gh-117431: Optimize str.startswith

[eendebakpt]

We apply two optimizations in tailmatch (which is used in both str.startswith and str.endswith).

• For one and two character arguments we avoid a call to memcmp as all characters have already been checked
• In the call to memcmp we can reduce the number of bytes compared since the first and last character have already been checked.

Notes:

Two possible optimizations not included in this PR:

• For the single character case we still do some double work as PyUnicode_READ(kind_self, data_self, offset) == PyUnicode_READ(kind_sub, data_sub, 0) and PyUnicode_READ(kind_self, data_self, offset + end_sub) == PyUnicode_READ(kind_sub, data_sub, end_sub) are equal in that case. We can eliminate that by adding something like

int first_character_equal = PyUnicode_READ(kind_self, data_self, offset) == PyUnicode_READ(kind_sub, data_sub, 0)
if (PyUnicode_GET_LENGTH(substring)==1) {
   return first_character_equal ;
...

This makes the code for the single character case a bit faster, but the code a bit more complex.

• We can make the number of bytes compared even smaller, but we would have calculate a different offset which does not seem worth the effort.

Benchmark (on top of #117466): python -m timeit -s "s = 'abcdef'" "s.startswith('a')"

main: 10000000 loops, best of 5: 27.2 nsec per loop
PR: 10000000 loops, best of 5: 26.3 nsec per loop

• Issue: Improve performance of startswith, endswith, count, *find, and *index methods for str, bytes and bytearray #117431

[erlend-aasland]

Could you add the other optimisations as separate commits?

[serhiy-storchaka]

The difference between 27.2 and 26.3 ns is too small and can be the result of unrelated factors. I get a nanosecond variation when run the same command several times.

[erlend-aasland]

The difference between 27.2 and 26.3 ns is too small and can be the result of unrelated factors. I get a nanosecond variation when run the same command several times.

Yes, so I'm curious about the other two mentioned optimisations that are not (yet) part of this PR. Perhaps they have a greater impact.

[eendebakpt]

The difference between 27.2 and 26.3 ns is too small and can be the result of unrelated factors. I get a nanosecond variation when run the same command several times.

Yes, so I'm curious about the other two mentioned optimisations that are not (yet) part of this PR. Perhaps they have a greater impact.

I created a PR with the other approach: #117782.

[eendebakpt]

Closing this in favor of the alternate PR.