Remove common expression out of the loop in reference_batchnorm_backward_nhwc_c

library/include/ck/library/reference_tensor_operation/cpu/reference_batchnorm_backward_nhwc_c.hpp

@@ -183,6 +183,10 @@ struct ReferenceBatchNormBwd_Input_N_H_W_C_Output_C
                 arg.p_dscale_[offset_C] = type_convert<ScaleDataType>(dscale);
                 arg.p_dbias_[offset_C]  = type_convert<BiasDataType>(dbias);
 
+                AccDataType scale = type_convert<AccDataType>(arg.p_scale_[offset_C]);
+                AccDataType multiplier =
+                    type_convert<AccDataType>(1.0f) / reduceSize * invVar * scale;
+
                 // 1) calculate tmp = dscale * (x - mean) * inv-variance
                 // 2) calculate dx = 1/nhw * inv-variance * scale * (nhw * dy - dbias - tmp)
                 for(index_t iN = 0; iN < arg.n_; iN++)
@@ -201,14 +205,12 @@ struct ReferenceBatchNormBwd_Input_N_H_W_C_Output_C
 
                             AccDataType norm_x = (x - mean) * invVar;
                             AccDataType dy     = type_convert<AccDataType>(arg.p_dy_[offset]);
-                            AccDataType scale  = type_convert<AccDataType>(arg.p_scale_[offset_C]);
 
                             arg.dy_elementwise_op_(dy, dy);
 
                             AccDataType tmpVal = norm_x * dscale;
 
-                            AccDataType dx = type_convert<AccDataType>(1.0f) / reduceSize * invVar *
-                                             scale * (reduceSize * dy - dbias - tmpVal);
+                            AccDataType dx = multiplier * (reduceSize * dy - dbias - tmpVal);
 
                             arg.p_dx_[offset] = type_convert<XDataType>(dx);
                         };