Fix std::midpoint for denormal values

Backport from mainline
2019-06-25  Jonathan Wakely  <jwakely@redhat.com>

	* include/std/numeric (midpoint(T, T)): Avoid std::abs in constexpr
	function.

Backport from mainline
2019-06-24  Jonathan Wakely  <jwakely@redhat.com>

	* include/std/numeric (midpoint(T, T)): Change implementation for
	floating-point types to avoid incorrect rounding of denormals.
	* testsuite/26_numerics/midpoint/floating.cc: Add check for correct
	rounding with denormals.
	* testsuite/26_numerics/gcd/gcd_neg.cc: Adjust dg-error line numbers.
	* testsuite/26_numerics/lcm/lcm_neg.cc: Likewise.

From-SVN: r273829

diff --git a/libstdc++-v3/ChangeLog b/libstdc++-v3/ChangeLog
index c75886da5a205c614e048525ec0225ce02d6809d..de7b156d9f5b5057345bb1fe49acd7df0fd58c54 100644 (file)
--- a/libstdc++-v3/ChangeLog
+++ b/libstdc++-v3/ChangeLog
@@ -1,3 +1,21 @@
+2019-07-26  Jonathan Wakely  <jwakely@redhat.com>
+
+       Backport from mainline
+       2019-06-25  Jonathan Wakely  <jwakely@redhat.com>
+
+       * include/std/numeric (midpoint(T, T)): Avoid std::abs in constexpr
+       function.
+
+       Backport from mainline
+       2019-06-24  Jonathan Wakely  <jwakely@redhat.com>
+
+       * include/std/numeric (midpoint(T, T)): Change implementation for
+       floating-point types to avoid incorrect rounding of denormals.
+       * testsuite/26_numerics/midpoint/floating.cc: Add check for correct
+       rounding with denormals.
+       * testsuite/26_numerics/gcd/gcd_neg.cc: Adjust dg-error line numbers.
+       * testsuite/26_numerics/lcm/lcm_neg.cc: Likewise.
+
 2019-07-03  Jonathan Wakely  <jwakely@redhat.com>
 
        PR libstdc++/91067

diff --git a/libstdc++-v3/include/std/numeric b/libstdc++-v3/include/std/numeric
index 3babfaa81079e28720aaba24d03ceec561b669b2..bef823bf5267e5cb8fe6db88110ea477ccaf5c4a 100644 (file)
--- a/libstdc++-v3/include/std/numeric
+++ b/libstdc++-v3/include/std/numeric
@@ -69,9 +69,8 @@
  * @defgroup numerics Numerics
  *
  * Components for performing numeric operations. Includes support for
- * for complex number types, random number generation, numeric
- * (n-at-a-time) arrays, generalized numeric algorithms, and special
- * math functions.
+ * complex number types, random number generation, numeric (n-at-a-time)
+ * arrays, generalized numeric algorithms, and mathematical special functions.
  */
 
 #if __cplusplus >= 201402L
@@ -156,11 +155,21 @@ namespace __detail
 
 #endif // C++17
 
+_GLIBCXX_END_NAMESPACE_VERSION
+} // namespace std
+
+#endif // C++14
+
 #if __cplusplus > 201703L
+#include <limits>
+
+namespace std _GLIBCXX_VISIBILITY(default)
+{
+_GLIBCXX_BEGIN_NAMESPACE_VERSION
   // midpoint
 # define __cpp_lib_interpolate 201902L
 
-template<typename _Tp>
+  template<typename _Tp>
     constexpr
     enable_if_t<__and_v<is_arithmetic<_Tp>, is_same<remove_cv_t<_Tp>, _Tp>,
                        __not_<is_same<_Tp, bool>>>,
@@ -182,11 +191,19 @@ template<typename _Tp>
            }
          return __a + __k * _Tp(_Up(__M - __m) / 2);
        }
-      else
+      else // is_floating
        {
-         return __builtin_isnormal(__a) && __builtin_isnormal(__b)
-           ? __a / 2 + __b / 2
-           : (__a + __b) / 2;
+         constexpr _Tp __lo = numeric_limits<_Tp>::min() * 2;
+         constexpr _Tp __hi = numeric_limits<_Tp>::max() / 2;
+         const _Tp __abs_a = __a < 0 ? -__a : __a;
+         const _Tp __abs_b = __b < 0 ? -__b : __b;
+         if (__abs_a <= __hi && __abs_b <= __hi) [[likely]]
+           return (__a + __b) / 2; // always correctly rounded
+         if (__abs_a < __lo) // not safe to halve __a
+           return __a + __b/2;
+         if (__abs_b < __lo) // not safe to halve __b
+           return __a/2 + __b;
+         return __a/2 + __b/2;     // otherwise correctly rounded
        }
     }
 
