fast tanh

Author: hlu1

topi/include/topi/elemwise.h

@@ -6,9 +6,9 @@
  * to you under the Apache License, Version 2.0 (the
  * "License"); you may not use this file except in compliance
  * with the License.  You may obtain a copy of the License at
- * 
+ *
  *   http://www.apache.org/licenses/LICENSE-2.0
- * 
+ *
  * Unless required by applicable law or agreed to in writing,
  * software distributed under the License is distributed on an
  * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
@@ -31,6 +31,7 @@
 #include "tvm/tvm.h"
 #include "tvm/ir.h"
 #include "tvm/ir_pass.h"
+#include "broadcast.h"
 
 namespace topi {
 using namespace tvm;
@@ -46,7 +47,6 @@ using namespace tvm;
   }
 
 TOPI_DECLARE_UNARY_OP(exp);
-TOPI_DECLARE_UNARY_OP(tanh);
 TOPI_DECLARE_UNARY_OP(sigmoid);
 TOPI_DECLARE_UNARY_OP(sqrt);
 TOPI_DECLARE_UNARY_OP(log);
@@ -56,6 +56,58 @@ TOPI_DECLARE_UNARY_OP(round);
 TOPI_DECLARE_UNARY_OP(trunc);
 TOPI_DECLARE_UNARY_OP(abs);
 
+/*!
+* \brief Creates an operation that returns hyperbolic tan of a given tensor
+*        Same as the fast_tanh_float implementation from Eigen
+*
+* \param in The input tensor
+* \param name The name of the operation
+* \param tag The tag to mark the operation
+*
+* \return A Tensor whose op member is tanh
+*/
+
+inline Tensor tanh(const Tensor& in,
+                   std::string name = "T_tanh",
+                   std::string tag = kElementWise) {
+  // Clamp the inputs to the range [-9, 9] since anything outside
+  // this range is +/-1.0f in single-precision.
+  auto x = maximum(minimum(in, make_const(in->dtype, 9.0)), make_const(in->dtype, -9.0));
+
+  // The monomial coefficients of the numerator polynomial (odd).
+  auto alpha_1 = make_const(in->dtype, 4.89352455891786e-03);
+  auto alpha_3 = make_const(in->dtype, 6.37261928875436e-04);
+  auto alpha_5 = make_const(in->dtype, 1.48572235717979e-05);
+  auto alpha_7 = make_const(in->dtype, 5.12229709037114e-08);
+  auto alpha_9 = make_const(in->dtype, -8.60467152213735e-11);
+  auto alpha_11 = make_const(in->dtype, 2.00018790482477e-13);
+  auto alpha_13 = make_const(in->dtype, -2.76076847742355e-16);
+
+  // The monomial coefficients of the denominator polynomial (even).
+  auto beta_0 = make_const(in->dtype, 4.89352518554385e-03);
+  auto beta_2 = make_const(in->dtype, 2.26843463243900e-03);
+  auto beta_4 = make_const(in->dtype, 1.18534705686654e-04);
+  auto beta_6 = make_const(in->dtype, 1.19825839466702e-06);
+
+  return compute(x->shape,
+                 [&](const Array<Var>& i) {
+                   auto x2 = x(i) * x(i);
+                   auto p = x2 * alpha_13 + alpha_11;
+                   p = x2 * p + alpha_9;
+                   p = x2 * p + alpha_7;
+                   p = x2 * p + alpha_5;
+                   p = x2 * p + alpha_3;
+                   p = x2 * p + alpha_1;
+                   p = x(i) * p;
+
+                   auto q = x2 * beta_6 + beta_4;
+                   q = x2 * q + beta_2;
+                   q = x2 * q + beta_0;
+                   return p / q;
+                 },
+                 name, tag);
+}
+
 /*!
 * \brief Creates an operation that returns identity of a given tensor
 *