[gcc r11-7369] libstdc++: More efficient date from days

[gcc r11-7369] libstdc++: More efficient date from days

[Jonathan Wakely]

https://gcc.gnu.org/g:3dfd5493cf9798d46dd24ac32becc54d5074271e

commit r11-7369-g3dfd5493cf9798d46dd24ac32becc54d5074271e
Author: Cassio Neri <cassio.neri@gmail.com>
Date:   Wed Feb 24 17:23:56 2021 +0000

    libstdc++: More efficient date from days
    
    This patch reimplements std::chrono::year_month_day::_S_from_days() which
    retrieves a date from the number of elapsed days since 1970/01/01.  The new
    implementation is based on Proposition 6.3 of Neri and Schneider, "Euclidean
    Affine Functions and Applications to Calendar Algorithms" available at
    https://arxiv.org/abs/2102.06959.
    
    The aforementioned paper benchmarks the implementation against several
    counterparts, including libc++'s (which is identical to the current
    implementation).  The results, shown in Figure 4, indicate the new algorithm is
    2.2 times faster than the current one.
    
    The patch adds a test which loops through all integers in [-12687428, 11248737],
    and for each of them, gets the corresponding date and compares the result
    against its expected value.  The latter is calculated using a much simpler and
    easy to understand algorithm but which is also much slower.
    
    The interval used in the test covers the full range of values for which a
    roundtrip must work [time.cal.ymd.members].  Despite its completeness the test
    runs in a matter of seconds.
    
    libstdc++-v3/ChangeLog:
    
            * include/std/chrono (year_month_day::_S_from_days): New
            implementation.
            * testsuite/std/time/year_month_day/3.cc: New test.

Diff:
---
 libstdc++-v3/include/std/chrono                    | 49 +++++++++++----
 .../testsuite/std/time/year_month_day/3.cc         | 71 ++++++++++++++++++++++
 2 files changed, 107 insertions(+), 13 deletions(-)

diff --git a/libstdc++-v3/include/std/chrono b/libstdc++-v3/include/std/chrono
index 7840099d743..085e487a7ed 100644
--- a/libstdc++-v3/include/std/chrono
+++ b/libstdc++-v3/include/std/chrono
@@ -2429,22 +2429,45 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       // TODO: Implement operator<<, from_stream.
     };
 
-    // Construct from days since 1970/01/01. Magic.
+    // Construct from days since 1970/01/01.
+    // Proposition 6.3 of Neri and Schneider,
+    // "Euclidean Affine Functions and Applications to Calendar Algorithms".
+    // https://arxiv.org/abs/2102.06959
     constexpr year_month_day
     year_month_day::_S_from_days(const days& __dp) noexcept
     {
-      const auto __z = __dp.count() + 719468;
-      const auto __era = (__z >= 0 ? __z : __z - 146096) / 146097;
-      const auto __doe = static_cast<unsigned>(__z - __era * 146097);
-      const auto __yoe
-	= (__doe - __doe / 1460 + __doe / 36524 - __doe / 146096) / 365;
-      const auto __y = static_cast<days::rep>(__yoe) + __era * 400;
-      const auto __doy = __doe - (365 * __yoe + __yoe / 4 - __yoe / 100);
-      const auto __mp = (5 * __doy + 2) / 153;
-      const auto __d = __doy - (153 * __mp + 2) / 5 + 1;
-      const auto __m = __mp < 10 ? __mp + 3 : __mp - 9;
-      return year_month_day{chrono::year(__y + (__m <= 2)),
-			    chrono::month(__m), chrono::day(__d)};
+      constexpr auto __z2    = static_cast<uint32_t>(-1468000);
+      constexpr auto __r2_e3 = static_cast<uint32_t>(536895458);
+
+      const auto __r0 = __dp.count() + __r2_e3;
+
+      const auto __n1 = 4 * __r0 + 3;
+      const auto __q1 = __n1 / 146097;
+      const auto __r1 = __n1 % 146097 / 4;
+
+      constexpr auto __p32 = static_cast<uint64_t>(1) << 32;
+      const auto __n2 = 4 * __r1 + 3;
+      const auto __u2 = static_cast<uint64_t>(2939745) * __n2;
+      const auto __q2 = static_cast<uint32_t>(__u2 / __p32);
+      const auto __r2 = static_cast<uint32_t>(__u2 % __p32) / 2939745 / 4;
+
+      constexpr auto __p16 = static_cast<uint32_t>(1) << 16;
+      const auto __n3 = 2141 * __r2 + 197913;
+      const auto __q3 = __n3 / __p16;
+      const auto __r3 = __n3 % __p16 / 2141;
+
+      const auto __y0 = 100 * __q1 + __q2;
+      const auto __m0 = __q3;
+      const auto __d0 = __r3;
+
+      const auto __j  = __r2 >= 306;
+      const auto __y1 = __y0 + __j;
+      const auto __m1 = __j ? __m0 - 12 : __m0;
+      const auto __d1 = __d0 + 1;
+
+      return year_month_day{chrono::year{__y1 + __z2}, chrono::month{__m1},
+			    chrono::day{__d1}
+      };
     }
 
     // Days since 1970/01/01. Magic.
diff --git a/libstdc++-v3/testsuite/std/time/year_month_day/3.cc b/libstdc++-v3/testsuite/std/time/year_month_day/3.cc
new file mode 100644
index 00000000000..6e6a61d9163
--- /dev/null
+++ b/libstdc++-v3/testsuite/std/time/year_month_day/3.cc
@@ -0,0 +1,71 @@
+// { dg-options "-std=gnu++2a" }
+// { dg-do run { target c++2a } }
+
+// Copyright (C) 2021 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// Class year_month_day [time.cal.year_month_day]
+
+#include <chrono>
+#include <testsuite_hooks.h>
+
+// Slow but very clear way of advancing one day.
+constexpr void
+advance(std::chrono::year_month_day& ymd) noexcept {
+
+  using namespace std::chrono;
+
+  auto y = ymd.year();
+  auto m = ymd.month();
+  auto d = ymd.day();
+
+  if (d != year_month_day_last{year{y}, month_day_last{m}}.day())
+    ++d;
+  else {
+    d = day{1};
+    if (m != December)
+      ++m;
+    else {
+      m = January;
+      ++y;
+    }
+  }
+  ymd = year_month_day{y, m, d};
+}
+
+void test01()
+{
+  using namespace std::chrono;
+
+  // [-12687428, 11248737] maps to [-32767y/January/1d, 32767y/December/31d]
+
+  auto n   = days{-12687428};
+  auto ymd = -32767y/January/1d;
+  while (n < days{11248737}) {
+    VERIFY( year_month_day{sys_days{n}} == ymd );
+    ++n;
+    advance(ymd);
+  }
+  // One more for n = 11248737 and ymd = 32767y/December/31d
+  VERIFY( 32767y/December/31d == year_month_day{sys_days{days{11248737}}} );
+}
+
+int main()
+{
+  test01();
+  return 0;
+}