Index: llvm/trunk/lib/Fuzzer/FuzzerDriver.cpp
===================================================================
--- llvm/trunk/lib/Fuzzer/FuzzerDriver.cpp
+++ llvm/trunk/lib/Fuzzer/FuzzerDriver.cpp
@@ -330,6 +330,7 @@
   Options.SaveArtifacts = !DoPlainRun;
   Options.PrintNewCovPcs = Flags.print_new_cov_pcs;
   Options.PrintFinalStats = Flags.print_final_stats;
+  Options.TruncateUnits = Flags.truncate_units;
 
   unsigned Seed = Flags.seed;
   // Initialize Seed.
Index: llvm/trunk/lib/Fuzzer/FuzzerFlags.def
===================================================================
--- llvm/trunk/lib/Fuzzer/FuzzerFlags.def
+++ llvm/trunk/lib/Fuzzer/FuzzerFlags.def
@@ -84,6 +84,7 @@
     "try to detect memory leaks during fuzzing (i.e. not only at shut down).")
 FUZZER_FLAG_INT(rss_limit_mb, 2048, "If non-zero, the fuzzer will exit upon"
     "reaching this limit of RSS memory usage.")
+FUZZER_FLAG_INT(truncate_units, 0, "Try truncated units when loading corpus.")
 
 FUZZER_DEPRECATED_FLAG(exit_on_first)
 FUZZER_DEPRECATED_FLAG(save_minimized_corpus)
Index: llvm/trunk/lib/Fuzzer/FuzzerInternal.h
===================================================================
--- llvm/trunk/lib/Fuzzer/FuzzerInternal.h
+++ llvm/trunk/lib/Fuzzer/FuzzerInternal.h
@@ -317,6 +317,7 @@
     bool PrintNewCovPcs = false;
     bool PrintFinalStats = false;
     bool DetectLeaks = true;
+    bool TruncateUnits = false;
   };
 
   // Aggregates all available coverage measurements.
@@ -354,6 +355,7 @@
   }
   size_t ChooseUnitIdxToMutate();
   const Unit &ChooseUnitToMutate() { return Corpus[ChooseUnitIdxToMutate()]; };
+  void TruncateUnits(std::vector<Unit> *NewCorpus);
   void Loop();
   void Drill();
   void ShuffleAndMinimize();
@@ -396,6 +398,9 @@
   void SetMaxLen(size_t MaxLen);
   void RssLimitCallback();
 
+  // Public for tests.
+  void ResetCoverage();
+
 private:
   void AlarmCallback();
   void CrashCallback();
@@ -416,7 +421,6 @@
   // Must be called whenever the corpus or unit weights are changed.
   void UpdateCorpusDistribution();
 
-  void ResetCoverage();
   bool UpdateMaxCoverage();
 
   // Trace-based fuzzing: we run a unit with some kind of tracing
Index: llvm/trunk/lib/Fuzzer/FuzzerLoop.cpp
===================================================================
--- llvm/trunk/lib/Fuzzer/FuzzerLoop.cpp
+++ llvm/trunk/lib/Fuzzer/FuzzerLoop.cpp
@@ -59,6 +59,7 @@
 
 namespace fuzzer {
 static const size_t kMaxUnitSizeToPrint = 256;
+static const size_t TruncateMaxRuns = 1000;
 
 static void MissingWeakApiFunction(const char *FnName) {
   Printf("ERROR: %s is not defined. Exiting.\n"
@@ -353,12 +354,54 @@
     });
 }
 
+// Tries random prefixes of corpus items.
+// Prefix length is chosen according to exponential distribution
+// to sample short lengths much more heavily.
+void Fuzzer::TruncateUnits(std::vector<Unit> *NewCorpus) {
+  size_t MaxCorpusLen = 0;
+  for (const auto &U : Corpus)
+    MaxCorpusLen = std::max(MaxCorpusLen, U.size());
+
+  if (MaxCorpusLen <= 1)
+    return;
+
+  // 50% of exponential distribution is Log[2]/lambda.
+  // Choose lambda so that median is MaxCorpusLen / 2.
+  double Lambda = 2.0 * log(2.0) / static_cast<double>(MaxCorpusLen);
+  std::exponential_distribution<> Dist(Lambda);
+  std::vector<double> Sizes;
+  size_t TruncatePoints = std::max(1ul, TruncateMaxRuns / Corpus.size());
+  Sizes.reserve(TruncatePoints);
+  for (size_t I = 0; I < TruncatePoints; ++I) {
+    Sizes.push_back(Dist(MD.GetRand().Get_mt19937()) + 1);
+  }
+  std::sort(Sizes.begin(), Sizes.end());
+
+  for (size_t S : Sizes) {
+    for (const auto &U : Corpus) {
+      if (S < U.size() && RunOne(U.data(), S)) {
+        Unit U1(U.begin(), U.begin() + S);
+        NewCorpus->push_back(U1);
+        WriteToOutputCorpus(U1);
+        PrintStatusForNewUnit(U1);
+      }
+    }
+  }
+  PrintStats("TRUNC  ");
+}
+
 void Fuzzer::ShuffleAndMinimize() {
   PrintStats("READ  ");
   std::vector<Unit> NewCorpus;
   if (Options.ShuffleAtStartUp)
     ShuffleCorpus(&Corpus);
 
+  if (Options.TruncateUnits) {
+    ResetCoverage();
+    TruncateUnits(&NewCorpus);
+    ResetCoverage();
+  }
+
   for (const auto &U : Corpus) {
     if (RunOne(U)) {
       NewCorpus.push_back(U);
Index: llvm/trunk/lib/Fuzzer/test/FuzzerUnittest.cpp
===================================================================
--- llvm/trunk/lib/Fuzzer/test/FuzzerUnittest.cpp
+++ llvm/trunk/lib/Fuzzer/test/FuzzerUnittest.cpp
@@ -13,6 +13,10 @@
   abort();
 }
 
+static int EmptyLLVMFuzzerTestOneInput(const uint8_t *Data, size_t Size) {
+  return 0;
+}
+
 TEST(Fuzzer, CrossOver) {
   Random Rand(0);
   MutationDispatcher MD(Rand);
@@ -423,3 +427,21 @@
     EXPECT_GT(Hist[i], TriesPerUnit / N / 3);
   }
 }
+
+TEST(Corpus, TruncateUnits) {
+  Random Rand(0);
+  MutationDispatcher MD(Rand);
+  Fuzzer::FuzzingOptions Options;
+  Options.OutputCorpus = ""; // stops from writing new units.
+  Fuzzer Fuzz(EmptyLLVMFuzzerTestOneInput, MD, Options);
+
+  Fuzz.AddToCorpus(Unit(1024, static_cast<uint8_t>(1)));
+  Fuzz.ResetCoverage();
+
+  std::vector<Unit> NewCorpus;
+  Fuzz.TruncateUnits(&NewCorpus);
+
+  // New corpus should have a shorter unit.
+  EXPECT_EQ(1ul, NewCorpus.size());
+  EXPECT_EQ(1ul, NewCorpus[0].size());
+}