Re: [PATCH] [PR24021] Implement PLUS_EXPR range-op entry for floats.

[Jakub Jelinek <jakub@redhat.com>]

On Fri, Nov 04, 2022 at 08:14:23PM +0100, Jakub Jelinek via Gcc-patches wrote:
> One thing that is clear is that real_isdenormal is never true for these
> (but then a question is if flush_denormals_to_zero actually works).

So, after some more investigation, I think that is actually the case,
real_isdenormal is only meaningful (will ever return true) right after
round_for_format.
The uses inside of real.cc are fine, real_to_target first calls
round_for_format and then calls fmt->encode in which the real_isdenormal
calls are done.  And, round_for_format is what transforms the
normalized way of expressing the number into the denormal form:
  /* Check the range of the exponent.  If we're out of range,
     either underflow or overflow.  */
  if (REAL_EXP (r) > emax2)
    goto overflow;
  else if (REAL_EXP (r) <= emin2m1)
    {
      int diff;

      if (!fmt->has_denorm)
        {
          /* Don't underflow completely until we've had a chance to round.  */
          if (REAL_EXP (r) < emin2m1)
            goto underflow;
        }
      else
        {
          diff = emin2m1 - REAL_EXP (r) + 1;
          if (diff > p2)
            goto underflow;

          /* De-normalize the significand.  */
          r->sig[0] |= sticky_rshift_significand (r, r, diff);
          SET_REAL_EXP (r, REAL_EXP (r) + diff);
        }
    }
But, real_to_target is one of the 4 callers of static round_for_format,
another one is real_convert, but that one undoes that immediately:
  round_for_format (fmt, r);
        
  /* Make resulting NaN value to be qNaN. The caller has the
     responsibility to avoid the operation if flag_signaling_nans
     is on.  */
  if (r->cl == rvc_nan)
    r->signalling = 0;
      
  /* round_for_format de-normalizes denormals.  Undo just that part.  */
  if (r->cl == rvc_normal)
    normalize (r);
and the last two are inside of encoding the IBM double double composite.
So, I think everywhere in the frange you'll see normalized REAL_VALUE_TYPE
and so real_isdenormal will always be false there.
You need to check for REAL_EXP (r) < fmt->emin or so (and ideally only on
REAL_VALUE_TYPE already real_converted to the right mode (your
frange_arithmetics does that already, which is good, but say conversions
and other unary ops might need it too).
If I in the hack from last mail replace
-	  fprintf (stderr, "%d %d %s %s\n", i, real_isdenormal (&result), buf, buf2);
+	  fprintf (stderr, "%d %d %s %s\n", i, REAL_EXP (&result) < REAL_MODE_FORMAT (mode)->emin, buf, buf2);
then it is 1 until:
102 1 0x0.8p-971 0x0.80000000000008p-971
103 1 0x0.8p-970 0x0.80000000000008p-970
104 1 0x0.8p-969 0x0.80000000000008p-969
105 0 0x0.8p-968 0x0.80000000000008p-968
106 0 0x0.8p-967 0x0.80000000000008p-967
107 0 0x0.8p-966 0x0.80000000000008p-966

Now, the __LDBL_MIN__ powerpc64 gcc announces is
2.00416836000897277799610805135016205e-292L
which is equivalent to:
0x1p-969
and that is equivalent to
0x0.8p-968, so I think that is exactly what we want.

	Jakub