[issue29737] Optimize concatenating empty tuples

[Serhiy Storchaka]

New submission from Serhiy Storchaka:

Since tuples are immutable, concatenating with empty tuple can be optimized by returning an opposite argument.

Microbenchmarks (the difference is larger for larger tuples):

$ ./python -m perf timeit --duplicate=100 -s 'a = (1, 2)' 'a + ()'
Unpatched:  Median +- std dev: 288 ns +- 12 ns
Patched:    Median +- std dev: 128 ns +- 5 ns

$ ./python -m perf timeit --duplicate=100 -s 'a = (1, 2)' '() + a'
Unpatched:  Median +- std dev: 285 ns +- 16 ns
Patched:    Median +- std dev: 128 ns +- 6 ns

Non-empty tuples are not affected:

$ ./python -m perf timeit --duplicate=100 -s 'a = (1, 2)' 'a + a'
Unpatched:  Median +- std dev: 321 ns +- 24 ns
Patched:    Median +- std dev: 317 ns +- 26 ns

----------
components: Interpreter Core
messages: 289129
nosy: haypo, serhiy.storchaka
priority: normal
severity: normal
stage: patch review
status: open
title: Optimize concatenating empty tuples
type: performance
versions: Python 3.7

_______________________________________
Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue29737>
_______________________________________