Index: lib/Fuzzer/FuzzerExtFunctions.def
===================================================================
--- lib/Fuzzer/FuzzerExtFunctions.def
+++ lib/Fuzzer/FuzzerExtFunctions.def
@@ -19,5 +19,10 @@
 EXT_FUNC(LLVMFuzzerCustomMutator, size_t,
          (uint8_t * Data, size_t Size, size_t MaxSize, unsigned int Seed),
          false);
+EXT_FUNC(LLVMFuzzerCustomCrossOver, size_t,
+         (const uint8_t * Data1, size_t Size1,
+          const uint8_t * Data2, size_t Size2,
+          uint8_t * Out, size_t MaxOutSize, unsigned int Seed),
+         false);
 
 // TODO: Sanitizer functions
Index: lib/Fuzzer/FuzzerInterface.h
===================================================================
--- lib/Fuzzer/FuzzerInterface.h
+++ lib/Fuzzer/FuzzerInterface.h
@@ -45,6 +45,15 @@
 size_t LLVMFuzzerCustomMutator(uint8_t *Data, size_t Size, size_t MaxSize,
                                unsigned int Seed);
 
+// Optional user-provided custom cross-over function.
+// Combines pieces of Data1 & Data2 together into Out.
+// Returns the new size, which is not greater than MaxOutSize.
+// Should produce the same mutation given the same Seed.
+size_t LLVMFuzzerCustomCrossOver(const uint8_t *Data1, size_t Size1,
+                                 const uint8_t *Data2, size_t Size2,
+                                 uint8_t *Out, size_t MaxOutSize,
+                                 unsigned int Seed);
+
 // Experimental, may go away in future.
 // libFuzzer-provided function to be used inside LLVMFuzzerTestOneInput.
 // Mutates raw data in [Data, Data+Size) inplace.
Index: lib/Fuzzer/FuzzerInternal.h
===================================================================
--- lib/Fuzzer/FuzzerInternal.h
+++ lib/Fuzzer/FuzzerInternal.h
@@ -215,6 +215,8 @@
   void RecordSuccessfulMutationSequence();
   /// Mutates data by invoking user-provided mutator.
   size_t Mutate_Custom(uint8_t *Data, size_t Size, size_t MaxSize);
+  /// Mutates data by invoking user-provided crossover.
+  size_t Mutate_CustomCrossOver(uint8_t *Data, size_t Size, size_t MaxSize);
   /// Mutates data by shuffling bytes.
   size_t Mutate_ShuffleBytes(uint8_t *Data, size_t Size, size_t MaxSize);
   /// Mutates data by erasing a byte.
Index: lib/Fuzzer/FuzzerMutate.cpp
===================================================================
--- lib/Fuzzer/FuzzerMutate.cpp
+++ lib/Fuzzer/FuzzerMutate.cpp
@@ -41,6 +41,10 @@
     Mutators.push_back({&MutationDispatcher::Mutate_Custom, "Custom"});
   else
     Mutators = DefaultMutators;
+
+  if (EF.LLVMFuzzerCustomCrossOver)
+    Mutators.push_back(
+        {&MutationDispatcher::Mutate_CustomCrossOver, "CustomCrossOver"});
 }
 
 static char FlipRandomBit(char X, Random &Rand) {
@@ -66,6 +70,25 @@
   return EF.LLVMFuzzerCustomMutator(Data, Size, MaxSize, Rand.Rand());
 }
 
