gh-139353: Add Objects/unicode_writer.c file

[vstinner]

Move the public PyUnicodeWriter API and the private _PyUnicodeWriter API to a new Objects/unicode_writer.c file.

Rename a few helper functions to share them between unicodeobject.c and unicode_writer.c, such as resize_compact() or unicode_result().

• Issue: Split large Objects/unicodeobject.c file into smaller files #139353

[vstinner]

cc @serhiy-storchaka @malemburg

[malemburg]

As mentioned before, I don't think turning these parts into separate object files is a good idea.

Have you benchmarked the effect of putting the writer into a separate object file vs. keeping it in the unicodeobject.c file (vie #includes )

[vstinner]

Have you benchmarked the effect of putting the writer into a separate object file vs. keeping it in the unicodeobject.c file (vie #includes )

I'm not sure if it's relevant since PyUnicodeWriter is not used by unicodeobject.c.

I ran a benchmark on repr(tuple) (which is implemented with PyUnicodeWriter): there is no impact on performance. The difference is just noise in the benchmark.

ref is the main branch and split is this PR:

Benchmark	ref	split
repr tuple-1	254 ns	265 ns: 1.04x slower
repr tuple-10	786 ns	783 ns: 1.00x faster
repr tuple-50	3.02 us	3.03 us: 1.00x slower
repr tuple-100	5.78 us	5.84 us: 1.01x slower
Geometric mean	(ref)	1.01x slower

Benchmark hidden because not significant (1): repr tuple-5

Note: I didn't use LTO+PGO optimizations which should reduce the noise even more.

import pyperf
runner = pyperf.Runner()
for size in (1, 5, 10, 50, 100):
    runner.timeit(f'repr tuple-{size}',
        setup=f't = (1,)*{size}',
        stmt='repr(t)')

[malemburg]

Thanks for running the benchmark.

I'm more concerned about uses of unicodeobject.c code in this new unicode_writerr.c, than use of the writer APIs in unicodeobject.c.