Some cleanups in qwp

diff --git a/passes/cmds/qwp.cc b/passes/cmds/qwp.cc
index f76de326a29bcefce5359416ad23a93037827c81..784076c372453141b1f0a3705639b64435ad935c 100644 (file)
--- a/passes/cmds/qwp.cc
+++ b/passes/cmds/qwp.cc
@@ -203,8 +203,8 @@ struct QwpWorker
 
        void solve(bool alt_mode = false)
        {
-               // A := observation_matrix
-               // y := observation_rhs_vector
+               // A := constraint_matrix
+               // y := constraint_rhs_vector
                //
                // AA = A' * A
                // Ay = A' * y
@@ -215,6 +215,10 @@ struct QwpWorker
                int N = GetSize(nodes), N1 = N+1;
                vector<double> M(N * N1);
 
+               // Edge constraints:
+               //   A[i,:] := [ 0 0 .... 0 weight 0 ... 0 -weight 0 ... 0 0], y[i] := 0
+               //
+               // i.e. nonzero columns in A[i,:] at the two node indices.
                for (auto &edge : edges)
                {
                        int idx1 = edge.first.first;
@@ -228,6 +232,14 @@ struct QwpWorker
                        M[idx2 + idx1*N1] += -weight * weight;
                }
 
+               // Node constraints:
+               //   A[i,:] := [ 0 0 .... 0 weight 0 ... 0 0], y[i] := weight * pos
+               //
+               // i.e. nonzero column in A[i,:] at the node index
+               //
+               // "tied" nodes have a large weight, pinning them in position. Untied
+               // nodes have a small weight, giving then a tiny preference to stay at
+               // the current position, making sure that AA never is singular.
                for (int idx = 0; idx < GetSize(nodes); idx++)
                {
                        auto &node = nodes[idx];
@@ -453,7 +465,7 @@ struct QwpWorker
                config.dump_file << stringf("</svg>\n");
        }
 
-       void run_worker(int indent, bool return_after_solve = false)
+       void run_worker(int indent)
        {
                int count_cells = 0;
 
@@ -481,9 +493,6 @@ struct QwpWorker
                solve();
                solve(true);
 
-               if (return_after_solve)
-                       return;
-
                // detect median position and check for break condition
 
                vector<pair<double, int>> sorted_pos;
@@ -769,7 +778,7 @@ struct QwpPass : public Pass {
                log("    -dump <html_file_name>\n");
                log("        Dump a protocol of the placement algorithm to the html file.\n");
                log("\n");
-               log("Note: This implementation of a quadratic wirelength placer uses unoptimized\n");
+               log("Note: This implementation of a quadratic wirelength placer uses exact\n");
                log("dense matrix operations. It is only a toy-placer for small circuits.\n");
                log("\n");
        }