The following testcase is miscompiled e.g. with -O2 -march=zEC12 -mtune=z13 on s390x-linux:

__attribute__((noipa)) void
bar (char *p)
{
  int i;
  for (i = 0; i < 6; i++)
    if (p[i] != "foobar"[i])
      __builtin_abort ();
  for (; i < 32; i++)
    if (p[i] != '\0')
      __builtin_abort ();
}

__attribute__((noipa)) void
foo (unsigned int x)
{
  char s[32] = { 'f', 'o', 'o', 'b', 'a', 'r', 0 };
  ((unsigned int *) s)[2] = __builtin_bswap32 (x);
  bar (s);
}

int
main ()
{
  foo (0);
  return 0;
}

The problem is that since that change we emit a store_by_pieces (8 bytes) followed by clear_storage (24 bytes), but the object we pass to the latter actually has S1, so DSE2 then happily removes it when it sees a further store of 4 bytes to s+8.