Index: lib/fuzzer/FuzzerDriver.cpp
===================================================================
--- lib/fuzzer/FuzzerDriver.cpp
+++ lib/fuzzer/FuzzerDriver.cpp
@@ -613,6 +613,7 @@
   Options.PrintNewCovPcs = Flags.print_pcs;
   Options.PrintNewCovFuncs = Flags.print_funcs;
   Options.PrintFinalStats = Flags.print_final_stats;
+  Options.PrintMutationStats = Flags.print_mutation_stats;
   Options.PrintCorpusStats = Flags.print_corpus_stats;
   Options.PrintCoverage = Flags.print_coverage;
   Options.DumpCoverage = Flags.dump_coverage;
Index: lib/fuzzer/FuzzerFlags.def
===================================================================
--- lib/fuzzer/FuzzerFlags.def
+++ lib/fuzzer/FuzzerFlags.def
@@ -153,3 +153,4 @@
 FUZZER_FLAG_INT(analyze_dict, 0, "Experimental")
 FUZZER_DEPRECATED_FLAG(use_clang_coverage)
 FUZZER_FLAG_STRING(data_flow_trace, "Experimental: use the data flow trace")
+FUZZER_FLAG_INT(print_mutation_stats, 0, "Experimental")
Index: lib/fuzzer/FuzzerLoop.cpp
===================================================================
--- lib/fuzzer/FuzzerLoop.cpp
+++ lib/fuzzer/FuzzerLoop.cpp
@@ -355,6 +355,8 @@
     TPC.DumpCoverage();
   if (Options.PrintCorpusStats)
     Corpus.PrintStats();
+  if (Options.PrintMutationStats)
+    MD.PrintMutationStats();
   if (!Options.PrintFinalStats)
     return;
   size_t ExecPerSec = execPerSec();
Index: lib/fuzzer/FuzzerMutate.h
===================================================================
--- lib/fuzzer/FuzzerMutate.h
+++ lib/fuzzer/FuzzerMutate.h
@@ -86,8 +86,11 @@
 
   Random &GetRand() { return Rand; }
 
-private:
+  void PrintMutationStats();
+
+  void RecordUsefulMutations();
 
+private:
   struct Mutator {
     size_t (MutationDispatcher::*Fn)(uint8_t *Data, size_t Size, size_t Max);
     const char *Name;
@@ -128,8 +131,8 @@
   // entries that led to successful discoveries in the past mutations.
   Dictionary PersistentAutoDictionary;
 
-  Vector<Mutator> CurrentMutatorSequence;
   Vector<DictionaryEntry *> CurrentDictionaryEntrySequence;
+  Vector<size_t> CurrentMutatorIdxSequence;
 
   static const size_t kCmpDictionaryEntriesDequeSize = 16;
   DictionaryEntry CmpDictionaryEntriesDeque[kCmpDictionaryEntriesDequeSize];
@@ -143,6 +146,11 @@
 
   Vector<Mutator> Mutators;
   Vector<Mutator> DefaultMutators;
+
+  // A total count of each mutation used in the fuzzing process.
+  Vector<uint64_t> TotalMutations;
+  // The number of each mutation that resulted in new coverage.
+  Vector<uint64_t> UsefulMutations;
 };
 
 }  // namespace fuzzer
Index: lib/fuzzer/FuzzerMutate.cpp
===================================================================
--- lib/fuzzer/FuzzerMutate.cpp
+++ lib/fuzzer/FuzzerMutate.cpp
@@ -58,6 +58,10 @@
   if (EF->LLVMFuzzerCustomCrossOver)
     Mutators.push_back(
         {&MutationDispatcher::Mutate_CustomCrossOver, "CustomCrossOver"});
+
+  // Initialize mutation statistic counters.
+  TotalMutations.resize(Mutators.size(), 0);
+  UsefulMutations.resize(Mutators.size(), 0);
 }
 
