Index: bolt/tools/merge-fdata/merge-fdata.cpp
===================================================================
--- bolt/tools/merge-fdata/merge-fdata.cpp
+++ bolt/tools/merge-fdata/merge-fdata.cpp
@@ -20,6 +20,7 @@
 #include "llvm/Support/ManagedStatic.h"
 #include "llvm/Support/PrettyStackTrace.h"
 #include "llvm/Support/Signals.h"
+#include "llvm/Support/ThreadPool.h"
 #include <unordered_map>
 
 using namespace llvm;
@@ -258,15 +259,17 @@
 void mergeLegacyProfiles(const SmallVectorImpl<std::string> &Filenames) {
   errs() << "Using legacy profile format.\n";
   std::optional<bool> BoltedCollection;
-  StringMap<uint64_t> Entries;
-  for (const std::string &Filename : Filenames) {
+  typedef StringMap<uint64_t> ProfileTy;
+
+  auto ParseProfile = [&](const std::string &Filename, auto &Profiles) {
+    const llvm::thread::id tid = llvm::this_thread::get_id();
+
     if (isYAML(Filename))
       report_error(Filename, "cannot mix YAML and legacy formats");
     ErrorOr<std::unique_ptr<MemoryBuffer>> MB =
         MemoryBuffer::getFileOrSTDIN(Filename);
     if (std::error_code EC = MB.getError())
       report_error(Filename, EC);
-    errs() << "Merging data from " << Filename << "...\n";
 
     StringRef Buf = MB.get()->getBuffer();
     // Check if the string "boltedcollection" is in the first line
@@ -295,15 +298,31 @@
       uint64_t Count;
       if (Line.substr(Pos + 1, Line.size() - Pos).getAsInteger(10, Count))
         report_error(Filename, "Malformed / corrupted profile counter");
-      Count += Entries.lookup(Signature);
-      Entries.insert_or_assign(Signature, Count);
+      uint64_t NewCount = Profiles[tid].lookup(Signature) + Count;
+      Profiles[tid].insert_or_assign(Signature, NewCount);
+    }
+  };
+
+  // The final reduction has non-trivial cost, make sure each thread has at
+  // least 4 tasks.
+  ThreadPoolStrategy S = optimal_concurrency(Filenames.size() / 4);
+  ThreadPool Pool(S);
+  DenseMap<llvm::thread::id, ProfileTy> ParsedProfiles(Pool.getThreadCount());
+  for (const auto &Filename : Filenames)
+    Pool.async(ParseProfile, std::cref(Filename), std::ref(ParsedProfiles));
+  Pool.wait();
+
+  ProfileTy MergedProfile;
+  for (const auto &[Thread, Profile] : ParsedProfiles)
+    for (const auto &[Key, Value] : Profile) {
+      uint64_t Count = MergedProfile.lookup(Key) + Value;
+      MergedProfile.insert_or_assign(Key, Count);
     }
-  }
 
   if (BoltedCollection)
     output() << "boltedcollection\n";
-  for (const auto &Entry : Entries)
-    output() << Entry.getKey() << " " << Entry.getValue() << "\n";
+  for (const auto &[Key, Value] : MergedProfile)
+    output() << Key << " " << Value << "\n";
 
   errs() << "Profile from " << Filenames.size() << " files merged.\n";
 }