@@ -197,12 +214,10 @@ template<typename _Tp>
     {
       return __a  + (__b - __a) / 2;
     }
-#endif // C++20
-
 _GLIBCXX_END_NAMESPACE_VERSION
 } // namespace std
 
-#endif // C++14
+#endif // C++20
 
 #if __cplusplus > 201402L
 // Parallel STL algorithms

diff --git a/libstdc++-v3/testsuite/26_numerics/gcd/gcd_neg.cc b/libstdc++-v3/testsuite/26_numerics/gcd/gcd_neg.cc
index 87a74988fa466be80714e552e053ca1789962ba2..05e21431313f8745aa88ab5ed4f4f318fd7e4136 100644 (file)
--- a/libstdc++-v3/testsuite/26_numerics/gcd/gcd_neg.cc
+++ b/libstdc++-v3/testsuite/26_numerics/gcd/gcd_neg.cc
@@ -46,9 +46,9 @@ test01()
   std::gcd<const int&, const int&>(0.1, 0.1);   // { dg-error "from here" }
 }
 
+// { dg-error "integers" "" { target *-*-* } 133 }
 // { dg-error "integers" "" { target *-*-* } 134 }
-// { dg-error "integers" "" { target *-*-* } 135 }
-// { dg-error "not bools" "" { target *-*-* } 136 }
-// { dg-error "not bools" "" { target *-*-* } 138 }
+// { dg-error "not bools" "" { target *-*-* } 135 }
+// { dg-error "not bools" "" { target *-*-* } 137 }
 // { dg-prune-output "deleted function" }
 // { dg-prune-output "invalid operands" }

diff --git a/libstdc++-v3/testsuite/26_numerics/lcm/lcm_neg.cc b/libstdc++-v3/testsuite/26_numerics/lcm/lcm_neg.cc
index 4db01ae364745f59484889119fae878aec7705a1..3a0f5bbe3eb35bf6e906ad5bdee0e28d266b5220 100644 (file)
--- a/libstdc++-v3/testsuite/26_numerics/lcm/lcm_neg.cc
+++ b/libstdc++-v3/testsuite/26_numerics/lcm/lcm_neg.cc
@@ -46,9 +46,9 @@ test01()
   std::lcm<const int&, const int&>(0.1, 0.1);   // { dg-error "from here" }
 }
 
+// { dg-error "integers" "" { target *-*-* } 147 }
 // { dg-error "integers" "" { target *-*-* } 148 }
-// { dg-error "integers" "" { target *-*-* } 149 }
-// { dg-error "not bools" "" { target *-*-* } 150 }
-// { dg-error "not bools" "" { target *-*-* } 152 }
+// { dg-error "not bools" "" { target *-*-* } 149 }
+// { dg-error "not bools" "" { target *-*-* } 151 }
 // { dg-prune-output "deleted function" }
 // { dg-prune-output "invalid operands" }

diff --git a/libstdc++-v3/testsuite/26_numerics/midpoint/floating.cc b/libstdc++-v3/testsuite/26_numerics/midpoint/floating.cc
index 9c6e4113b1820243b4e015fbeaa17ac5675e9aa3..f09f586f7b8ea377d4826753af9bcfb981b465cf 100644 (file)
--- a/libstdc++-v3/testsuite/26_numerics/midpoint/floating.cc
+++ b/libstdc++-v3/testsuite/26_numerics/midpoint/floating.cc
@@ -20,6 +20,7 @@
 
 #include <numeric>
 #include <limits>
+#include <cfloat>
 #include <testsuite_hooks.h>
 
 void
@@ -57,6 +58,19 @@ test03()
   VERIFY( std::midpoint(9e9l, -9e9l) == 0.0l );
 }
 
+namespace test04
+{
+  // https://gcc.gnu.org/ml/libstdc++/2019-03/msg00065.html
+  constexpr double d = DBL_MIN + __DBL_DENORM_MIN__;
+  static_assert( std::midpoint(d, d) == d );
+
+  constexpr float f = FLT_MIN + __FLT_DENORM_MIN__;
+  static_assert( std::midpoint(f, f) == f );
+
+  constexpr long double l = LDBL_MIN + __LDBL_DENORM_MIN__;
+  static_assert( std::midpoint(l, l) == l );
+}
+
 int main()
 {
   test01();