wc: count characters as bytes in single-byte locales

[darkraider01]

In single-byte locales (where MB_CUR_MAX == 1), characters should be counted as bytes for wc -m.

Previously, this was inferred by checking if the locale was named "C" or "POSIX", which does not match GNU semantics on platforms where the C locale resolves to UTF-8, nor does it handle other single-byte locales (such as ISO-8859-1).

This updates wc to detect whether the active locale encoding is genuinely single-byte via native libc locale properties (MB_CUR_MAX == 1), counting bytes directly for -m across all dispatch paths while preserving multibyte counting for UTF-8 and other multibyte locales.

Once #15139 merges, I'll rebase this follow-up ^^

Closes #14928.

[darkraider01]

Byte counting is limited to C/POSIX because other non-UTF-8 locales can still use multibyte encodings. Those locales keep their existing counting paths.

[github-actions]

GNU testsuite comparison:

Congrats! The gnu test tests/wc/wc is no longer failing!

[codspeed]

Merging this PR will regress 4 benchmarks

⚡ 16 improved benchmarks
❌ 4 regressed benchmarks
✅ 310 untouched benchmarks
⏩ 123 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	five_38_bit_primes	1.7 s	2.2 s	-21.78%
❌	Simulation	wc_chars_large_line_count[100000]	2.3 ms	2.8 ms	-18.01%
❌	Simulation	sort_dictionary_order[500000]	1.9 s	2.1 s	-6.54%
❌	Simulation	rm_recursive_tree	13.7 ms	14.4 ms	-5.02%
⚡	Simulation	cut_fields_custom_delim	65.9 ms	51.9 ms	+27%
⚡	Simulation	cut_fields_tab	57.4 ms	45.5 ms	+26.24%
⚡	Simulation	cut_bytes	18.1 ms	15.5 ms	+16.69%
⚡	Simulation	check_sorted_utf8_locale	491.1 ms	421.8 ms	+16.45%
⚡	Simulation	sort_numeric_utf8_locale	40.8 ms	36.1 ms	+12.98%
⚡	Simulation	merge_single_file_utf8_locale	133.4 ms	119.3 ms	+11.85%
⚡	Simulation	cut_characters	25.4 ms	22.9 ms	+10.88%
⚡	Simulation	three_39_bit_primes	545.9 ms	514.8 ms	+6.05%
⚡	Simulation	merge_pre_sorted_files_utf8_locale	251.9 ms	238 ms	+5.82%
⚡	Simulation	merge_pre_sorted_files	252.5 ms	238.6 ms	+5.82%
⚡	Simulation	sort_german_de_locale	295 ms	282.1 ms	+4.6%
⚡	Simulation	sort_case_sensitive[500000]	334.9 ms	321 ms	+4.32%
⚡	Simulation	sort_mixed_utf8_locale	82.1 ms	79.4 ms	+3.52%
⚡	Simulation	sort_reverse_utf8_locale	81.9 ms	79.2 ms	+3.45%
⚡	Simulation	sort_spill_to_tmp_files_utf8_locale	332.1 ms	321 ms	+3.45%
⚡	Simulation	sort_unique_utf8_locale	84.9 ms	82.2 ms	+3.33%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

---

_{Comparing darkraider01:wc-c-locale-character-counts (aee391f) with main (8f6a838)²}

Footnotes

1. 123 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

2. No successful run was found on main (ca3e965) during the generation of this report, so 8f6a838 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

[sylvestre]

sorry but it needs to be rebased

[darkraider01]

right, sorry, on it !!!!

[sylvestre]

Should be done in a different pr

[darkraider01]

done!

[darkraider01]

i'll rebase this pr again, once #15139 lands.
/cc @sylvestre