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Re: [PATCH] Implement "P0631R4 Math Constants" for C++20


On 31/07/19 19:07 +0200, Marc Glisse wrote:
On Wed, 31 Jul 2019, Jonathan Wakely wrote:

The values of the constants are taken from Glibc where the equivalent
constant exists, or by rounding the actual constant to the same number
of digits as the Glibc constants have.

How does it behave with __float128? I think that with -std=gnu++2a, it counts as a floating point type, but the constant first becomes a long double and thus loses precision.

Indeed it does:

#if !defined(__STRICT_ANSI__) && defined(_GLIBCXX_USE_FLOAT128)
 template<>
   struct __is_floating_point_helper<__float128>
   : public true_type { };
#endif

We could add explicit specializations for __float128 and use the Q
suffix. That's not valid when __STRICT_ANSI__ is defined (unless -fext-numeric-literals is also used) but as the snippet above shows,
__float128 isn't a floating-point type when __STRICT_ANSI__ is
defined.

So something like the attached patch.  The glibc <math.h> says the
values I used have enough digits for IEEE quad-precision:

/* The above constants are not adequate for computation using `long double's.
  Therefore we provide as an extension constants with similar names as a
  GNU extension.  Provide enough digits for the 128-bit IEEE quad.  */

I don't know if we need more accuracy for IBM double double.


diff --git a/libstdc++-v3/include/std/numbers b/libstdc++-v3/include/std/numbers
index b8e38dd8080..314b130fbd5 100644
--- a/libstdc++-v3/include/std/numbers
+++ b/libstdc++-v3/include/std/numbers
@@ -133,6 +133,72 @@ namespace numbers
   inline constexpr double egamma = egamma_v<double>;
   inline constexpr double phi = phi_v<double>;
 
+#if !defined(__STRICT_ANSI__) && defined(_GLIBCXX_USE_FLOAT128)
+  template<>
+    inline constexpr __float128 e_v<__float128>
+      = 2.718281828459045235360287471352662498Q;
+
+  /// log_2 e
+  template<>
+    inline constexpr __float128 log2e_v<__float128>
+      = 1.442695040888963407359924681001892137Q;
+
+  /// log_10 e
+  template<>
+    inline constexpr __float128 log10e_v<__float128>
+      = 0.434294481903251827651128918916605082Q;
+
+  /// pi
+  template<>
+    inline constexpr __float128 pi_v<__float128>
+      = 3.141592653589793238462643383279502884Q;
+
+  /// 1/pi
+  template<>
+    inline constexpr __float128 inv_pi_v<__float128>
+      = 0.318309886183790671537767526745028724Q;
+
+  /// 1/sqrt(pi)
+  template<>
+    inline constexpr __float128 inv_sqrtpi_v<__float128>
+      = 0.564189583547756286948079451560772586Q;
+
+  /// log_e 2
+  template<>
+    inline constexpr __float128 ln2_v<__float128>
+      = 0.693147180559945309417232121458176568Q;
+
+  /// log_e 10
+  template<>
+    inline constexpr __float128 ln10_v<__float128>
+      = 2.302585092994045684017991454684364208Q;
+
+  /// sqrt(2)
+  template<>
+    inline constexpr __float128 sqrt2_v<__float128>
+      = 1.414213562373095048801688724209698079Q;
+
+  /// sqrt(3)
+  template<>
+    inline constexpr __float128 sqrt3_v<__float128>
+      = 1.732050807568877293527446341505872367Q;
+
+  /// 1/sqrt(3)
+  template<>
+    inline constexpr __float128 inv_sqrt3_v<__float128>
+      = 0.577350269189625764509148780501957456Q;
+
+  /// The Euler-Mascheroni constant
+  template<>
+    inline constexpr __float128 egamma_v<__float128>
+      = 0.577215664901532860606512090082402431Q;
+
+  /// The golden ratio, (1+sqrt(5))/2
+  template<>
+    inline constexpr __float128 phi_v<__float128>
+      = 1.618033988749894848204586834365638118Q;
+#endif // USE_FLOAT128
+
 } // namespace numbers
 /// @}
 _GLIBCXX_END_NAMESPACE_VERSION

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