[issue12805] Optimzations for bytes.join() et. al

[John O'Connor]

New submission from John O'Connor <tehjcon at gmail.com>:

I noticed there are a few more common cases that can be sped up in bytes.join().

When:
 - The sequence length and separator length are both 0
 - The separator length is 0 or 1
 - The separator string contains only 1 distinct byte

These could also be applied to other sequence types.

for i in {{0..5}}; do
./python -m timeit -s "f=open('/usr/share/dict/words', 'rb'); L=f.readlines()" \
"b' '.join(L)"; 
done

- BEFORE -
100 loops, best of 3: 3.79 msec per loop
100 loops, best of 3: 3.5 msec per loop
100 loops, best of 3: 3.75 msec per loop
- AFTER -
100 loops, best of 3: 2.81 msec per loop
100 loops, best of 3: 2.81 msec per loop
100 loops, best of 3: 3.03 msec per loop

~20% speedup

Same test on a smaller list (lines from LICENSE file) yields a similar 15-20% speedup.
Same test on L = [b'1', b'2', b'3'] yields 10% speedup

----------
components: Interpreter Core
files: bytes_join_optimization.patch
keywords: patch
messages: 142650
nosy: benjamin.peterson, ezio.melotti, haypo, jcon, pitrou
priority: normal
severity: normal
status: open
title: Optimzations for bytes.join() et. al
type: performance
versions: Python 3.3
Added file: http://bugs.python.org/file22976/bytes_join_optimization.patch

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