apply PR comments for drawing interposer

1) early exit
2) commenting

Author: soheilshahrouz

vpr/src/draw/draw_interposer.cpp

@@ -13,20 +13,33 @@ void draw_interposer_cuts(ezgl::renderer* g) {
     const DeviceGrid& grid = device_ctx.grid;
     const t_draw_coords* draw_coords = get_draw_coords_vars();
 
-    const std::vector<std::vector<int>>& horizontal_cuts = grid.get_horizontal_interposer_cuts();
-    const std::vector<std::vector<int>>& vertical_cuts = grid.get_vertical_interposer_cuts();
-
-    std::vector<std::pair<ezgl::point2d, ezgl::point2d>> lines_to_draw;
+    if (draw_state->pic_on_screen == e_pic_type::NO_PICTURE) {
+        return;
+    }
+    VTR_ASSERT_SAFE(draw_state->pic_on_screen == e_pic_type::PLACEMENT || draw_state->pic_on_screen == e_pic_type::ROUTING);
 
-    const ezgl::rectangle world{{draw_coords->tile_x.front() - draw_coords->get_tile_width(), draw_coords->tile_y.front() - draw_coords->get_tile_height()},
-                                {draw_coords->tile_x.back() + 2 * draw_coords->get_tile_width(), draw_coords->tile_y.back() + 2 * draw_coords->get_tile_height()}};
+    // A rectangle containing the FPGA fabric with some padding.
+    // Used to determine the start and end coordinates of an interposer cut whose x or y position is fixed.
+    const ezgl::rectangle world{
+        {draw_coords->tile_x.front() - draw_coords->get_tile_width(),
+         draw_coords->tile_y.front() - draw_coords->get_tile_height()},
+        {draw_coords->tile_x.back() + 2 * draw_coords->get_tile_width(),
+         draw_coords->tile_y.back() + 2 * draw_coords->get_tile_height()}
+    };
 
     g->set_color(ezgl::BLACK, 255);
     g->set_line_dash(ezgl::line_dash::asymmetric_5_3);
     g->set_line_width(2);
 
+    // Offset factor used to slightly shift the interposer cut lines away from tile boundaries.
+    // During placement, we draw the line in the middle of a routing channel.
+    // During routing, we draw the line closer to the tile that does not own the channel.
     const float offset_factor = draw_state->pic_on_screen == e_pic_type::PLACEMENT ? -0.5f : -0.5f / device_ctx.chan_width.max;
 
+    const std::vector<std::vector<int>>& horizontal_cuts = grid.get_horizontal_interposer_cuts();
+    const std::vector<std::vector<int>>& vertical_cuts = grid.get_vertical_interposer_cuts();
+    std::vector<std::pair<ezgl::point2d, ezgl::point2d>> lines_to_draw;
+
     for (size_t layer = 0; layer < grid.get_num_layers(); layer++) {
         if (!draw_state->draw_layer_display[layer].visible) {
             continue;
@@ -43,12 +56,8 @@ void draw_interposer_cuts(ezgl::renderer* g) {
         }
     }
 
-    if (draw_state->pic_on_screen == e_pic_type::PLACEMENT || draw_state->pic_on_screen == e_pic_type::ROUTING) {
-        for (const auto& [start, end] : lines_to_draw) {
-            g->draw_line(start, end);
-        }
-    } else {
-        VTR_ASSERT(draw_state->pic_on_screen == e_pic_type::NO_PICTURE);
+    for (const auto& [start, end] : lines_to_draw) {
+        g->draw_line(start, end);
     }
 }