Add PyType_FastSubclass for float

[mattip]

BPO	46131
Nosy	@rhettinger, @serhiy-storchaka, @mattip
PRs	bpo-46131: add fastpath for PyFloat_Check() #30200 Revert "bpo-46131: add fastpath for PyFloat_Check()" #30208

^{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 2021-12-19.20:24:14.736>
created_at = <Date 2021-12-19.10:38:20.315>
labels = ['expert-C-API']
title = 'Add PyType_FastSubclass for float'
updated_at = <Date 2021-12-19.21:07:50.529>
user = 'https://github.com/mattip'

bugs.python.org fields:

activity = <Date 2021-12-19.21:07:50.529>
actor = 'rhettinger'
assignee = 'none'
closed = True
closed_date = <Date 2021-12-19.20:24:14.736>
closer = 'rhettinger'
components = ['C API']
creation = <Date 2021-12-19.10:38:20.315>
creator = 'mattip'
dependencies = []
files = []
hgrepos = []
issue_num = 46131
keywords = ['patch']
message_count = 8.0
messages = ['408890', '408891', '408896', '408923', '408924', '408925', '408926', '408927']
nosy_count = 3.0
nosy_names = ['rhettinger', 'serhiy.storchaka', 'mattip']
pr_nums = ['30200', '30208']
priority = 'normal'
resolution = 'rejected'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue46131'
versions = []

[mattip]

The basic classes int, dict, list, tuple ... all have a fast path for Py_*Check(obj):

#define PyLong_Check(op) \
        PyType_FastSubclass(Py_TYPE(op), Py_TPFLAGS_LONG_SUBCLASS)

except for float. I propose to add a Py_TPFLAGS_FLOAT_SUBCLASS enum and use it to implement PyType_FastSubclass for float.

[mattip]

"complex" is also missing a fast path

[serhiy-storchaka]

It is because PyFloat_Check() and PyComplex_Check() are rarely used in comparison with checks for integers, strings, etc. The flags space is limited, so it is better to use it for something more important.

There were reasons for not adding such flags at first place and I think that they are still valid.

[rhettinger]

Thanks for the suggestion, but I also concur with Serhiy.

[rhettinger]

New changeset 2ef06d4 by Matti Picus in branch 'main':
bpo-46131: add fastpath for PyFloat_Check() (bpo-30200)
2ef06d4

[mattip]

What do ya'all think of widen the flags field to int64_t so that there is more room for optimizations like this?

[rhettinger]

New changeset 29ea68b by Raymond Hettinger in branch 'main':
Revert "bpo-46131: add fastpath for PyFloat_Check() (GH-30200)" (GH-30208)
29ea68b

[rhettinger]

That would not change that slot space is at a premium and that we prefer to use that space for high payoff optimizations.