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.

(cherry picked from commit b011535456396a6846ff24fb5b1baea8fe0a33b1)

diff --git a/libstdc++-v3/include/std/chrono b/libstdc++-v3/include/std/chrono
index f5a88331b1b50ac3ad2539d50db8148ebc80860a..5c1b7a8daf4584cb5aea70b1666074fba06ab1e0 100644 (file)
--- a/libstdc++-v3/include/std/chrono
+++ b/libstdc++-v3/include/std/chrono
@@ -1468,22 +1468,26 @@ _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