[C API] Py_IS_INFINITY() macro doesn't work in the limited C API if isinf() is not defined

[vstinner]

BPO	45440
Nosy	@vstinner
PRs	bpo-45440: Require math.h isinf() to build #28894 bpo-45440: Remove pymath.c fallbacks #28977

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2021-10-13.21:30:01.697>
created_at = <Date 2021-10-11.23:51:20.093>
labels = ['expert-C-API', '3.11']
title = "[C API] Py_IS_INFINITY() macro doesn't work in the limited C API if isinf() is not defined"
updated_at = <Date 2021-10-15.17:45:37.921>
user = 'https://github.com/vstinner'

bugs.python.org fields:

activity = <Date 2021-10-15.17:45:37.921>
actor = 'vstinner'
assignee = 'none'
closed = True
closed_date = <Date 2021-10-13.21:30:01.697>
closer = 'vstinner'
components = ['C API']
creation = <Date 2021-10-11.23:51:20.093>
creator = 'vstinner'
dependencies = []
files = []
hgrepos = []
issue_num = 45440
keywords = ['patch']
message_count = 5.0
messages = ['403704', '403706', '403873', '403874', '404041']
nosy_count = 1.0
nosy_names = ['vstinner']
pr_nums = ['28894', '28977']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue45440'
versions = ['Python 3.11']

[vstinner]

If the HAVE_DECL_ISINF macro is not defined in pyconfig.h, the Py_IS_INFINITY macro is defined as:

#define Py_IS_INFINITY(X) \
    ((X) && (Py_FORCE_DOUBLE(X)*0.5 == Py_FORCE_DOUBLE(X)))

Problem: Py_FORCE_DOUBLE() is excluded from the limited C API (and the stable ABI).

I see different options:

• Implement Py_IS_INFINITY() as an opaque function if the HAVE_DECL_ISINF macro is not defined. I did something similar in Py_INCREF() to support the limited C API with a debug build of Python: call _Py_IncRef() opaque function.

• Make Py_FORCE_DOUBLE() private and add it to the limited C API as an implementation detail.

• Add Py_FORCE_DOUBLE() macro to the limited C API: the current implementation is fragile, it depends on how Python.h is included. Also, I dislike macros in the limited C API.

I would prefer to *remove* Py_FORCE_DOUBLE() to all APIs, than adding it to the limited C API.

[vstinner]

The Py_FORCE_DOUBLE() macro was added in bpo-5724 by the change:

commit e05e840
Author: Mark Dickinson <dickinsm@gmail.com>
Date: Mon May 4 13:30:43 2009 +0000

Issue bpo-5724: Fix cmath failures on Solaris 10.

[vstinner]

New changeset 194a952 by Victor Stinner in branch 'main':
bpo-45440: Require math.h isinf() to build (GH-28894)
194a952

[vstinner]

[C API] Py_IS_INFINITY() macro doesn't work in the limited C API if isinf() is not defined

Well, the final fix is to remove the code path when isinf() is not available and require it to build Python ;-)

[vstinner]

New changeset 00ffc45 by Victor Stinner in branch 'main':
bpo-45440: Remove pymath.c fallbacks (GH-28977)
00ffc45