Slow functions calls for C-coded functions, accepting positional-or-kwarg argument vs positional-only

Bug report

Bug description:

I don't see measurable difference on pure-Python level, e.g. for

# a.py
import pyperf

def f1(x):
    return 1

def f2(x, /):
    return 1

runner = pyperf.Runner()
runner.bench_func("f1(1)", f1, 1)
runner.bench_func("f2(1)", f2, 1)

I got on 3.13:

$ python a.py -q
f1(1): Mean +- std dev: 233 ns +- 2 ns
f2(1): Mean +- std dev: 232 ns +- 2 ns

But for functions, implemented in C, using the Argument Clinic - the difference is bigger:

$ python bench.py -q -o patch.json
isfinite(1.0): Mean +- std dev: 152 ns +- 1 ns
$ git checkout master && make -s
Checked 114 modules (35 built-in, 78 shared, 1 n/a on linux-x86_64, 0 disabled, 0 missing, 0 failed on import)
$ python bench.py -q -o ref.json
isfinite(1.0): Mean +- std dev: 136 ns +- 1 ns

(In the second version - the math.isfinite() accept also keyword argument x)

diff --git a/Modules/mathmodule.c b/Modules/mathmodule.c
index 7c2a421dd6..e33281ac8b 100644
--- a/Modules/mathmodule.c
+++ b/Modules/mathmodule.c
@@ -3156,14 +3156,13 @@ math_radians_impl(PyObject *module, double x)
 math.isfinite
 
     x: double
-    /
 
 Return True if x is neither an infinity nor a NaN, and False otherwise.
 [clinic start generated code]*/
 
 static PyObject *
 math_isfinite_impl(PyObject *module, double x)
-/*[clinic end generated code: output=8ba1f396440c9901 input=46967d254812e54a]*/
+/*[clinic end generated code: output=8ba1f396440c9901 input=a47ad6e72998b81d]*/
 {
     return PyBool_FromLong((long)isfinite(x));
 }

This performance penalty affects API decisions (e.g. https://github.com/python/cpython/pull/131886) and that looks as a bug.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-136732
• gh-137202

Start with the benchmark in bench.py and the pyperf examples, comparing math.isfinite(1.0) for the positional-only and positional-or-keyword forms. Inspect Modules/mathmodule.c and the generated Argument Clinic output from Tools/clinic/clinic.py; done means the C-coded call no longer has the reported regression while the benchmark and API behavior remain correct.