[Bug fortran/107294] Missed optimization: multiplying real with complex number in Fortran (only)

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107294

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2022-10-17

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
The issue is that the fortran frontend produces

  complex(kind=4) __result_csmul;

  __result_csmul = COMPLEX_EXPR <a, 0.0> * b;
  return __result_csmul;

while the C frontend does

  return COMPLEX_EXPR <SAVE_EXPR <a> * REALPART_EXPR <SAVE_EXPR <b>>, SAVE_EXPR
<a> * IMAGPART_EXPR <SAVE_EXPR <b>>>;

the fortran IL then gets optimized to

  <bb 2> [local count: 1073741824]:
  b$real_6 = REALPART_EXPR <b_4(D)>;
  b$imag_7 = IMAGPART_EXPR <b_4(D)>;
  _8 = a_3(D) * b$real_6;
  _9 = b$imag_7 * 0.0;
  _10 = a_3(D) * b$imag_7;
  _11 = b$real_6 * 0.0;
  _12 = _8 - _9;
  _13 = _10 + _11;
  _2 = COMPLEX_EXPR <_12, _13>;
  return _2;

and here we have to stop because with singed zeros the multiply by zero
is ambiguous and the resulting sign important(?).  With -fno-signed-zeros
the optimization result is the same but really the C frontend
interprets scalar * complex multiplication differently here.

Not sure who is correct, or if both are and we can optimize the multiply
by zero here (maybe in the context of a - b * 0.0 and a + b * 0.0).

It looks like the C17 standard in the G.5.1 annex gives us leeway to
implement multiplication in the pre-simplified form we do.  Somebody
would have to check the Fortran standard how to treat this case.