[PATCH] D50609: [LLD][ELF] - Simplify Call-Chain Clustering implementation a bit.

[George Rimar via Phabricator via llvm-commits]

grimar created this revision.
grimar added reviewers: ruiu, Bigcheese.
Herald added subscribers: arichardson, emaste.
Herald added a reviewer: espindola.

Looking at the current implementation and algorithm description,
it does not seem we need to keep vector with all edges for
each cluster and can just remember the best one.

At least with the current implementation, this is NFC change I believe.

This allows to simplify the code a bit, patch shows the change I am suggesting.


https://reviews.llvm.org/D50609

Files:
  ELF/CallGraphSort.cpp


Index: ELF/CallGraphSort.cpp
===================================================================
--- ELF/CallGraphSort.cpp
+++ ELF/CallGraphSort.cpp
@@ -72,7 +72,7 @@
   size_t Size = 0;
   uint64_t Weight = 0;
   uint64_t InitialWeight = 0;
-  std::vector<Edge> Preds;
+  Edge BestPred = {-1, 0};
 };
 
 class CallGraphSort {
@@ -136,8 +136,12 @@
     if (From == To)
       continue;
 
-    // Add an edge
-    Clusters[To].Preds.push_back({From, Weight});
+    // Remember the best edge.
+    Cluster& ToC = Clusters[To];
+    if (ToC.BestPred.From == -1 || ToC.BestPred.Weight < Weight) {
+      ToC.BestPred.From = From;
+      ToC.BestPred.Weight = Weight;
+    }
   }
   for (Cluster &C : Clusters)
     C.InitialWeight = C.Weight;
@@ -181,21 +185,11 @@
     // been merged into another cluster yet.
     Cluster &C = Clusters[SI];
 
-    int BestPred = -1;
-    uint64_t BestWeight = 0;
-
-    for (Edge &E : C.Preds) {
-      if (BestPred == -1 || E.Weight > BestWeight) {
-        BestPred = E.From;
-        BestWeight = E.Weight;
-      }
-    }
-
-    // don't consider merging if the edge is unlikely.
-    if (BestWeight * 10 <= C.InitialWeight)
+    // Don't consider merging if the edge is unlikely.
+    if (C.BestPred.From == -1 || C.BestPred.Weight * 10 <= C.InitialWeight)
       continue;
 
-    Cluster *PredC = SecToCluster[BestPred];
+    Cluster *PredC = SecToCluster[C.BestPred.From];
     if (PredC == &C)
       continue;
 


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