diff --git a/compiler-rt/lib/fuzzer/FuzzerMutate.cpp b/compiler-rt/lib/fuzzer/FuzzerMutate.cpp
--- a/compiler-rt/lib/fuzzer/FuzzerMutate.cpp
+++ b/compiler-rt/lib/fuzzer/FuzzerMutate.cpp
@@ -425,26 +425,26 @@
   if (!CrossOverWith) return 0;
   const Unit &O = *CrossOverWith;
   if (O.empty()) return 0;
-  MutateInPlaceHere.resize(MaxSize);
-  auto &U = MutateInPlaceHere;
   size_t NewSize = 0;
   switch(Rand(3)) {
     case 0:
-      NewSize = CrossOver(Data, Size, O.data(), O.size(), U.data(), U.size());
+      MutateInPlaceHere.resize(MaxSize);
+      NewSize = CrossOver(Data, Size, O.data(), O.size(),
+                          MutateInPlaceHere.data(), MaxSize);
+      memcpy(Data, MutateInPlaceHere.data(), NewSize);
       break;
     case 1:
-      NewSize = InsertPartOf(O.data(), O.size(), U.data(), U.size(), MaxSize);
+      NewSize = InsertPartOf(O.data(), O.size(), Data, Size, MaxSize);
       if (!NewSize)
-        NewSize = CopyPartOf(O.data(), O.size(), U.data(), U.size());
+        NewSize = CopyPartOf(O.data(), O.size(), Data, Size);
       break;
     case 2:
-      NewSize = CopyPartOf(O.data(), O.size(), U.data(), U.size());
+      NewSize = CopyPartOf(O.data(), O.size(), Data, Size);
       break;
     default: assert(0);
   }
   assert(NewSize > 0 && "CrossOver returned empty unit");
   assert(NewSize <= MaxSize && "CrossOver returned overisized unit");
-  memcpy(Data, U.data(), NewSize);
   return NewSize;
 }
 
diff --git a/compiler-rt/test/fuzzer/CrossOverTest.cpp b/compiler-rt/test/fuzzer/CrossOverTest.cpp
--- a/compiler-rt/test/fuzzer/CrossOverTest.cpp
+++ b/compiler-rt/test/fuzzer/CrossOverTest.cpp
@@ -4,10 +4,11 @@
 
 // Test for a fuzzer. The fuzzer must find the string
 // ABCDEFGHIJ
-// We use it as a test for CrossOver functionality
-// by passing two inputs to it:
-// ABCDE00000
-// ZZZZZFGHIJ
+// We use it as a test for each of CrossOver functionalities
+// by passing the following sets of two inputs to it:
+// {ABCDE00000, ZZZZZFGHIJ}
+// {ABCDEHIJ, ZFG} to specifically test InsertPartOf
+// {ABCDE00HIJ, ZFG} to specifically test CopyPartOf
 //
 #include <assert.h>
 #include <cstddef>
@@ -42,13 +43,11 @@
 
 extern "C" int LLVMFuzzerTestOneInput(const uint8_t *Data, size_t Size) {
   // fprintf(stderr, "ExpectedHash: %x\n", ExpectedHash);
-  if (Size != 10) return 0;
+  if (Size == 10 && ExpectedHash == simple_hash(Data, Size))
+    *NullPtr = 0;
   if (*Data == 'A')
     Sink++;
   if (*Data == 'Z')
     Sink--;
-  if (ExpectedHash == simple_hash(Data, Size))
-    *NullPtr = 0;
   return 0;
 }
-
diff --git a/compiler-rt/test/fuzzer/cross_over.test b/compiler-rt/test/fuzzer/cross_over.test
--- a/compiler-rt/test/fuzzer/cross_over.test
+++ b/compiler-rt/test/fuzzer/cross_over.test
@@ -12,7 +12,7 @@
 RUN: echo -n ZZZZZFGHIJ > %t-corpus/B
 
 
-RUN: not %run %t-CrossOverTest -max_len=10 -seed=1 -runs=10000000 %t-corpus
+RUN: not %run %t-CrossOverTest -max_len=10 -reduce_inputs=0 -seed=1 -runs=10000000 %t-corpus
 
 # Test the same thing but using -seed_inputs instead of passing the corpus dir.
-RUN: not %run %t-CrossOverTest -max_len=10 -seed=1 -runs=10000000 -seed_inputs=%t-corpus/A,%t-corpus/B
+RUN: not %run %t-CrossOverTest -max_len=10 -reduce_inputs=0 -seed=1 -runs=10000000 -seed_inputs=%t-corpus/A,%t-corpus/B
diff --git a/compiler-rt/test/fuzzer/cross_over_copy.test b/compiler-rt/test/fuzzer/cross_over_copy.test
new file mode 100644
--- /dev/null
+++ b/compiler-rt/test/fuzzer/cross_over_copy.test
@@ -0,0 +1,20 @@
+# Tests CrossOver CopyPartOf.
+# We want to make sure that the test can find the input
+# ABCDEFGHIJ when given two other inputs in the seed corpus:
+#    ABCDE00HIJ and
+# (Z)     FG
+#
+RUN: %cpp_compiler %S/CrossOverTest.cpp -o %t-CrossOverTest
+
+RUN: rm -rf %t-corpus
+RUN: mkdir %t-corpus
+RUN: echo -n ABCDE00HIJ > %t-corpus/A
+RUN: echo -n ZFG > %t-corpus/B
+
+
+RUN: not %run %t-CrossOverTest -mutate_depth=1 -max_len=1024 -reduce_inputs=0 -seed=1 -runs=10000000 %t-corpus 2>&1 | FileCheck %s
+
+# Test the same thing but using -seed_inputs instead of passing the corpus dir.
+RUN: not %run %t-CrossOverTest -mutate_depth=1 -max_len=1024 -reduce_inputs=0 -seed=1 -runs=10000000 -seed_inputs=%t-corpus/A,%t-corpus/B 2>&1 | FileCheck %s
+
+CHECK: MS: 1 CrossOver-
diff --git a/compiler-rt/test/fuzzer/cross_over_insert.test b/compiler-rt/test/fuzzer/cross_over_insert.test
new file mode 100644
--- /dev/null
+++ b/compiler-rt/test/fuzzer/cross_over_insert.test
@@ -0,0 +1,20 @@
+# Tests CrossOver InsertPartOf.
+# We want to make sure that the test can find the input
+# ABCDEFGHIJ when given two other inputs in the seed corpus:
+#    ABCDE  HIJ and
+# (Z)     FG
+#
+RUN: %cpp_compiler %S/CrossOverTest.cpp -o %t-CrossOverTest
+
+RUN: rm -rf %t-corpus
+RUN: mkdir %t-corpus
+RUN: echo -n ABCDEHIJ > %t-corpus/A
+RUN: echo -n ZFG > %t-corpus/B
+
+
+RUN: not %run %t-CrossOverTest -mutate_depth=1 -max_len=1024 -reduce_inputs=0 -seed=1 -runs=10000000 %t-corpus 2>&1 | FileCheck %s
+
+# Test the same thing but using -seed_inputs instead of passing the corpus dir.
+RUN: not %run %t-CrossOverTest -mutate_depth=1 -max_len=1024 -reduce_inputs=0 -seed=1 -runs=10000000 -seed_inputs=%t-corpus/A,%t-corpus/B 2>&1 | FileCheck %s
+
+CHECK: MS: 1 CrossOver-