[llvm] [CGData][MachineOutliner] Global Outlining (PR #90074)

[Kyungwoo Lee via llvm-commits]

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@@ -695,6 +859,39 @@ void MachineOutliner::findCandidates(
   }
 }
 
+void MachineOutliner::computeAndPublishHashSequence(MachineFunction &MF,
+                                                    unsigned CandSize) {
+  // Compute the hash sequence for the outlined function.
+  SmallVector<stable_hash> OutlinedHashSequence;
+  for (auto &MBB : MF) {
+    for (auto &NewMI : MBB) {
+      stable_hash Hash = stableHashValue(NewMI);
+      if (!Hash) {
+        OutlinedHashSequence.clear();
+        break;
+      }
+      OutlinedHashSequence.push_back(Hash);
+    }
+  }
+
+  // Append a unique name based on the non-empty hash sequence.
+  if (AppendContentHashToOutlinedName && !OutlinedHashSequence.empty()) {
+    auto CombinedHash = stable_hash_combine(OutlinedHashSequence);
+    auto NewName =
+        MF.getName().str() + ".content." + std::to_string(CombinedHash);
+    MF.getFunction().setName(NewName);
+  }
+
+  // Publish the non-empty hash sequence to the local hash tree.
+  if (OutlinerMode == CGDataMode::Write) {
+    StableHashAttempts++;
+    if (!OutlinedHashSequence.empty())
+      LocalHashTree->insert({OutlinedHashSequence, CandSize});
----------------
kyulee-com wrote:

We track the frequency of each outlined hash sequence in the LocalHashTree (in write mode), which helps us understand how often each sequence appears in a specific module. This frequency is represented as `Terminals` -- https://github.com/llvm/llvm-project/blob/main/llvm/lib/CGData/OutlinedHashTree.cpp#L87.
When combining data from all modules, this terminal count shows the total occurrences of each sequence across all modules.

In this PR, when we create a `GlobalOutlinedFunction` using CGData from previous runs (in read mode), we initialize a `GlobalOccurrenceCount` from the `Terminals` in the global hash tree, which has been merged. This count helps us decide which candidates are most important to outline based on their frequency across all modules.

https://github.com/llvm/llvm-project/pull/90074