On Sat, 11 Nov 2023 at 16:46, Cassio Neri <[email protected]> wrote:
>
> When year_month_day_last::day() was implemented, Dr. Matthias Kretz realised
> that the operation "& 1" wasn't necessary but we did not patch it at that
> time. This patch removes the unnecessary operation.
>
> libstdc++-v3/ChangeLog:
>
> * include/std/chrono: Remove &1 from year_month_day_last::day().
> ---
> Previous versions of this patch failed to apply. I hope it works this time.
>
> OK for trunk?
Yes, thanks - pushed to trunk.
>
> libstdc++-v3/include/std/chrono | 24 ++++++++++++++----------
> 1 file changed, 14 insertions(+), 10 deletions(-)
>
> diff --git a/libstdc++-v3/include/std/chrono b/libstdc++-v3/include/std/chrono
> index 10e868e5a03..a826982803b 100644
> --- a/libstdc++-v3/include/std/chrono
> +++ b/libstdc++-v3/include/std/chrono
> @@ -1800,22 +1800,26 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> {
> const auto __m = static_cast<unsigned>(month());
>
> - // Excluding February, the last day of month __m is either 30 or 31
> or,
> - // in another words, it is 30 + b = 30 | b, where b is in {0, 1}.
> + // The result is unspecified if __m < 1 or __m > 12. Hence, assume
> + // 1 <= __m <= 12. For __m != 2, day() == 30 or day() == 31 or, in
> + // other words, day () == 30 | b, where b is in {0, 1}.
>
> - // If __m in {1, 3, 4, 5, 6, 7}, then b is 1 if, and only if __m is
> odd.
> - // Hence, b = __m & 1 = (__m ^ 0) & 1.
> + // If __m in {1, 3, 4, 5, 6, 7}, then b is 1 if, and only if, __m is
> + // odd. Hence, b = __m & 1 = (__m ^ 0) & 1.
>
> - // If __m in {8, 9, 10, 11, 12}, then b is 1 if, and only if __m is
> even.
> - // Hence, b = (__m ^ 1) & 1.
> + // If __m in {8, 9, 10, 11, 12}, then b is 1 if, and only if, __m is
> + // even. Hence, b = (__m ^ 1) & 1.
>
> // Therefore, b = (__m ^ c) & 1, where c = 0, if __m < 8, or c = 1 if
> // __m >= 8, that is, c = __m >> 3.
>
> - // The above mathematically justifies this implementation whose
> - // performance does not depend on look-up tables being on the L1
> cache.
> - return chrono::day{__m != 2 ? ((__m ^ (__m >> 3)) & 1) | 30
> - : _M_y.is_leap() ? 29 : 28};
> + // Since 30 = (11110)_2 and __m <= 31 = (11111)_2, the "& 1" in b's
> + // calculation is unnecessary.
> +
> + // The performance of this implementation does not depend on look-up
> + // tables being on the L1 cache.
> + return chrono::day{__m != 2 ? (__m ^ (__m >> 3)) | 30
> + : _M_y.is_leap() ? 29 : 28};
> }
>
> constexpr
> --
> 2.41.0
>
