bpo-46848: Optimize mmap.find() with memmem(3)

[rumpelsepp]

This merge requests optimizes mmap.find() by using libc's memmem(3) function (which is optimized) instead of using a trivial for loop. I could not find an equivalent solution for mmap.rfind() yet.

The following snippet runs 100% faster with this patch applied. The file used is a logfile of 2.3 GB size

with open(sys.argv[1], 'r+b') as file_:
    with mmap.mmap(file_.fileno(), 0, access=mmap.ACCESS_READ) as file:
        cur_offset = 0
        while True:
            cur_offset = file.find(b"\n", cur_offset)
            if cur_offset == -1:
                break
            cur_offset += 1

With this patch applied:

rumpelsepp@kronos ~/P/v/c/debug (main)> time ./python test.py /tmp/log

________________________________________________________
Executed in    3,12 secs    fish           external
   usr time    2,94 secs    1,34 millis    2,94 secs
   sys time    0,15 secs    0,00 millis    0,15 secs

Without this patch:

rumpelsepp@kronos ~/P/v/c/debug (mmap)> time ./python test.py /tmp/log

________________________________________________________
Executed in    6,47 secs    fish           external
   usr time    6,53 secs  248,91 millis    6,29 secs
   sys time    0,18 secs   32,26 millis    0,15 secs

https://bugs.python.org/issue46848

[rumpelsepp]

Will fix the failing tests ASAP.

[sweeneyde]

Another thing you could try out and compare is using the fastsearch functions defined in Objects/stringlib, which should generally be faster than that naive loop and also platform independent.

[tiran]

The function is not specified in POSIX standard. Please add a feature check to configure.ac and check for HAVE_MEMMEM.

[rumpelsepp]

Done.

[bedevere-bot]

A Python core developer has requested some changes be made to your pull request before we can consider merging it. If you could please address their requests along with any other requests in other reviews from core developers that would be appreciated.

Once you have made the requested changes, please leave a comment on this pull request containing the phrase I have made the requested changes; please review again. I will then notify any core developers who have left a review that you're ready for them to take another look at this pull request.

[rumpelsepp]

#31625 provides a platform independent proposal.