Don't rely on errno values, coming from platform's math functions

提案とリンクされたDiscourseの議論から始め、その後 Modules/cmathmodule.c、Objects/floatobject.c、Include/internal/pycore_pymath.h を調べてください。既存の errno 処理と調整ヘルパーを、リンクされた platform-libm のバグ報告と比較してください。完了の条件は、影響を受ける算術処理全体で浮動小数点エラーを一貫してテスト可能な、方針が決定されたアプローチがあることです。

Feature or enhancement

Proposal:

In the cmath module most functions don't check errno values. See cosh() example below, which set nonzero errno in some cases, but don't read errno (e.g. to detect overflow in libm's cosh/sinh()). Here we essentially assume, that platform implements Annex F of the C standard:
https://github.com/python/cpython/blob/04242c027feeff726acb15b6463422897b489bcf/Modules/cmathmodule.c#L441-L490

What if we extend this approach to the rest of CPython's floating-point arithmetics? Now this seems natural, as we require that platforms C double has IEEE 754 binary64 format. On practice, that means much more, i.e. that platform supports Annex F (modulo bugs). And we can utilize that, assuming that for invalid input - NaN's produced, for overflows - infinities.

For example, instead of current code of float_pow() that handle finite input
https://github.com/python/cpython/blob/04242c027feeff726acb15b6463422897b489bcf/Objects/floatobject.c#L788-L801
we could use something much more simple:

    ix = pow(iv, iw);
    if (negate_result)
        ix = -ix;
    if (isinf(ix)) {
        PyErr_SetString(PyExc_OverflowError,
                        "float exponentiation result out of range");
        return NULL;
    }

(_Py_ADJUST_ERANGE1 and _Py_ADJUST_ERANGE2 helpers will be removed.)

See recent d.p.o thread for illustration of subtle issues, that can be introduced trying to fix errno, coming from platforms functions. We also have occurring bugreports, when platform libm set errno wrongly, e.g. https://github.com/python/cpython/issues/153144.

Disclaimer: this issue filled, based on my humble understanding of the proposal by @mdickinson. It seems, there are volunteers to work on this (CC @hpkfft), hence issue was opened.

Has this already been discussed elsewhere?

I have already discussed this feature proposal on Discourse

Links to previous discussion of this feature:

https://discuss.python.org/t/108539/

Linked PRs

• gh-156551