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Currently str.find() and similar methods can make a copy of self or searched string if they have different kinds. In some cases this is redundant because the result can be known before trying to search. Longer string can't be found in shorter string and wider string can't be found in narrower string. Proposed patch avoid creating temporary widened copies in such corner cases. It also adds special cases for searching 1-character strings.
Some sample microbenchmark results:
$ ./python -m timeit -s "a = 'x'; b = 'x\U00012345'" -- "b.find(a)"
Unpatched: 1000000 loops, best of 3: 1.92 usec per loop
Patched: 1000000 loops, best of 3: 1.03 usec per loop
$ ./python -m timeit -s "a = 'x'; b = 'x\U00012345'" -- "a in b"
Unpatched: 1000000 loops, best of 3: 0.543 usec per loop
Patched: 1000000 loops, best of 3: 0.25 usec per loop
$ ./python -m timeit -s "a = '\U00012345'; b = 'x'*1000" -- "b.find(a)"
Unpatched: 100000 loops, best of 3: 4.58 usec per loop
Patched: 1000000 loops, best of 3: 0.969 usec per loop
$ ./python -m timeit -s "a = 'x'*1000; b = '\U00012345'" -- "b.find(a)"
Unpatched: 100000 loops, best of 3: 3.77 usec per loop
Patched: 1000000 loops, best of 3: 0.97 usec per loop
$ ./python -m timeit -s "a = 'x'*1000; b = '\U00012345'" -- "a in b"
Unpatched: 100000 loops, best of 3: 2.4 usec per loop
Patched: 1000000 loops, best of 3: 0.225 usec per loop

New changeset 6db9d7c1be29 by Serhiy Storchaka in branch 'default':
Issue #23573: Increased performance of string search operations (str.find,
https://hg.python.org/cpython/rev/6db9d7c1be29 

Looks as this patch makes buildbots crash.

> Looks as this patch makes buildbots crash.
Yep. It took me some minutes to find that the crash was caused by this issue :-p
http://buildbot.python.org/all/builders/AMD64%20Windows7%20SP1%203.x/builds/5930/steps/test/logs/stdio
...
[117/393/1] test_bigmem
Assertion failed: 0, file c:\buildbot.python.org3円.x.kloth-win64\build\objects\stringlib/fastsearch.h, line 76
Fatal Python error: Aborted
Current thread 0x000010ec (most recent call first):
 File "C:\buildbot.python.org3円.x.kloth-win64\build\lib\test\test_bigmem.py", line 294 in test_rfind
 File "C:\buildbot.python.org3円.x.kloth-win64\build\lib\test\support\__init__.py", line 1641 in wrapper
 File "C:\buildbot.python.org3円.x.kloth-win64\build\lib\unittest\case.py", line 577 in run
 File "C:\buildbot.python.org3円.x.kloth-win64\build\lib\unittest\case.py", line 625 in __call__
 File "C:\buildbot.python.org3円.x.kloth-win64\build\lib\unittest\suite.py", line 122 in run
 File "C:\buildbot.python.org3円.x.kloth-win64\build\lib\unittest\suite.py", line 84 in __call__
 File "C:\buildbot.python.org3円.x.kloth-win64\build\lib\unittest\suite.py", line 122 in run
 File "C:\buildbot.python.org3円.x.kloth-win64\build\lib\unittest\suite.py", line 84 in __call__
 File "C:\buildbot.python.org3円.x.kloth-win64\build\lib\unittest\runner.py", line 176 in run
 File "C:\buildbot.python.org3円.x.kloth-win64\build\lib\test\support\__init__.py", line 1773 in _run_suite
 File "C:\buildbot.python.org3円.x.kloth-win64\build\lib\test\support\__init__.py", line 1807 in run_unittest
 File "C:\buildbot.python.org3円.x.kloth-win64\build\lib\test\test_bigmem.py", line 1252 in test_main
...

The problem is that Windows has no memrchr() function, and so fastsearch_memchr_1char() only supports FAST_SEARCH on Windows.

New changeset 3ac58de829ef by Victor Stinner in branch 'default':
Issue #23573: Fix bytes.rfind() and bytearray.rfind() on Windows
https://hg.python.org/cpython/rev/3ac58de829ef 

It looks like fastsearch_memchr_1char() manipulate pointers for memory alignment. It's not necessary when looking for ASCII or Latin1 characters or for bytes.
I propose to add a new fastsearch_memchr_1byte() function which would be used by bytes and bytearray, but also by str for ASCII and Latin1 strings.
Are you interested to implement this idea Serhiy?
For Windows without memrchr(), the code can be a simple loop.

Many thanks Victor for fixing crashes. Unfortunately I couldn't reproduce a 
crash on my computers, perhaps it is was 64-bit only.
Yes, I'll look how the code can be optimized.

Here is a patch that restores optimization of bytes.rfind() and bytearray.rfind() with 1-byte argument on Linux (it also reverts bc1a178b3bc8).

New changeset 311a4d28631b by Serhiy Storchaka in branch '3.5':
Issue #23573: Restored optimization of bytes.rfind() and bytearray.rfind()
https://hg.python.org/cpython/rev/311a4d28631b
New changeset c06410c68217 by Serhiy Storchaka in branch 'default':
Issue #23573: Restored optimization of bytes.rfind() and bytearray.rfind()
https://hg.python.org/cpython/rev/c06410c68217