reuse copy size

Author: tensor-tang

paddle/fluid/operators/math/im2col.cc

@@ -54,14 +54,15 @@ class Im2ColFunctor<paddle::operators::math::ColFormat::kCFO,
     if (stride[0] == 1 && stride[1] == 1 && dilation[0] == 1 &&
         dilation[1] == 1 && padding[0] == 0 && padding[1] == 0) {
       int col_matrix_width = output_width * output_height;
+      size_t copy_size = sizeof(T) * output_width;
       for (int oh = 0; oh < output_height; ++oh) {
         const T* im_data_start = im_data + oh * im_width;
         T* dst_data = col_data + oh * output_width;
         for (int ic = 0; ic < im_channels; ++ic) {
           const T* src_data = im_data_start + ic * im_height * im_width;
           for (int kh = 0; kh < filter_height; ++kh) {
             for (int kw = 0; kw < filter_width; ++kw) {
-              std::memcpy(dst_data, src_data + kw, sizeof(T) * output_width);
+              std::memcpy(dst_data, src_data + kw, copy_size);
               dst_data = dst_data + col_matrix_width;
             }
             src_data = src_data + im_width;