On Wed, 7 Jun 2023, 18:17 Jakub Jelinek via Libstdc++, <
libstd...@gcc.gnu.org> wrote:
> Hi!
>
> 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.
>
> Bootstrapped/regtested on x86_64-linux and i686-linux, additionally
> tested on the latter with
> make check RUNTESTFLAGS='--target_board=unix/-fexcess-precision=standard
> conformance.exp=to_chars/double.cc'
> Ok for trunk?
>
Yes, OK.
Thanks for solving this puzzle!
> 2023-06-07 Jakub Jelinek <ja...@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.
>
> --- libstdc++-v3/testsuite/20_util/to_chars/double.cc.jj 2022-11-03
> 22:16:08.542329555 +0100
> +++ libstdc++-v3/testsuite/20_util/to_chars/double.cc 2023-06-07
> 15:41:44.275604870 +0200
> @@ -40,6 +40,7 @@
> #include <cstring>
> #include <limits>
> #include <optional>
> +#include <cfloat>
>
> #include <testsuite_hooks.h>
>
> @@ -1968,9 +1969,19 @@ inline constexpr double_to_chars_testcas
> {1e125, chars_format::fixed,
>
>
> "9999999999999999248677616189928820425446708698348384614392259722252941999757930266031634937628176537515300"
> "5841365553228283904"},
> +#if FLT_EVAL_METHOD >= 0 && FLT_EVAL_METHOD <= 1
> + // When long double is Intel extended and double constants are
> evaluated in precision of
> + // long double, this value is initialized to double(1e126L), which is
> 0x1.7a2ecc414a040p+418 due to
> + // double rounding of 0x1.7a2ecc414a03f7ff6p+418L first to
> 0x1.7a2ecc414a03f800p+418L and
> + // then to 0x1.7a2ecc414a040p+418, while when double constants are
> evaluated in precision of
> + // IEEE double, this is 0x1.7a2ecc414a03fp+418 which the test
> expects. See PR110145.
> {1e126, chars_format::fixed,
>
>
> "9999999999999999248677616189928820425446708698348384614392259722252941999757930266031634937628176537515300"
> "58413655532282839040"},
> +#endif
> + {0x1.7a2ecc414a03fp+418, chars_format::fixed,
> +
>
> "9999999999999999248677616189928820425446708698348384614392259722252941999757930266031634937628176537515300"
> + "58413655532282839040"},
> {1e127, chars_format::fixed,
>
>
> "9999999999999999549291066784979473595300225087383524118479625982517885450291174622154390152298057300868772"
> "377386949310916067328"},
> @@ -2816,8 +2827,12 @@ inline constexpr double_to_chars_testcas
> {0x1.a6c767640cd71p+879, chars_format::scientific,
> "6.6564021122018745e+264"},
>
> // Incorrectly handled by dtoa_milo() (Grisu2), which doesn't achieve
> shortest round-trip.
> +#if FLT_EVAL_METHOD >= 0 && FLT_EVAL_METHOD <= 1
> {4.91e-6, chars_format::scientific, "4.91e-06"},
> {5.547e-6, chars_format::scientific, "5.547e-06"},
> +#endif
> + {0x1.4981285e98e79p-18, chars_format::scientific, "4.91e-06"},
> + {0x1.7440bbff418b9p-18, chars_format::scientific, "5.547e-06"},
>
> // Test hexfloat corner cases.
> {0x1.728p+0, chars_format::hex, "1.728p+0"}, // instead of "2.e5p-1"
> @@ -5537,10 +5552,16 @@ inline constexpr double_to_chars_testcas
> "9."
>
>
> "9999999999999992486776161899288204254467086983483846143922597222529419997579302660316349376281765375153005"
> "841365553228283904e+124"},
> +#if FLT_EVAL_METHOD >= 0 && FLT_EVAL_METHOD <= 1
> {1e+126, chars_format::scientific, 124,
> "9."
>
>
> "9999999999999992486776161899288204254467086983483846143922597222529419997579302660316349376281765375153005"
> "841365553228283904e+125"},
> +#endif
> + {0x1.7a2ecc414a03fp+418, chars_format::scientific, 124,
> + "9."
> +
>
> "9999999999999992486776161899288204254467086983483846143922597222529419997579302660316349376281765375153005"
> + "841365553228283904e+125"},
> {1e+127, chars_format::scientific, 126,
> "9."
>
>
> "9999999999999995492910667849794735953002250873835241184796259825178854502911746221543901522980573008687723"
> @@ -29579,9 +29600,14 @@ inline constexpr double_to_chars_testcas
> {1e+125, chars_format::fixed, 0,
>
>
> "9999999999999999248677616189928820425446708698348384614392259722252941999757930266031634937628176537515300"
> "5841365553228283904"},
> +#if FLT_EVAL_METHOD >= 0 && FLT_EVAL_METHOD <= 1
> {1e+126, chars_format::fixed, 0,
>
>
> "9999999999999999248677616189928820425446708698348384614392259722252941999757930266031634937628176537515300"
> "58413655532282839040"},
> +#endif
> + {0x1.7a2ecc414a03fp+418, chars_format::fixed, 0,
> +
>
> "9999999999999999248677616189928820425446708698348384614392259722252941999757930266031634937628176537515300"
> + "58413655532282839040"},
> {1e+127, chars_format::fixed, 0,
>
>
> "9999999999999999549291066784979473595300225087383524118479625982517885450291174622154390152298057300868772"
> "377386949310916067328"},
>
> Jakub
>
>
