[Bug libgcc/78067] New: liggcc2 calls count_leading_zero with 0

[bernd.edlinger at hotmail dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78067

            Bug ID: 78067
           Summary: liggcc2 calls count_leading_zero with 0
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libgcc
          Assignee: unassigned at gcc dot gnu.org
          Reporter: bernd.edlinger at hotmail dot de
  Target Milestone: ---

in libgcc2.c, __floatdidf, line 1637 I see:

  /* If there are no high bits set, fall back to one conversion.  */
  if ((Wtype)u == u)
    return (FSTYPE)(Wtype)u;

  /* Otherwise, find the power of two.  */
  Wtype hi = u >> W_TYPE_SIZE;
  if (hi < 0)
    hi = -(UWtype) hi;

  UWtype count, shift;
  count_leading_zeros (count, hi);

If u is 1 << W_TYPE_SIZE - 1
hi will be 0
and count_leading_zeros will be called with 0.

In that case, see longling.h:

   5) count_leading_zeros(count, x) counts the number of zero-bits from the
   msb to the first nonzero bit in the UWtype X.  This is the number of
   steps X needs to be shifted left to set the msb.  Undefined for X == 0,
   unless the symbol COUNT_LEADING_ZEROS_0 is defined to some value.

we should check that COUNT_LEADING_ZEROS_0 is defined and identical
to W_TYPE_SIZE.

like this:

  UWtype count, shift;
#if defined (COUNT_LEADING_ZEROS_0) && COUNT_LEADING_ZEROS_0 == W_TYPE_SIZE
  if (hi == 0)
    count = W_TYPE_SIZE;
  else
#endif
  count_leading_zeros (count, hi);