 static char RandCh(Random &Rand) {
@@ -261,9 +265,9 @@
     DE = MakeDictionaryEntryFromCMP(X.A, X.B, Data, Size);
   } break;
   case 3: if (Options.UseMemmem) {
-    auto X = TPC.MMT.Get(Rand.Rand());
-    DE = DictionaryEntry(X);
-  } break;
+      auto X = TPC.MMT.Get(Rand.Rand());
+      DE = DictionaryEntry(X);
+    } break;
   default:
     assert(0);
   }
@@ -431,18 +435,18 @@
   auto &U = MutateInPlaceHere;
   size_t NewSize = 0;
   switch(Rand(3)) {
-    case 0:
-      NewSize = CrossOver(Data, Size, O.data(), O.size(), U.data(), U.size());
-      break;
-    case 1:
-      NewSize = InsertPartOf(O.data(), O.size(), U.data(), U.size(), MaxSize);
-      if (!NewSize)
-        NewSize = CopyPartOf(O.data(), O.size(), U.data(), U.size());
-      break;
-    case 2:
+  case 0:
+    NewSize = CrossOver(Data, Size, O.data(), O.size(), U.data(), U.size());
+    break;
+  case 1:
+    NewSize = InsertPartOf(O.data(), O.size(), U.data(), U.size(), MaxSize);
+    if (!NewSize)
       NewSize = CopyPartOf(O.data(), O.size(), U.data(), U.size());
-      break;
-    default: assert(0);
+    break;
+  case 2:
+    NewSize = CopyPartOf(O.data(), O.size(), U.data(), U.size());
+    break;
+  default: assert(0);
   }
   assert(NewSize > 0 && "CrossOver returned empty unit");
   assert(NewSize <= MaxSize && "CrossOver returned overisized unit");
@@ -451,7 +455,7 @@
 }
 
 void MutationDispatcher::StartMutationSequence() {
-  CurrentMutatorSequence.clear();
+  CurrentMutatorIdxSequence.clear();
   CurrentDictionaryEntrySequence.clear();
 }
 
@@ -465,6 +469,7 @@
     if (!PersistentAutoDictionary.ContainsWord(DE->GetW()))
       PersistentAutoDictionary.push_back({DE->GetW(), 1});
   }
+  RecordUsefulMutations();
 }
 
 void MutationDispatcher::PrintRecommendedDictionary() {
@@ -484,9 +489,9 @@
 }
 
 void MutationDispatcher::PrintMutationSequence() {
-  Printf("MS: %zd ", CurrentMutatorSequence.size());
-  for (auto M : CurrentMutatorSequence)
-    Printf("%s-", M.Name);
+  Printf("MS: %zd ", CurrentMutatorIdxSequence.size());
+  for (auto M : CurrentMutatorIdxSequence)
+    Printf("%s-", Mutators[M].Name);
   if (!CurrentDictionaryEntrySequence.empty()) {
     Printf(" DE: ");
     for (auto DE : CurrentDictionaryEntrySequence) {
@@ -514,12 +519,14 @@
   // in which case they will return 0.
   // Try several times before returning un-mutated data.
   for (int Iter = 0; Iter < 100; Iter++) {
-    auto M = Mutators[Rand(Mutators.size())];
+    size_t MutatorIdx = Rand(Mutators.size());
+    auto M = Mutators[MutatorIdx];
     size_t NewSize = (this->*(M.Fn))(Data, Size, MaxSize);
     if (NewSize && NewSize <= MaxSize) {
       if (Options.OnlyASCII)
         ToASCII(Data, NewSize);
-      CurrentMutatorSequence.push_back(M);
+      CurrentMutatorIdxSequence.push_back(MutatorIdx);
+      TotalMutations[MutatorIdx]++;
       return NewSize;
     }
   }
@@ -532,4 +539,23 @@
       {W, std::numeric_limits<size_t>::max()});
 }
 
+void MutationDispatcher::RecordUsefulMutations() {
+  for (const size_t M : CurrentMutatorIdxSequence)
+    UsefulMutations[M]++;
+}
+
+void MutationDispatcher::PrintMutationStats() {
+  Printf("\nstat::mutation_usefulness:      ");
+  for (size_t i = 0; i < Mutators.size(); i++) {
+    double UsefulPercentage =
+        TotalMutations[i]
+            ? (100.0 * UsefulMutations[i]) / TotalMutations[i]
+            : 0;
+    Printf("%.3f", UsefulPercentage);
+    if (i < Mutators.size() - 1)
+      Printf(",");
+  }
+  Printf("\n");
+}
+
 }  // namespace fuzzer
Index: lib/fuzzer/FuzzerOptions.h
===================================================================
--- lib/fuzzer/FuzzerOptions.h
+++ lib/fuzzer/FuzzerOptions.h
@@ -52,6 +52,7 @@
   bool PrintNewCovPcs = false;
   int PrintNewCovFuncs = 0;
   bool PrintFinalStats = false;
+  bool PrintMutationStats = false;
   bool PrintCorpusStats = false;
   bool PrintCoverage = false;
   bool DumpCoverage = false;
Index: test/fuzzer/fuzzer-mutationstats.test
===================================================================
--- /dev/null
+++ test/fuzzer/fuzzer-mutationstats.test
@@ -0,0 +1,5 @@
+RUN: %cpp_compiler %S/SimpleTest.cpp -o %t-MutationStatsTest
+RUN: not %run %t-MutationStatsTest -print_mutation_stats=1 2>&1 | FileCheck %s
+
+# Ensures there are some non-zero values in the usefulness percentages printed.
+CHECK: stat::mutation_usefulness:   {{[0-9]+\.[0-9]+}}