[Bug tree-optimization/61964] New: [4.8 regression] krb5 database propagation enters infinite loop; reduced test case

[andersk at mit dot edu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61964

            Bug ID: 61964
           Summary: [4.8 regression] krb5 database propagation enters
                    infinite loop; reduced test case
           Product: gcc
           Version: 4.8.3
            Status: UNCONFIRMED
          Severity: major
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: andersk at mit dot edu

Kerberos is miscompiled by gcc-4.8.  The impact is detailed at
https://bugs.launchpad.net/bugs/1347147, but here is a reduced test case.  The
expected return is 0, but when compiled with gcc-4.8 -O2, it returns 1.

$ cat bug.c

struct node { struct node *next, *prev; } node;
struct head { struct node *first; } heads[5];
int k = 2;
struct head *head = &heads[2];

int main()
{
  node.prev = (void *)head;
  head->first = &node;

  struct node *n = head->first;
  struct head *h = &heads[k];

  if (n->prev == (void *)h)
    h->first = n->next;
  else
    n->prev->next = n->next;

  n->next = h->first;
  return n->next == &node;
}

$ gcc-4.7 -Wall -O2 bug.c -o bug; ./bug; echo $?
0
$ gcc-4.8 -Wall -O2 bug.c -o bug; ./bug; echo $?
1
$ gcc-4.9 -Wall -O2 bug.c -o bug; ./bug; echo $?
0
$ dpkg -l gcc-4.7 gcc-4.8 gcc-4.9
[…]
ii  gcc-4.7  4.7.4-2ubuntu1  amd64  GNU C compiler
ii  gcc-4.8  4.8.3-6ubuntu1  amd64  GNU C compiler
ii  gcc-4.9  4.9.1-3ubuntu2  amd64  GNU C compiler

I bisected the point where the problem disappeared between 4.8 and 4.9 at
r202525.  However, I don’t understand why.  I’m scared by the fact that r202525
was intended to fix a “missed-optimization” bug (bug 58404).