Avoid temporary `varargs` tuple creation in argument passing

[colorfulappl]

BPO	46212
Nosy	@larryhastings, @pablogsal, @isidentical, @erlend-aasland, @colorfulappl
PRs	gh-90370: Avoid temporary varargs tuple creation in argument passing #30312
Files	bench-print.py

^{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 = None
created_at = <Date 2021-12-31.10:05:47.113>
labels = ['3.11', 'expert-argument-clinic', 'performance']
title = 'Avoid temporary `varargs` tuple creation in argument passing'
updated_at = <Date 2022-01-21.14:06:02.177>
user = 'https://github.com/colorfulappl'

bugs.python.org fields:

activity = <Date 2022-01-21.14:06:02.177>
actor = 'erlendaasland'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Argument Clinic']
creation = <Date 2021-12-31.10:05:47.113>
creator = 'colorfulappl'
dependencies = []
files = ['50533']
hgrepos = []
issue_num = 46212
keywords = ['patch']
message_count = 6.0
messages = ['409412', '409413', '409414', '409659', '411129', '411130']
nosy_count = 5.0
nosy_names = ['larry', 'pablogsal', 'BTaskaya', 'erlendaasland', 'colorfulappl']
pr_nums = ['30312']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'performance'
url = 'https://bugs.python.org/issue46212'
versions = ['Python 3.11']

Linked PRs

• gh-90370: avoid temporary tuple creation for vararg in AC #126064

[colorfulappl]

When "Augument Clinic generated code" are parsing arguments, the args are packed to a tuple before passing to callee. This may be unnecessary.

Pass a raw pointer which points to on-stack varargs, and a varargssize integer to indicate how many varargs are passed, can save the time of tuple creation/destruction and value copy.

[colorfulappl]

I wrote some microbenchs.

Patch: b68176d

Environment:
macOS 12.1
clang 13.0.0
configure with --enable-optimizations

Result on microbench:

+--------------------------------------------+-------------------------+------------------------+
| Benchmark | ./opt_baseline/res.json | ./opt_patched/res.json |
+============================================+=========================+========================+
| print(a, b, c) | 917 ns | 820 ns: 1.12x faster |
+--------------------------------------------+-------------------------+------------------------+
| print(a, b, c, *v) | 1.56 us | 1.62 us: 1.04x slower |
+--------------------------------------------+-------------------------+------------------------+
| print(a, sep='', file=stdout) | 376 ns | 295 ns: 1.27x faster |
+--------------------------------------------+-------------------------+------------------------+
| print(*v, sep='', flush=True, file=stdout) | 2.02 us | 1.94 us: 1.04x faster |
+--------------------------------------------+-------------------------+------------------------+
| Geometric mean | (ref) | 1.05x faster |
+--------------------------------------------+-------------------------+------------------------+

Benchmark hidden because not significant (3): print(a), print(a, sep='', flush=True, file=stdout), print(a, b, c, *v, sep='', flush=True, file=stdout)

[erlend-aasland]

Note that _PyArg_UnpackKeywordsWithVararg is defined with PyAPI_FUNC. Changing its argument spec is strictly a backwards incompatible change, IIUC.

[colorfulappl]

I am a rookie in Python, did not notice changing PyAPI_FUNC means breaking backward compatibility.

I have reverted _PyArg_UnpackKeywordsWithVararg and committed again.

[isidentical]

Note that _PyArg_UnpackKeywordsWithVararg is defined with PyAPI_FUNC. Changing its argument spec is strictly a backwards incompatible change, IIUC.

AFAIK we have committed _PyArg_UnpackKeywordsWithVararg on 3.11 alpha, so I think it should be fine. Also CC: @pablogsal

[erlend-aasland]

AFAIK we have committed _PyArg_UnpackKeywordsWithVararg on 3.11 alpha, so I think it should be fine.

I see, so no ABI worries then.