refine im2col

paddle/operators/math/im2col.cc

@@ -61,19 +61,10 @@ class Im2ColFunctor<paddle::operators::math::ColFormat::kCFO,
 
     const T* im_data = im.data<T>();
     T* col_data = col->data<T>();
-    int w_offset = -1;
-    int h_offset = 0;
-    int c_im = 0;
     for (int c = 0; c < channels_col; ++c) {
-      ++w_offset;
-      if (w_offset == filter_width) {
-        w_offset = 0;
-        ++h_offset;
-        if (h_offset == filter_height) {
-          h_offset = 0;
-          ++c_im;
-        }
-      }
+      int w_offset = c % filter_width;
+      int h_offset = (c / filter_width) % filter_height;
+      int c_im = c / (filter_width * filter_height);
       for (int h = 0; h < col_height; ++h) {
         int im_row_idx = h * stride[0] - padding[0] + h_offset * dilation[0];
         for (int w = 0; w < col_width; ++w) {