clusterlin: unidirectional MakeTopological initially (optimization)

diff --git a/src/cluster_linearize.h b/src/cluster_linearize.h
index 164ee0df..7e20f802 100644
--- a/src/cluster_linearize.h
+++ b/src/cluster_linearize.h
@@ -1129,6 +1129,16 @@ public:
     void MakeTopological() noexcept
     {
         Assume(m_suboptimal_chunks.empty());
+        /** What direction to initially merge chunks in; one of the two directions is enough. This
+         *  is sufficient because if a non-topological inactive dependency exists between two
+         *  chunks, at least one of the two chunks will eventually be processed in a direction that
+         *  discovers it - either the lower chunk tries upward, or the upper chunk tries downward.
+         *  Chunks that are the result of the merging are always tried in both directions. */
+        unsigned init_dir = m_rng.randbool();
+        /** Which chunks are the result of merging, and thus need merge attempts in both
+         *  directions. */
+        SetType merged_chunks;
+        // Mark chunks as suboptimal.
         m_suboptimal_idxs = m_chunk_idxs;
         for (auto chunk_idx : m_chunk_idxs) {
             m_suboptimal_chunks.emplace_back(chunk_idx);
@@ -1147,9 +1157,12 @@ public:
             // If what was popped is not currently a chunk, continue. This may
             // happen when it was merged with something else since being added.
             if (!m_chunk_idxs[chunk_idx]) continue;
+            /** What direction(s) to attempt merging in. 1=up, 2=down, 3=both. */
+            unsigned direction = merged_chunks[chunk_idx] ? 3 : init_dir + 1;
             int flip = m_rng.randbool();
             for (int i = 0; i < 2; ++i) {
                 if (i ^ flip) {
+                    if (!(direction & 1)) continue;
                     // Attempt to merge the chunk upwards.
                     auto result_up = MergeStep<false>(chunk_idx);
                     if (result_up != INVALID_SET_IDX) {
@@ -1157,9 +1170,11 @@ public:
                             m_suboptimal_idxs.Set(result_up);
                             m_suboptimal_chunks.push_back(result_up);
                         }
+                        merged_chunks.Set(result_up);
                         break;
                     }
                 } else {
+                    if (!(direction & 2)) continue;
                     // Attempt to merge the chunk downwards.
                     auto result_down = MergeStep<true>(chunk_idx);
                     if (result_down != INVALID_SET_IDX) {
@@ -1167,6 +1182,7 @@ public:
                             m_suboptimal_idxs.Set(result_down);
                             m_suboptimal_chunks.push_back(result_down);
                         }
+                        merged_chunks.Set(result_down);
                         break;
                     }
                 }
diff --git a/src/test/util/cluster_linearize.h b/src/test/util/cluster_linearize.h
index d5686068..3814c3fc 100644
--- a/src/test/util/cluster_linearize.h
+++ b/src/test/util/cluster_linearize.h
@@ -403,13 +403,13 @@ inline uint64_t MaxOptimalLinearizationIters(DepGraphIndex cluster_count)
     static constexpr uint64_t ITERS[65] = {
         0,
         0, 4, 10, 34, 76, 156, 229, 380,
-        432, 517, 678, 896, 1037, 1366, 1464, 1792,
-        2060, 2542, 3068, 3116, 4029, 3467, 5324, 5512,
-        6481, 7161, 7441, 8183, 8843, 9353, 11104, 11269,
-        11791, 11871, 13367, 14259, 14229, 12397, 13581, 18152,
-        18737, 16581, 23217, 24044, 29597, 29030, 34069, 34594,
-        33630, 26227, 34471, 38815, 40814, 31182, 40305, 34019,
-        36582, 55659, 39994, 41277, 42365, 52822, 59733, 67035
+        441, 517, 678, 933, 1037, 1366, 1464, 1711,
+        2111, 2542, 3068, 3116, 4029, 3467, 5324, 5402,
+        6481, 7161, 7441, 8183, 8843, 9353, 11104, 11455,
+        11791, 12570, 13480, 14259, 14525, 12426, 14477, 20201,
+        18737, 16581, 23622, 28486, 30652, 33021, 32942, 32745,
+        34046, 26227, 34662, 38019, 40814, 31113, 41448, 33968,
+        35024, 59207, 42872, 41277, 42365, 51833, 63410, 67035
     };
     assert(cluster_count < std::size(ITERS));
     // Multiply the table number by two, to account for the fact that they are not absolutes.