Revert the specialization for scalar_logistic_op<float> introduced in:

https://bitbucket.org/eigen/eigen/commits/77b447c24e3344e43ff64eb932d4bb35a2db01ce


While providing a 50% speedup on Haswell+ processors, the large relative error outside [-18, 18] in this approximation causes problems, e.g., when computing gradients of activation functions like softplus in neural networks.

1 files changed, 2 insertions, 71 deletions

diff --git a/Eigen/src/Core/functors/UnaryFunctors.h b/Eigen/src/Core/functors/UnaryFunctors.h
index dd256d16d..cf19c5ce9 100644
--- a/Eigen/src/Core/functors/UnaryFunctors.h
+++ b/Eigen/src/Core/functors/UnaryFunctors.h
@@ -905,83 +905,14 @@ struct scalar_logistic_op {
   }
 };
 
-/** \internal
-  * \brief Template specialization of the logistic function for float.
-  *
-  *  Uses just a 9/10-degree rational interpolant which
-  *  interpolates 1/(1+exp(-x)) - 0.5 up to a couple of ulp in the range
-  *  [-18, 18], outside of which the fl(logistic(x)) = {0|1}. The shifted
-  *  logistic is interpolated because it was easier to make the fit converge.
-  *
-  */
-
-template <>
-struct scalar_logistic_op<float> {
-  EIGEN_EMPTY_STRUCT_CTOR(scalar_logistic_op)
-  EIGEN_DEVICE_FUNC EIGEN_STRONG_INLINE float operator()(const float& x) const {
-    if (x < -18.0f) return 0.0f;
-    else if (x > 18.0f) return 1.0f;
-    else return 1.0f / (1.0f + numext::exp(-x));
-  }
-
-  template <typename Packet> EIGEN_DEVICE_FUNC EIGEN_STRONG_INLINE
-  Packet packetOp(const Packet& _x) const {
-    // Clamp the inputs to the range [-18, 18] since anything outside
-    // this range is 0.0f or 1.0f in single-precision.
-    const Packet x = pmax(pmin(_x, pset1<Packet>(18.0)), pset1<Packet>(-18.0));
-
-    // The monomial coefficients of the numerator polynomial (odd).
-    const Packet alpha_1 = pset1<Packet>(2.48287947061529e-01);
-    const Packet alpha_3 = pset1<Packet>(8.51377133304701e-03);
-    const Packet alpha_5 = pset1<Packet>(6.08574864600143e-05);
-    const Packet alpha_7 = pset1<Packet>(1.15627324459942e-07);
-    const Packet alpha_9 = pset1<Packet>(4.37031012579801e-11);
-
-    // The monomial coefficients of the denominator polynomial (even).
-    const Packet beta_0 = pset1<Packet>(9.93151921023180e-01);
-    const Packet beta_2 = pset1<Packet>(1.16817656904453e-01);
-    const Packet beta_4 = pset1<Packet>(1.70198817374094e-03);
-    const Packet beta_6 = pset1<Packet>(6.29106785017040e-06);
-    const Packet beta_8 = pset1<Packet>(5.76102136993427e-09);
-    const Packet beta_10 = pset1<Packet>(6.10247389755681e-13);
-
-    // Since the polynomials are odd/even, we need x^2.
-    const Packet x2 = pmul(x, x);
-
-    // Evaluate the numerator polynomial p.
-    Packet p = pmadd(x2, alpha_9, alpha_7);
-    p = pmadd(x2, p, alpha_5);
-    p = pmadd(x2, p, alpha_3);
-    p = pmadd(x2, p, alpha_1);
-    p = pmul(x, p);
-
-    // Evaluate the denominator polynomial p.
-    Packet q = pmadd(x2, beta_10, beta_8);
-    q = pmadd(x2, q, beta_6);
-    q = pmadd(x2, q, beta_4);
-    q = pmadd(x2, q, beta_2);
-    q = pmadd(x2, q, beta_0);
-
-    // Divide the numerator by the denominator and shift it up.
-    return pmax(pmin(padd(pdiv(p, q), pset1<Packet>(0.5)), pset1<Packet>(1.0)),
-                pset1<Packet>(0.0));
-  }
-};
-
 template <typename T>
 struct functor_traits<scalar_logistic_op<T> > {
   enum {
     Cost = scalar_div_cost<T, packet_traits<T>::HasDiv>::value +
-           (internal::is_same<T, float>::value
-                ? NumTraits<T>::AddCost * 12 + NumTraits<T>::MulCost * 11
-                : NumTraits<T>::AddCost * 2 +
-                      functor_traits<scalar_exp_op<T> >::Cost),
+        NumTraits<T>::AddCost * 2 + functor_traits<scalar_exp_op<T> >::Cost,
     PacketAccess =
         packet_traits<T>::HasAdd && packet_traits<T>::HasDiv &&
-        (internal::is_same<T, float>::value
-             ? packet_traits<T>::HasMul && packet_traits<T>::HasMax &&
-                   packet_traits<T>::HasMin
-             : packet_traits<T>::HasNegate && packet_traits<T>::HasExp)
+        packet_traits<T>::HasNegate && packet_traits<T>::HasExp
   };
 };