[PATCH 23/25] math: Use erfc from CORE-MATH

[DJ Delorie]

Adhemerval Zanella <adhemerval.zanella@linaro.org> writes:
> The CORE-MATH implementation is correctly rounded for any rounding mode.
> The code was adapted to glibc style and to use the definition of
> math_config.h (to handle errno, overflow, and underflow).
>
> Benchtest on x64_64 (Ryzen 9 5900X, gcc 14.2.1), aarch64 (Neoverse-N1,
> gcc 13.3.1), and powerpc (POWER10, gcc 13.2.1) shows:
>
> reciprocal-throughput        master        patched   improvement
> x86_64                      49.0980       267.0660      -443.94%
> x86_64v2                    49.3220       257.6310      -422.34%
> x86_64v3                    42.9539        84.9571       -97.79%
> aarch64                     28.7266        52.9096       -84.18%
> power10                     14.1673        25.1273       -77.36%
>
> Latency                      master        patched   improvement
> x86_64                      95.6640       269.7060      -181.93%
> x86_64v2                    95.8296       260.4860      -171.82%
> x86_64v3                    91.1658       112.7150       -23.64%
> aarch64                     37.0745        58.6791       -58.27%
> power10                     23.3197        31.5737       -35.39%

These are much worse than usual... Do we *want* to slow down this
function that much, just for a few ULPs?

> +  for (int i = 0; i < 22; i++)
> +    if (x == exceptions[i][0])
> +      return exceptions[i][1] + exceptions[i][2];

IMHO the need for this code implies that all your other algorithms are
faulty, if you need to fix their errors...

The exception table could have been sorted, too.

> +  /* subnormal exceptions */
> +  if (x == 0x1.a8f7bfbd15495p+4)
> +    return fma (0x1p-1074, -0.25, 0x1.99ef5883f656cp-1024);

Could have been part of the exceptions table?