This patch reimplements std::chrono::year_month_day_last:day() which yields the last day of a particular month. The current implementation uses a look-up table implemented as an unsigned[12] array. The new implementation instead is based on the fact that a month m in [1, 12], except for m == 2 (February), is either 31 or 30 days long and m's length depends on two things: m's parity and whether m >= 8 or not. These two conditions are determined by the 0th and 3th bit of m and, therefore, cheap and straightforward bit-twiddling can provide the right result.
Measurements in x86_64 [1] suggest a 10% performance boost. Although this does not seem to be huge, notice that measurements are done in hot L1 cache conditions which might not be very representative of production runs. Also freeing L1 cache from holding the look-up table might allow performance improvements elsewhere. References: [1] https://github.com/cassioneri/calendar libstdc++-v3/ChangeLog: * include/std/chrono: --- libstdc++-v3/include/std/chrono | 23 +++++++++++++++++------ 1 file changed, 17 insertions(+), 6 deletions(-) diff --git a/libstdc++-v3/include/std/chrono b/libstdc++-v3/include/std/chrono index 7840099d743..35a7a5e4382 100644 --- a/libstdc++-v3/include/std/chrono +++ b/libstdc++-v3/include/std/chrono @@ -1269,9 +1269,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION inline constexpr unsigned __days_per_month[12] = { 31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 }; - - inline constexpr unsigned __last_day[12] - = { 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 }; } // DAY @@ -2526,9 +2523,23 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION constexpr chrono::day day() const noexcept { - if (!_M_mdl.ok() || (month() == February && _M_y.is_leap())) - return chrono::day{29}; - return chrono::day{__detail::__last_day[unsigned(month()) - 1]}; + 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}. + + // 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. + + // 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}; } constexpr -- 2.29.2
