I noticed the following 2 tests are failing with -O1
-D_FORTIFY_SOURCE=1:

[   44s] FAILED: whitespace-1.out: unifdef -DFOO whitespace.c
[   44s] FAILED: whitespace-2.out: unifdef -DBAR whitespace.c

It's caused by fact that:

fgets returns '\0' if n == 1:

char *
_IO_fgets (char *buf, int n, FILE *fp)
{
  size_t count;
  char *result;
  int old_error;
  CHECK_FILE (fp, NULL);
  if (n <= 0)
    return NULL;
  if (__glibc_unlikely (n == 1))
    {
      /* Another irregular case: since we have to store a NUL byte and
	 there is only room for exactly one byte, we don't have to
	 read anything.  */
      buf[0] = '\0';
      return buf;
    }