| Message ID | 20231111164458.46230-1-cassio.neri@gmail.com |
|---|---|
| State | New |
| Headers | show |
| Series | [v4] libstdc++: Remove unnecessary "& 1" from year_month_day_last::day(). | expand |
On Sat, 11 Nov 2023 at 16:46, Cassio Neri <cassio.neri@gmail.com> 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 >
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