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

[Rahman Lavaee via llvm-commits]

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@@ -699,28 +719,44 @@ class ExtTSPImpl {
     /// Deterministically compare pairs of chains.
     auto compareChainPairs = [](const ChainT *A1, const ChainT *B1,
                                 const ChainT *A2, const ChainT *B2) {
-      if (A1 != A2)
-        return A1->Id < A2->Id;
-      return B1->Id < B2->Id;
+      return std::make_tuple(A1->Id, B1->Id) < std::make_tuple(A2->Id, B2->Id);
     };
 
+    double PrevScore = 1e9;
     while (HotChains.size() > 1) {
       ChainT *BestChainPred = nullptr;
       ChainT *BestChainSucc = nullptr;
       MergeGainT BestGain;
       // Iterate over all pairs of chains.
       for (ChainT *ChainPred : HotChains) {
+        // Since the score of merging doesn't increase, we can stop early when
+        // the newly found merge is as good as the previous one.
+        if (BestGain.score() == PrevScore)
+          break;
         // Get candidates for merging with the current chain.
         for (const auto &[ChainSucc, Edge] : ChainPred->Edges) {
           // Ignore loop edges.
-          if (ChainPred == ChainSucc)
+          if (Edge->isSelfEdge())
             continue;
-
           // Stop early if the combined chain violates the maximum allowed size.
           if (ChainPred->numBlocks() + ChainSucc->numBlocks() >= MaxChainSize)
             continue;
+          // Don't merge the chains if they have vastly different densities.
+          // We stop early if the ratio between the densities exceeds
+          // MaxMergeDensityRatio. Smaller values of the option result in
+          // fewer merges (hence, more chains), which in turn typically yields
+          // smaller size of the hot code section.
+          double minDensity =
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rlavaee wrote:

Use `std::minmax`.

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