[Bug debug/63284] New: [4.9/5 Regression] -fcompare-debug issue due to redirection to __builtin_unreachable ()

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63284

            Bug ID: 63284
           Summary: [4.9/5 Regression] -fcompare-debug issue due to
                    redirection to __builtin_unreachable ()
           Product: gcc
           Version: 4.9.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: debug
          Assignee: jakub at gcc dot gnu.org
          Reporter: jakub at gcc dot gnu.org

/* { dg-do compile } */
/* { dg-options "-O2 -fcompare-debug" } */

int a[10], *b, *d, c, f;
int fn2 (void);
void fn3 (void);
void fn4 (int);

static int
fn1 (int x)
{
  int e = a[0];
  if (e)
    return 1;
  if (b)
    switch (x)
      {
      case 1:
        if (d)
          e = fn2 ();
        else
          fn3 ();
        break;
      case 0:
        if (d)
          {
            fn3 ();
            if (c)
              fn4 (1);
          }
        else
          fn4 (0);
      }
  return e;
}

void
fn6 (void)
{
  f = fn1 (0);
}

fails on the 4.9 branch, it happens to succeed on the trunk, but when one looks
at *.optimized dumps differences, there are still undesirable differences in
the order of predecessors of one bb (and thus also PHI argument order).