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| author | Cassio Neri <cassio.neri@gmail.com> | 2023-11-11 16:44:58 +0000 |
|---|---|---|
| committer | Jonathan Wakely <jwakely@redhat.com> | 2023-11-14 22:32:08 +0000 |
| commit | b011535456396a6846ff24fb5b1baea8fe0a33b1 (patch) | |
| tree | f59cc1d26193097d52eaad037b0962076ba90ee4 | |
| parent | 1c15303375f7089ff985b085ab877b11ebfbc4b7 (diff) | |
| download | gcc-b011535456396a6846ff24fb5b1baea8fe0a33b1.zip gcc-b011535456396a6846ff24fb5b1baea8fe0a33b1.tar.gz gcc-b011535456396a6846ff24fb5b1baea8fe0a33b1.tar.bz2 | |
libstdc++: Remove unnecessary "& 1" from year_month_day_last::day()
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 (year_month_day_last::day): Remove &1.
| -rw-r--r-- | libstdc++-v3/include/std/chrono | 24 |
1 files changed, 14 insertions, 10 deletions
diff --git a/libstdc++-v3/include/std/chrono b/libstdc++-v3/include/std/chrono index 10e868e..a826982 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 |