+size_t MutationDispatcher::Mutate_CustomCrossOver(uint8_t *Data, size_t Size,
+                                                  size_t MaxSize) {
+  if (!Corpus || Corpus->size() < 2 || Size == 0)
+    return 0;
+  size_t Idx = Rand(Corpus->size());
+  const Unit &Other = (*Corpus)[Idx];
+  if (Other.empty())
+    return 0;
+  MutateInPlaceHere.resize(MaxSize);
+  auto &U = MutateInPlaceHere;
+  size_t NewSize = EF.LLVMFuzzerCustomCrossOver(
+      Data, Size, Other.data(), Other.size(), U.data(), U.size(), Rand.Rand());
+  if (!NewSize)
+    return 0;
+  assert(NewSize <= MaxSize && "CustomCrossOver returned overisized unit");
+  memcpy(Data, U.data(), NewSize);
+  return NewSize;
+}
+
 size_t MutationDispatcher::Mutate_ShuffleBytes(uint8_t *Data, size_t Size,
                                                size_t MaxSize) {
   assert(Size);
Index: lib/Fuzzer/test/CMakeLists.txt
===================================================================
--- lib/Fuzzer/test/CMakeLists.txt
+++ lib/Fuzzer/test/CMakeLists.txt
@@ -66,6 +66,7 @@
   BufferOverflowOnInput
   CallerCalleeTest
   CounterTest
+  CustomCrossOverTest
   CustomMutatorTest
   EmptyTest
   FourIndependentBranchesTest
Index: lib/Fuzzer/test/CustomCrossOverTest.cpp
===================================================================
--- /dev/null
+++ lib/Fuzzer/test/CustomCrossOverTest.cpp
@@ -0,0 +1,57 @@
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+
+// Simple test for a cutom mutator.
+#include <assert.h>
+#include <cstddef>
+#include <cstdint>
+#include <cstdlib>
+#include <iostream>
+#include <random>
+#include <string.h>
+
+#include "FuzzerInterface.h"
+
+static const char *Separator = "-_^_-";
+static const char *Target = "012-_^_-abc";
+
+extern "C" int LLVMFuzzerTestOneInput(const uint8_t *Data, size_t Size) {
+  assert(Data);
+  std::string Str(reinterpret_cast<const char *>(Data), Size);
+
+  if (Str.find(Target) != std::string::npos) {
+    std::cout << "BINGO; Found the target, exiting\n";
+    exit(1);
+  }
+  return 0;
+}
+
+extern "C" size_t LLVMFuzzerCustomCrossOver(const uint8_t *Data1, size_t Size1,
+                                            const uint8_t *Data2, size_t Size2,
+                                            uint8_t *Out, size_t MaxOutSize,
+                                            unsigned int Seed) {
+  static bool Printed;
+  static size_t SeparatorLen = strlen(Separator);
+
+  if (!Printed) {
+    std::cerr << "In LLVMFuzzerCustomCrossover\n";
+    Printed = true;
+  }
+
+  std::mt19937 R(Seed);
+
+  size_t Offset1 = 0;
+  size_t Len1 = R() % (Size1 - Offset1);
+  size_t Offset2 = 0;
+  size_t Len2 = R() % (Size2 - Offset2);
+  size_t Size = Len1 + Len2 + SeparatorLen;
+
+  if (Size > MaxOutSize)
+    return 0;
+
+  memcpy(Out, Data1 + Offset1, Len1);
+  memcpy(Out + Len1, Separator, SeparatorLen);
+  memcpy(Out + Len1 + SeparatorLen, Data2 + Offset2, Len2);
+
+  return Len1 + Len2 + SeparatorLen;
+}
Index: lib/Fuzzer/test/fuzzer-customcrossover.test
===================================================================
--- /dev/null
+++ lib/Fuzzer/test/fuzzer-customcrossover.test
@@ -0,0 +1,10 @@
+RUN: rm -rf %t/CustomCrossover
+RUN: mkdir -p %t/CustomCrossover
+RUN: echo "0123456789" > %t/CustomCrossover/digits
+RUN: echo "abcdefghij" > %t/CustomCrossover/chars
+RUN: not LLVMFuzzer-CustomCrossOverTest -seed=1 -use_memcmp=0 -runs=100000 -prune_corpus=0 %t/CustomCrossover 2>&1 | FileCheck %s --check-prefix=LLVMFuzzerCustomCrossover
+RUN: rm -rf %t/CustomCrossover
+
+LLVMFuzzerCustomCrossover: In LLVMFuzzerCustomCrossover
+LLVMFuzzerCustomCrossover: BINGO
+