Merge pull request #1298 from valerioedu/selu-lambda-clean

Fix SeLU lambda precision (Follow-up to #1287)

Author: jmitrevs

hls4ml/templates/vivado/nnet_utils/nnet_activation.h

@@ -717,19 +717,27 @@ template <class data_T, class res_T, typename CONFIG_T> void selu(data_T data[CO
         initialized = true;
     }
 
-    #pragma HLS PIPELINE
+    typedef ap_ufixed<16, 1> selu_const_t;
+    static const selu_const_t lambda = 1.0507009873554805;
 
-    data_T datareg;
-    // Index into the lookup table based on data
-    int index;
+    #pragma HLS PIPELINE
     for (int ii = 0; ii < CONFIG_T::n_in; ii++) {
-        datareg = data[ii];
+        data_T datareg = data[ii];
+
         if (datareg >= 0) {
-            res[ii] = res_T(1.0507009873554804934193349852946) * datareg;
+            // Positive branch  y = λ · x
+            res[ii] = lambda * datareg;
         } else {
-            index = datareg * CONFIG_T::table_size / -8;
-            if (index > CONFIG_T::table_size - 1)
+            // Negative branch  y = table(x)
+            int index = datareg * CONFIG_T::table_size / -8;
+
+            // clamp index to [0, table_size-1]
+            if (index < 0)
+                index = 0;
+            else if (index > CONFIG_T::table_size - 1) {
                 index = CONFIG_T::table_size - 1;
+            }
+
             res[ii] = selu_table[index];
         }
     }