Index: lib/fuzzer/FuzzerDriver.cpp
===================================================================
--- lib/fuzzer/FuzzerDriver.cpp
+++ lib/fuzzer/FuzzerDriver.cpp
@@ -615,6 +615,7 @@
   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.PrintUnstableStats = Flags.print_unstable_stats;
Index: lib/fuzzer/FuzzerFlags.def
===================================================================
--- lib/fuzzer/FuzzerFlags.def
+++ lib/fuzzer/FuzzerFlags.def
@@ -155,3 +155,4 @@
 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
@@ -360,6 +360,7 @@
     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
@@ -86,11 +86,16 @@
 
   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;
+    uint64_t UsefulCount;
+    uint64_t TotalCount;
   };
 
   size_t AddWordFromDictionary(Dictionary &D, uint8_t *Data, size_t Size,
@@ -128,8 +133,8 @@
   // entries that led to successful discoveries in the past mutations.
   Dictionary PersistentAutoDictionary;
 
-  Vector<Mutator> CurrentMutatorSequence;
   Vector<DictionaryEntry *> CurrentDictionaryEntrySequence;
+  Vector<Mutator *> CurrentMutatorSequence;
 
   static const size_t kCmpDictionaryEntriesDequeSize = 16;
   DictionaryEntry CmpDictionaryEntriesDeque[kCmpDictionaryEntriesDequeSize];
Index: lib/fuzzer/FuzzerMutate.cpp
===================================================================
--- lib/fuzzer/FuzzerMutate.cpp
+++ lib/fuzzer/FuzzerMutate.cpp
@@ -30,34 +30,36 @@
   DefaultMutators.insert(
       DefaultMutators.begin(),
       {
-          {&MutationDispatcher::Mutate_EraseBytes, "EraseBytes"},
-          {&MutationDispatcher::Mutate_InsertByte, "InsertByte"},
+          {&MutationDispatcher::Mutate_EraseBytes, "EraseBytes", 0, 0},
+          {&MutationDispatcher::Mutate_InsertByte, "InsertByte", 0, 0},
           {&MutationDispatcher::Mutate_InsertRepeatedBytes,
-           "InsertRepeatedBytes"},
-          {&MutationDispatcher::Mutate_ChangeByte, "ChangeByte"},
-          {&MutationDispatcher::Mutate_ChangeBit, "ChangeBit"},
-          {&MutationDispatcher::Mutate_ShuffleBytes, "ShuffleBytes"},
-          {&MutationDispatcher::Mutate_ChangeASCIIInteger, "ChangeASCIIInt"},
-          {&MutationDispatcher::Mutate_ChangeBinaryInteger, "ChangeBinInt"},
-          {&MutationDispatcher::Mutate_CopyPart, "CopyPart"},
-          {&MutationDispatcher::Mutate_CrossOver, "CrossOver"},
+           "InsertRepeatedBytes", 0, 0},
+          {&MutationDispatcher::Mutate_ChangeByte, "ChangeByte", 0, 0},
+          {&MutationDispatcher::Mutate_ChangeBit, "ChangeBit", 0, 0},
+          {&MutationDispatcher::Mutate_ShuffleBytes, "ShuffleBytes", 0, 0},
+          {&MutationDispatcher::Mutate_ChangeASCIIInteger, "ChangeASCIIInt", 0,
+           0},
+          {&MutationDispatcher::Mutate_ChangeBinaryInteger, "ChangeBinInt", 0,
+           0},
+          {&MutationDispatcher::Mutate_CopyPart, "CopyPart", 0, 0},
+          {&MutationDispatcher::Mutate_CrossOver, "CrossOver", 0, 0},
           {&MutationDispatcher::Mutate_AddWordFromManualDictionary,
-           "ManualDict"},
+           "ManualDict", 0, 0},
           {&MutationDispatcher::Mutate_AddWordFromPersistentAutoDictionary,
-           "PersAutoDict"},
+           "PersAutoDict", 0, 0},
       });
   if(Options.UseCmp)
     DefaultMutators.push_back(
-        {&MutationDispatcher::Mutate_AddWordFromTORC, "CMP"});
+        {&MutationDispatcher::Mutate_AddWordFromTORC, "CMP", 0, 0});
 
   if (EF->LLVMFuzzerCustomMutator)
-    Mutators.push_back({&MutationDispatcher::Mutate_Custom, "Custom"});
+    Mutators.push_back({&MutationDispatcher::Mutate_Custom, "Custom", 0, 0});
   else
     Mutators = DefaultMutators;
 
   if (EF->LLVMFuzzerCustomCrossOver)
     Mutators.push_back(
-        {&MutationDispatcher::Mutate_CustomCrossOver, "CustomCrossOver"});
+        {&MutationDispatcher::Mutate_CustomCrossOver, "CustomCrossOver", 0, 0});
 }
 
 static char RandCh(Random &Rand) {
@@ -464,6 +466,7 @@
     if (!PersistentAutoDictionary.ContainsWord(DE->GetW()))
       PersistentAutoDictionary.push_back({DE->GetW(), 1});
   }
+  RecordUsefulMutations();
 }
 
 void MutationDispatcher::PrintRecommendedDictionary() {
@@ -484,8 +487,7 @@
 
 void MutationDispatcher::PrintMutationSequence() {
   Printf("MS: %zd ", CurrentMutatorSequence.size());
-  for (auto M : CurrentMutatorSequence)
-    Printf("%s-", M.Name);
+  for (auto M : CurrentMutatorSequence) Printf("%s-", M->Name);
   if (!CurrentDictionaryEntrySequence.empty()) {
     Printf(" DE: ");
     for (auto DE : CurrentDictionaryEntrySequence) {
@@ -513,12 +515,13 @@
   // 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 NewSize = (this->*(M.Fn))(Data, Size, MaxSize);
+    auto M = &Mutators[Rand(Mutators.size())];
+    size_t NewSize = (this->*(M->Fn))(Data, Size, MaxSize);
     if (NewSize && NewSize <= MaxSize) {
       if (Options.OnlyASCII)
         ToASCII(Data, NewSize);
       CurrentMutatorSequence.push_back(M);
+      M->TotalCount++;
       return NewSize;
     }
   }
@@ -531,4 +534,21 @@
       {W, std::numeric_limits<size_t>::max()});
 }
 
+void MutationDispatcher::RecordUsefulMutations() {
+  for (auto M : CurrentMutatorSequence) M->UsefulCount++;
+}
+
+void MutationDispatcher::PrintMutationStats() {
+  Printf("\nstat::mutation_usefulness:      ");
+  for (size_t i = 0; i < Mutators.size(); i++) {
+    double UsefulPercentage =
+        Mutators[i].TotalCount
+            ? (100.0 * Mutators[i].UsefulCount) / Mutators[i].TotalCount
+            : 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 PrintUnstableStats = 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]+}}