C function calls: use Py_ssize_t rather than C int for number of arguments

[vstinner]

BPO	27848
Nosy	@pitrou, @scoder, @vstinner, @serhiy-storchaka
Files	Py_ssize_t.patch

^{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 2016-08-24.22:45:00.150>
created_at = <Date 2016-08-24.12:11:41.572>
labels = ['interpreter-core', 'type-feature']
title = 'C function calls: use Py_ssize_t rather than C int for number of arguments'
updated_at = <Date 2016-08-24.22:45:00.149>
user = 'https://github.com/vstinner'

bugs.python.org fields:

activity = <Date 2016-08-24.22:45:00.149>
actor = 'vstinner'
assignee = 'none'
closed = True
closed_date = <Date 2016-08-24.22:45:00.150>
closer = 'vstinner'
components = ['Interpreter Core']
creation = <Date 2016-08-24.12:11:41.572>
creator = 'vstinner'
dependencies = []
files = ['44209']
hgrepos = []
issue_num = 27848
keywords = ['patch']
message_count = 3.0
messages = ['273549', '273603', '273608']
nosy_count = 5.0
nosy_names = ['pitrou', 'scoder', 'vstinner', 'python-dev', 'serhiy.storchaka']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = None
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue27848'
versions = ['Python 3.6']

[vstinner]

I recently added new functions to call functions in C, especially _PyObject_FastCallDict(). I started with the C int type for the number of positional arguments: "int nargs".

But slowly, when I started to patch code to use _PyObject_FastCallDict(), I used more and more Py_ssize_t variables for the number of arguments, variable downcasted to int to call _PyObject_FastCallDict().

It is similar to the old issue bpo-18295.

I propose to avoid any risk of integer overflow by using Py_ssize_t everywhere. It *might* produce more efficient machine code, since "i++" may require extra code to handle overflow when the int rather is used. But I'm not sure that Py_ssize_t is better, especially when -fwrapv option of the GCC compiler is used.

Attached patch implements this idea.

The patch also uses Py_ssize_t for some BUILD_xxx opcodes like BUILD_LIST_UNPACK. The change is not directly related. I wrote it in the hope that machine code could be more efficient, and avoid having to ask myself if types could overflow or not.

[python-dev]

New changeset 9479da828358 by Victor Stinner in branch 'default':
Use Py_ssize_t type for number of arguments
https://hg.python.org/cpython/rev/9479da828358

[vstinner]

I pushed Py_ssize_t.patch without the "const" ;-) Thanks for the review Antoine.