[PATCH] libstdc++: Remove unnecessary "& 1" from year_month_day_last::day().

[PATCH] libstdc++: Remove unnecessary "& 1" from year_month_day_last::day().

[Cassio Neri]

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?

 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