gh-124502: Optimize unicode_eq()

[vstinner]

• Cleanup unicode_compare_eq() code.
• Copy unicode_compare_eq() code in unicode_eq(): the two functions are now identical.

• Issue: [C API] Add PyUnicode_Equal() function #124502

[vstinner]

Microbenchmark:

import pyperf
import _testcapi

str1 = "a" * 10
str2 = str1.encode().decode()
if str2 is str1:
    raise Exception

runner = pyperf.Runner()
runner.bench_time_func('_PyUnicode_EQ', _testcapi.bench_eq, str1, str2)
runner.bench_time_func('_PyUnicode_Equal', _testcapi.bench__equal, str1, str2)

Result on Python built with PGO+LTO (without CPU isolation):

_PyUnicode_EQ: Mean +- std dev: 7.45 ns +- 0.02 ns
_PyUnicode_Equal: Mean +- std dev: 7.44 ns +- 0.02 ns

The performance is the same.

[vstinner]

cc @serhiy-storchaka

[serhiy-storchaka]

Why change unicode_compare_eq()?

[vstinner]

For comparison, results on the main branch, also with PGO+LTO:

_PyUnicode_EQ: Mean +- std dev: 8.52 ns +- 0.02 ns
_PyUnicode_Equal: Mean +- std dev: 7.45 ns +- 0.02 ns

[vstinner]

Why change unicode_compare_eq()?

I like to reduce the scope of variables by only declaring them where they are first assigned.

[serhiy-storchaka]

If this PR is only about unique_eq(), change only it. Otherwise simply use unique_eq() instead of unique_compare_eq(). There are no reasons to have two identical static functions with different names.

[vstinner]

There are no reasons to have two identical static functions with different names.

Once this PR will be merged, I plan to write a following PR to remove unicode_compare_eq() and _PyUnicode_EQ().

I limited this PR to optimizing unicode_eq() to be able to measure the speedup. Otherwise, it was too difficult to measure it.

[serhiy-storchaka]

If you plan to remove unicode_compare_eq(), do not make cosmetic changes to it.

s/unicode_compare_eq/unicode_eq/

[vstinner]

If you plan to remove unicode_compare_eq(), do not make cosmetic changes to it.

As you wish: I reverted unicode_compare_eq() changes.

[serhiy-storchaka]

LGTM. Although I think that #125070 was clearer, because it shown the larger picture.

[vstinner]

Merged, thanks for reviews.

Follow-up PR: #125114 removes _PyUnicode_EQ().