Scale HPLW cost function by sqrt(n).

The HPWL cost function under-estimates net cost for nets with fan-out greater
than 3. The error grows at a rate proportional to sqrt(n). By scaling the cost
by this amount, the error is reduced for larger-fan-out nets.

README.md

@@ -21,7 +21,7 @@ The C library is tested using using the
 [check](http://libcheck.github.io/check/) library. The test suite can be built
 using the following command:
 
-	$ gcc -std=c99 -g -o run_tests -Irig_c_sa tests/*.c rig_c_sa/*.c -lm $(pkg-config --cflags --libs check)
+	$ gcc -std=c99 -g -o run_tests -Irig_c_sa tests/*.c rig_c_sa/sa.c -lm $(pkg-config --cflags --libs check)
 
 The test suite should then be run under valgrind to ensure any memory leaks are found:
 

rig_c_sa/sa.c

@@ -531,7 +531,7 @@ double sa_get_net_cost(sa_state_t *state, sa_net_t *net) {
 		// From this we can work out the bounding box size and thus the HPWL.
 		bbox_width = (int)state->width - max_delta_x;
 		bbox_height = (int)state->height - max_delta_y;
-		return (bbox_width + bbox_height) * net->weight;
+		return sqrt(net->num_vertices) * (bbox_width + bbox_height) * net->weight;
 	} else {
 		// Non-toriodal network: Compute bounding box
 		min_x = net->vertices[0]->x;
@@ -550,7 +550,7 @@ double sa_get_net_cost(sa_state_t *state, sa_net_t *net) {
 		}
 
 		// Compute weighted HPWL
-		return ((max_x - min_x) + (max_y - min_y)) * net->weight;
+		return sqrt(net->num_vertices) * ((max_x - min_x) + (max_y - min_y)) * net->weight;
 	}
 }
 

tests/test_sa_algorithm.c

@@ -121,14 +121,14 @@ START_TEST (test_get_net_cost)
 
 	// Without wrap-around we have a 17x8 boundingbox at weight 2.0.
 	s->has_wrap_around_links = false;
-	ck_assert_msg(sa_get_net_cost(s, n) == (17.0 + 8.0) * 2.0,
-	              "%f != %f", sa_get_net_cost(s, n), (17.0 + 8.0) * 2.0);
+	ck_assert_msg(sa_get_net_cost(s, n) == sqrt(4) * (17.0 + 8.0) * 2.0,
+	              "%f != %f", sa_get_net_cost(s, n), sqrt(4) * (17.0 + 8.0) * 2.0);
 
 	// With wrap-around the bounding box wraps giving a 8x4 bounding box at
 	// weight 2.0
 	s->has_wrap_around_links = true;
-	ck_assert_msg(sa_get_net_cost(s, n) == (8.0 + 4.0) * 2.0,
-	              "%f != %f", sa_get_net_cost(s, n), (8.0 + 4.0) * 2.0);
+	ck_assert_msg(sa_get_net_cost(s, n) == sqrt(4) * (8.0 + 4.0) * 2.0,
+	              "%f != %f", sa_get_net_cost(s, n), sqrt(4) * (8.0 + 4.0) * 2.0);
 
 	/* Set alternative vertex positions. Note that:
 	 * * The wrapping and non-wrapping bounding box is the same
@@ -153,11 +153,11 @@ START_TEST (test_get_net_cost)
 
 	// Without wrap-around we have a 2x2 boundingbox at weight 2.0.
 	s->has_wrap_around_links = false;
-	ck_assert_msg(sa_get_net_cost(s, n) == (2.0 + 2.0) * 2.0,
-	              "%f != %f", sa_get_net_cost(s, n), (2.0 + 2.0) * 2.0);
+	ck_assert_msg(sa_get_net_cost(s, n) == sqrt(4) * (2.0 + 2.0) * 2.0,
+	              "%f != %f", sa_get_net_cost(s, n), sqrt(4) * (2.0 + 2.0) * 2.0);
 	s->has_wrap_around_links = true;
-	ck_assert_msg(sa_get_net_cost(s, n) == (2.0 + 2.0) * 2.0,
-	              "%f != %f", sa_get_net_cost(s, n), (2.0 + 2.0) * 2.0);
+	ck_assert_msg(sa_get_net_cost(s, n) == sqrt(4) * (2.0 + 2.0) * 2.0,
+	              "%f != %f", sa_get_net_cost(s, n), sqrt(4) * (2.0 + 2.0) * 2.0);
 
 	sa_free(s);
 }
@@ -207,7 +207,8 @@ START_TEST (test_get_swap_cost)
 
 	// Nets x and y should have their cost reduced by 1 for a total cost saving
 	// of 2.
-	ck_assert(sa_get_swap_cost(s, 0, 0, va, 1, 0, vb) == -2.0);
+	double swap_cost = sa_get_swap_cost(s, 0, 0, va, 1, 0, vb);
+	ck_assert(abs(swap_cost - (sqrt(2) * -2.0)) < 0.001);
 
 	sa_free(s);
 }
@@ -344,7 +345,7 @@ START_TEST (test_step_bad_cost)
 			num_swapped++;
 
 			// Cost should have increased
-			ck_assert(cost == 1.0);
+			ck_assert(cost == sqrt(2) * 1.0);
 
 			// Vertices should have moved
 			ck_assert(sa_get_chip_vertex(s, 0, 0) == NULL);
@@ -451,7 +452,7 @@ START_TEST (test_run_steps)
 	ck_assert(cost_delta_sd < 1.0);
 
 	// The cost change overall should drop from 6 to 1.
-	ck_assert_msg(cost_delta == -5.0, "%f == %f", cost_delta, -5.0);
+	ck_assert_msg(abs(cost_delta - (sqrt(2) * -5.0)) < 0.001, "%f == %f", cost_delta, sqrt(2) * -5.0);
 
 	sa_free(s);
 }