Added a markdown file to keep track of uses of xlow (for RRGraphView)

Signed-off-by: Ethan Rogers <[email protected]>

Author: ethanroj23

doc/rr_graph_view/uses_of_xlow.md

@@ -0,0 +1,85 @@
+## Uses of xlow, xhigh, ylow, yhigh
+
+| Function | Use Scenario | One Usage (Location) | Count |
+| --       | --           | --       | -- |
+| xlow | retrieve physical tile from device_ctx.grid | test_fasm.cpp:193 | 16 |
+| xlow | Obtain length of wire | stats.cpp:313 | 3 |
+| xlow | Determine switchpoint | draw.cpp:1475 | 1 |
+| xlow | Draw from one edge to the other | draw.cpp:1934 | 1 |
+| xlow | Create bounding box | draw.cpp:2083 | 1 |
+| xlow | Check if nodes are adjacent | check_route.cpp:481 | 1 |
+| xlow | Check if node is initialized | check_route.cpp:64 | 3 |
+| xlow | Include x value in console output | check_route.cpp:258 | 5 |
+| xlow | Create a serial num for the wire  x value in console output | check_route.cpp:198 | 3 |
+| xlow | Count cblocks | rr_graph_area.cpp:229 | 3 |
+| xlow | Return the segment number (distance along the channel) of the connection box from from_rr_type (CHANX or CHANY) to to_node (IPIN) | rr_graph_util.cpp:21 | 1 |
+| xlow | Check direction going from one node to another| rr_graph_util.cpp:45 | 1 |
+| xlow | Write Coordinates | VprTimingGraphResolver.cpp:345 | 1 |
+<br><br><br>
+
+## Retrieve Physical Tile from device_ctx.grid
+```cpp
+static std::string get_pin_feature (size_t inode) {
+    auto& device_ctx = g_vpr_ctx.device();
+
+    // Get tile physical tile and the pin number
+    int ilow = device_ctx.rr_nodes[inode].xlow(); // <----------------------------- Use of xlow()
+    int jlow = device_ctx.rr_nodes[inode].ylow();
+    auto physical_tile = device_ctx.grid[ilow][jlow].type;
+    int pin_num = device_ctx.rr_nodes[inode].ptc_num();
+
+    // Get the sub tile (type, not instance) and index of its pin that matches
+    // the node index.
+    const t_sub_tile* sub_tile_type = nullptr;
+    int sub_tile_pin = -1;
+
+    for (auto& sub_tile : physical_tile->sub_tiles) {
+        auto max_inst_pins = sub_tile.num_phy_pins / sub_tile.capacity.total();
+        for (int pin = 0; pin < sub_tile.num_phy_pins; pin++) {
+            if (sub_tile.sub_tile_to_tile_pin_indices[pin] == pin_num) {
+                sub_tile_type = &sub_tile;
+                sub_tile_pin  = pin % max_inst_pins;
+                break;
+            }
+        }
+
+        if (sub_tile_type != nullptr) {
+            break;
+        }
+    }
+```
+
+<br><br><br>  
+
+## Obtain length of wire
+```cpp
+while (tptr != nullptr) {
+        inode = tptr->index;
+        curr_type = device_ctx.rr_nodes[inode].type();
+
+        if (curr_type == SINK) { /* Starting a new segment */
+            tptr = tptr->next;   /* Link to existing path - don't add to len. */
+            if (tptr == nullptr)
+                break;
+
+            curr_type = device_ctx.rr_nodes[tptr->index].type();
+        }
+
+        else if (curr_type == CHANX || curr_type == CHANY) {
+            segments++;
+            length += 1 + device_ctx.rr_nodes[inode].xhigh() - device_ctx.rr_nodes[inode].xlow() // <----------------------------- Use of xlow()
+                      + device_ctx.rr_nodes[inode].yhigh() - device_ctx.rr_nodes[inode].ylow();
+
+            if (curr_type != prev_type && (prev_type == CHANX || prev_type == CHANY))
+                bends++;
+        }
+
+        prev_type = curr_type;
+        tptr = tptr->next;
+    }
+
+    *bends_ptr = bends;
+    *len_ptr = length;
+    *segments_ptr = segments;
+}
+```