Updated some comments related to reading in constraints file

Author: sfkhalid

vpr/src/base/read_place.cpp

@@ -139,8 +139,11 @@ void read_place(const char* net_file,
     place_ctx.placement_id = vtr::secure_digest_file(place_file);
 }
 
+/** Reads in blocks whose locations are specified in a constraints file. Constraint file is in .place format.
+ * All blocks specified in this file will be locked down at their specified x, y, subtile locations.
+ * (Will not be moved by optimizers during placement
+ */
 void read_user_block_loc(const char* constraints_file) {
-    /* Reads in the locations of the blocks from a file. */
     t_hash **hash_table, *h_ptr;
     int xtmp, ytmp;
     FILE* fp;

vpr/src/place/initial_placement.cpp

@@ -321,8 +321,12 @@ static void initial_placement_blocks(std::vector<std::vector<int>>& free_locatio
     }
 
     for (auto blk_id : sorted_blocks) {
-        if (place_ctx.block_locs[blk_id].loc.x != -1) { // -1 is a sentinel for an empty block
-            // block placed.
+        /* -1 is a sentinel for a non-placed block, which the code in this routine will choose a location for.
+         * If the x value is not -1, we assume something else has already placed this block and we should leave it there.
+         * For example, if the user constrained it to a certain location, the block has already been placed.
+         */
+        if (place_ctx.block_locs[blk_id].loc.x != -1) {
+            //put VTR assert
             continue;
         }