2 and above
(but not with -fno-strict-aliasing).
The common_head = NULL initialization is removed as dead during 1st DCE pass,
but from quick skimming the reason for that seems to be incorrect aliasing
information.


-- 
           Summary: [4.1 Regression] Aliasing miscompilation
           Product: gcc
           Version: 4.1.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: jakub at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30708