[llvm] [CodeLayout] Faster basic block reordering, ext-tsp (PR #68617)

[Rahman Lavaee via llvm-commits]

================
@@ -836,35 +875,36 @@ class ExtTSPImpl {
     Gain.updateIfLessThan(
         computeMergeGain(ChainPred, ChainSucc, Jumps, 0, MergeTypeT::X_Y));
 
-    if (EnableChainSplitAlongJumps) {
-      // Attach (a part of) ChainPred before the first node of ChainSucc.
-      for (JumpT *Jump : ChainSucc->Nodes.front()->InJumps) {
-        const NodeT *SrcBlock = Jump->Source;
-        if (SrcBlock->CurChain != ChainPred)
-          continue;
-        size_t Offset = SrcBlock->CurIndex + 1;
-        tryChainMerging(Offset, {MergeTypeT::X1_Y_X2, MergeTypeT::X2_X1_Y});
-      }
+    // Attach (a part of) ChainPred before the first node of ChainSucc.
+    for (JumpT *Jump : ChainSucc->Nodes.front()->InJumps) {
+      const NodeT *SrcBlock = Jump->Source;
+      if (SrcBlock->CurChain != ChainPred)
+        continue;
+      size_t Offset = SrcBlock->CurIndex + 1;
+      tryChainMerging(Offset, {MergeTypeT::X1_Y_X2, MergeTypeT::X2_X1_Y});
+    }
 
-      // Attach (a part of) ChainPred after the last node of ChainSucc.
-      for (JumpT *Jump : ChainSucc->Nodes.back()->OutJumps) {
-        const NodeT *DstBlock = Jump->Target;
-        if (DstBlock->CurChain != ChainPred)
-          continue;
-        size_t Offset = DstBlock->CurIndex;
-        tryChainMerging(Offset, {MergeTypeT::X1_Y_X2, MergeTypeT::Y_X2_X1});
-      }
+    // Attach (a part of) ChainPred after the last node of ChainSucc.
+    for (JumpT *Jump : ChainSucc->Nodes.back()->OutJumps) {
+      const NodeT *DstBlock = Jump->Target;
+      if (DstBlock->CurChain != ChainPred)
+        continue;
+      size_t Offset = DstBlock->CurIndex;
+      tryChainMerging(Offset, {MergeTypeT::X1_Y_X2, MergeTypeT::Y_X2_X1});
     }
 
     // Try to break ChainPred in various ways and concatenate with ChainSucc.
-    if (ChainPred->Nodes.size() <= ChainSplitThreshold) {
-      for (size_t Offset = 1; Offset < ChainPred->Nodes.size(); Offset++) {
-        // Try to split the chain in different ways. In practice, applying
-        // X2_Y_X1 merging is almost never provides benefits; thus, we exclude
-        // it from consideration to reduce the search space.
-        tryChainMerging(Offset, {MergeTypeT::X1_Y_X2, MergeTypeT::Y_X2_X1,
-                                 MergeTypeT::X2_X1_Y});
-      }
+    // In practice, applying X2_Y_X1 merging is almost never provides benefits;
+    // thus, we exclude it from consideration to reduce the search space.
+    for (size_t Offset = 1; Offset < ChainPred->Nodes.size(); Offset++) {
+      // Do not split the chain along a jump.
+      const NodeT *BB = ChainPred->Nodes[Offset - 1];
+      const NodeT *BB2 = ChainPred->Nodes[Offset];
+      if (BB->isSuccessor(BB2))
----------------
rlavaee wrote:

What's the rationale for this? Sometimes a loop is rotated to improve overall ext-tsp score even though the chain is split along a jump.

https://github.com/llvm/llvm-project/pull/68617