libstdc++: Fix up 20_util/to_chars/double.cc test for excess precision [PR110145]

This test apparently contains 3 problematic floating point constants,
1e126, 4.91e-6 and 5.547e-6.  These constants suffer from double rounding
when -fexcess-precision=standard evaluates double constants in the precision
of Intel extended 80-bit long double.
As written in the PR, e.g. the first one is
0x1.7a2ecc414a03f7ff6ca1cb527787b130a97d51e51202365p+418
in the precision of GCC's internal format, 80-bit long double has
63-bit precision, so the above constant rounded to long double is
0x1.7a2ecc414a03f800p+418L
(the least significant bit in the 0 before p isn't there already).
0x1.7a2ecc414a03f800p+418L rounded to IEEE double is
0x1.7a2ecc414a040p+418.
Now, if excess precision doesn't happen and we round the GCC's internal
format number directly to double, it is
0x1.7a2ecc414a03fp+418 and that is the number the test expects.
One can see it on x86-64 (where excess precision to long double doesn't
happen) where double(1e126L) != 1e126.
The other two constants suffer from the same problem.

The following patch tweaks the testcase, such that those problematic
constants are used only if FLT_EVAL_METHOD is 0 or 1 (i.e. when we have
guarantee the constants will be evaluated in double precision),
plus adds corresponding tests with hexadecimal constants which don't
suffer from this excess precision problem, they are exact in double
and long double can hold all double values.

2023-06-07  Jakub Jelinek  <jakub@redhat.com>

	PR libstdc++/110145
	* testsuite/20_util/to_chars/double.cc: Include <cfloat>.
	(double_to_chars_test_cases,
	double_scientific_precision_to_chars_test_cases_2,
	double_fixed_precision_to_chars_test_cases_2): #if out 1e126, 4.91e-6
	and 5.547e-6 tests if FLT_EVAL_METHOD is negative or larger than 1.
	Add unconditional tests with corresponding double constants
	0x1.7a2ecc414a03fp+418, 0x1.4981285e98e79p-18 and
	0x1.7440bbff418b9p-18.

0 files changed, 0 insertions, 0 deletions