range-op-float: frange_arithmetic tweaks for MODE_COMPOSITE_P

range-op-float: frange_arithmetic tweaks for MODE_COMPOSITE_P

As mentioned in PR107967, ibm-ldouble-format documents that
+- has 1ulp accuracy, * 2ulps and / 3ulps.
So, even if the result is exact, we need to widen the range a little bit.

The following patch does that.  I just wonder what it means for reverse
division (the op1_range case), which we implement through multiplication,
when division has 3ulps error and multiplication just 2ulps.  In any case,
this format is a mess and for non-default rounding modes can't be trusted
at all, instead of +inf or something close to it it happily computes -inf.

2022-12-08  Jakub Jelinek  <jakub@redhat.com>

	* range-op-float.cc (frange_nextafter): For MODE_COMPOSITE_P from
	denormal or zero, use real_nextafter on DFmode with conversions
	around it.
	(frange_arithmetic): For mode_composite, on top of rounding in the
	right direction accept extra 1ulp error for PLUS/MINUS_EXPR, extra
	2ulps error for MULT_EXPR and extra 3ulps error for RDIV_EXPR.