[Bug middle-end/37850] infinite recursive call to __mulsc3 when multiplying not-constant complexs

[rguenth at gcc dot gnu dot org]

------- Comment #7 from rguenth at gcc dot gnu dot org  2008-10-18 12:43 -------
This is because the implementation of __muldc3 uses

  return x + I * y;

which if built with -O0 results in a call to __muldc3 ... because this
is what complex lowering at -O0 produces:

<bb 2>:
  D.1235 = COMPLEX_EXPR <x, 0.0>;
  D.1236 = COMPLEX_EXPR <y, 0.0>;
  D.1241 = REALPART_EXPR <D.1236>;
  D.1242 = IMAGPART_EXPR <D.1236>;
  D.1237 = __muldc3 (D.1241, D.1242, 0.0, 1.0e+0);
  D.1243 = REALPART_EXPR <D.1235>;
  D.1244 = IMAGPART_EXPR <D.1235>;
  D.1245 = REALPART_EXPR <D.1237>;
  D.1246 = IMAGPART_EXPR <D.1237>;
  D.1247 = D.1243 + D.1245;
  D.1248 = D.1244 + D.1246;
  D.1238 = COMPLEX_EXPR <D.1247, D.1248>;
  <retval> = D.1238;
  return <retval>;

That said - you shouldn't build libgcc itself without optimization.

Still I see a missed optimization in that we do not fold

COMPLEX_EXPR <x, 0.0> + COMPLEX_EXPR <y, 0.0> * __complex__ (0.0, 1.0e+0);

I thought I added this capability.  And I did:

          /* Fold z * +-I to __complex__ (-+__imag z, +-__real z).
             This is not the same for NaNs or if signed zeros are
             involved.  */
          if (!HONOR_NANS (TYPE_MODE (TREE_TYPE (arg0)))
              && !HONOR_SIGNED_ZEROS (TYPE_MODE (TREE_TYPE (arg0)))
              && COMPLEX_FLOAT_TYPE_P (TREE_TYPE (arg0))
              && TREE_CODE (arg1) == COMPLEX_CST
              && real_zerop (TREE_REALPART (arg1)))

The question is whether the standard requires actual complex multiplication
carried out for y * I, as it seems to allow an imaginary type used for I
as well.

A probably better writing of the return statement is

  CTYPE res;
  __real res = x;
  __imag res = y;
  return res;

which should be valid, as the NaN cases are checked earlier.

Joseph?


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jsm28 at gcc dot gnu dot org
           Severity|normal                      |enhancement
             Status|UNCONFIRMED                 |NEW
          Component|c                           |middle-end
     Ever Confirmed|0                           |1
           Keywords|                            |build, missed-optimization
   Last reconfirmed|0000-00-00 00:00:00         |2008-10-18 12:43:35
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37850