Improve efficiency of mutator by allowing Copy/Clone only in CrossOver

That's very libFuzzer specific tweek
libFuzzer calls LLVMFuzzerCustomMutator and LLVMFuzzerCustomCrossOver
with same probability. However LLVMFuzzerCustomCrossOver is
just a variation of Copy/Clone. So before the patch all mutator activity
was more skewed toward Copy/Clone.

This results in 2x increases in new feature discovery per iteration.

2 files changed, 3 insertions, 1 deletions

diff --git a/src/mutator.cc b/src/mutator.cc
index 6866b50..0459ab3 100644
--- a/src/mutator.cc
+++ b/src/mutator.cc
@@ -684,6 +684,8 @@ bool Mutator::MutateImpl(const ConstMessages& sources, const Messages& messages,
     mutations[static_cast<size_t>(Mutation::Delete)] = true;
   } else {
     mutations.set();
+    mutations[static_cast<size_t>(Mutation::Copy)] = false;
+    mutations[static_cast<size_t>(Mutation::Clone)] = false;
   }
   while (mutations.any()) {
     MutationSampler mutation(keep_initialized_, mutations, &random_);
diff --git a/src/mutator_test.cc b/src/mutator_test.cc
index 83e08a6..63367e8 100644
--- a/src/mutator_test.cc
+++ b/src/mutator_test.cc
@@ -555,7 +555,7 @@ void MutatorFieldTest::TestCopyField() {
     to.add_repeated_msg()->CopyFrom(*m1_);
     to.add_repeated_msg()->CopyFrom(*m1_);
     to.mutable_repeated_msg(1)->add_repeated_string("repeated_string");
-    EXPECT_TRUE(Mutate(from, to));
+    EXPECT_TRUE(CrossOver(from, from, to));
   }
 }