[PATCH] D147013: [CSSPGO][Preinliner] Trim cold call edges of the profiled call graph for a more stable profile generation.

[Hongtao Yu via Phabricator via llvm-commits]

hoy updated this revision to Diff 509042.
hoy added a comment.

Addressing feedbacks.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D147013/new/

https://reviews.llvm.org/D147013

Files:
  llvm/include/llvm/Transforms/IPO/ProfiledCallGraph.h
  llvm/tools/llvm-profgen/CSPreInliner.cpp


Index: llvm/tools/llvm-profgen/CSPreInliner.cpp
===================================================================
--- llvm/tools/llvm-profgen/CSPreInliner.cpp
+++ llvm/tools/llvm-profgen/CSPreInliner.cpp
@@ -76,7 +76,12 @@
 
 std::vector<StringRef> CSPreInliner::buildTopDownOrder() {
   std::vector<StringRef> Order;
-  ProfiledCallGraph ProfiledCG(ContextTracker);
+  // Trim cold edges to get a more stable call graph. This allows for a more
+  // stable top-down order which in turns helps the stablity of the generated
+  // profile from run to run.
+  uint64_t ColdCountThreshold = ProfileSummaryBuilder::getColdCountThreshold(
+      (Summary->getDetailedSummary()));
+  ProfiledCallGraph ProfiledCG(ContextTracker, ColdCountThreshold + 1);
 
   // Now that we have a profiled call graph, construct top-down order
   // by building up SCC and reversing SCC order.
Index: llvm/include/llvm/Transforms/IPO/ProfiledCallGraph.h
===================================================================
--- llvm/include/llvm/Transforms/IPO/ProfiledCallGraph.h
+++ llvm/include/llvm/Transforms/IPO/ProfiledCallGraph.h
@@ -64,16 +64,22 @@
   using iterator = ProfiledCallGraphNode::iterator;
 
   // Constructor for non-CS profile.
-  ProfiledCallGraph(SampleProfileMap &ProfileMap) {
+  ProfiledCallGraph(SampleProfileMap &ProfileMap,
+                    uint64_t IgnoreColdCallThreshold = 0) {
     assert(!FunctionSamples::ProfileIsCS &&
            "CS flat profile is not handled here");
     for (const auto &Samples : ProfileMap) {
       addProfiledCalls(Samples.second);
     }
+
+    // Trim edges with weight lower than `IgnoreColdCallThreshold`. This aims
+    // for a more stable call graph with "determinstic" edges from run to run.
+    trimColdEges(IgnoreColdCallThreshold);
   }
 
   // Constructor for CS profile.
-  ProfiledCallGraph(SampleContextTracker &ContextTracker) {
+  ProfiledCallGraph(SampleContextTracker &ContextTracker,
+                    uint64_t IgnoreColdCallThreshold = 0) {
     // BFS traverse the context profile trie to add call edges for calls shown
     // in context.
     std::queue<ContextTrieNode *> Queue;
@@ -121,11 +127,16 @@
                         ContextTracker.getFuncNameFor(Callee), Weight);
       }
     }
+
+    // Trim edges with weight lower than `IgnoreColdCallThreshold`. This aims
+    // for a more stable call graph with "determinstic" edges from run to run.
+    trimColdEges(IgnoreColdCallThreshold);
   }
 
   iterator begin() { return Root.Edges.begin(); }
   iterator end() { return Root.Edges.end(); }
   ProfiledCallGraphNode *getEntryNode() { return &Root; }
+
   void addProfiledFunction(StringRef Name) {
     if (!ProfiledFunctions.count(Name)) {
       // Link to synthetic root to make sure every node is reachable
@@ -148,8 +159,9 @@
     auto EdgeIt = Edges.find(Edge);
     if (EdgeIt == Edges.end()) {
       Edges.insert(Edge);
-    } else if (EdgeIt->Weight < Edge.Weight) {
-      // Replace existing call edges with same target but smaller weight.
+    } else {
+      // Accumulate weight to the existing edge.
+      Edge.Weight += EdgeIt->Weight;
       Edges.erase(EdgeIt);
       Edges.insert(Edge);
     }
@@ -175,6 +187,23 @@
     }
   }
 
+  // Trim edges with weight lower than `Threshold`.
+  void trimColdEges(uint64_t Threshold = 0) {
+    if (!Threshold)
+      return;
+
+    for (auto &Node : ProfiledFunctions) {
+      auto &Edges = Node.second.Edges;
+      auto I = Edges.begin();
+      while (I != Edges.end()) {
+        if (I->Weight < Threshold)
+          I = Edges.erase(I);
+        else
+          I++;
+      }
+    }
+  }
+
   ProfiledCallGraphNode Root;
   StringMap<ProfiledCallGraphNode> ProfiledFunctions;
 };


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