[lld] r340806 - [LLD][ELF] - Simplify Call-Chain Clustering implementation a bit.

[George Rimar via llvm-commits]

Author: grimar
Date: Tue Aug 28 01:49:40 2018
New Revision: 340806

URL: http://llvm.org/viewvc/llvm-project?rev=340806&view=rev
Log:
[LLD][ELF] - Simplify Call-Chain Clustering implementation a bit.

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. This is NFC change.

Differential revision: https://reviews.llvm.org/D50609

Modified:
    lld/trunk/ELF/CallGraphSort.cpp

Modified: lld/trunk/ELF/CallGraphSort.cpp
URL: http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/CallGraphSort.cpp?rev=340806&r1=340805&r2=340806&view=diff
==============================================================================
--- lld/trunk/ELF/CallGraphSort.cpp (original)
+++ lld/trunk/ELF/CallGraphSort.cpp Tue Aug 28 01:49:40 2018
@@ -72,7 +72,7 @@ struct Cluster {
   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 @@ CallGraphSort::CallGraphSort() {
     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 @@ void CallGraphSort::groupClusters() {
     // 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;