Jeff BaileyandRaman Tenneti 2705f4613a [libc] Implement fast date conversion algorithm of Ben Joffe (#208312) ...

Replaced the loop-based year/month/day extraction in update_from_seconds
with Ben Joffe's "Very Fast 64-bit" date algorithm using multiply-shift
division (4 multiplications, 0 hardware divisions). Also replaced the
mktime inverse with Ben Joffe's inverse date algorithm.

Updated is_leap_year to use the Drepper-Neri-Schneider algorithm (% 25
instead of % 100). Replaced the O(n) yday month-length loop with a
constant-time cumulative-days table lookup.

Uses UInt128 for portable 128-bit support across 64-bit (native) and
32-bit (software fallback) targets.

Raman's benchmarks on x86-64 show a ~13% wall-time improvement:

  Before: real 2.215s, user 2.214s
  After:  real 1.919s, user 1.913s

Algorithm references:
  https://www.benjoffe.com/fast-date-64
  https://www.benjoffe.com/fast-date#inverse
  https://www.benjoffe.com/fast-leap-year#drepper-neri-schneider

---------

Co-authored-by: Raman Tenneti <rtenneti@google.com>

2026-07-10 21:04:22 +01:00

..

building_docs.md

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builtin_compatibility.md

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code_style.md

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config_options.md

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date_and_time.md

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entrypoints.md

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fuzzing.md

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header_generation.md

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implementation_standard.md

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implementing_a_function.md

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index.md

[libc] Implement fast date conversion algorithm of Ben Joffe (#208312)

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modular_format.md

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printf_behavior.md

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source_tree_layout.md

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syscall_wrapper_refactor.md

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undefined_behavior.md

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