Index: tools/llvm-opt-fuzzer/llvm-opt-fuzzer.cpp
===================================================================
--- tools/llvm-opt-fuzzer/llvm-opt-fuzzer.cpp
+++ tools/llvm-opt-fuzzer/llvm-opt-fuzzer.cpp
@@ -65,15 +65,37 @@
 
   Mutator->mutateModule(*M, Seed, Size, MaxSize);
 
-#ifndef NDEBUG
   if (verifyModule(*M, &errs())) {
     errs() << "mutation result doesn't pass verification\n";
     M->dump();
-    abort();
+    // Avoid adding incorrect test cases to the corpus.
+    return 0;
+  }
+  
+  std::string Buf;
+  {
+    raw_string_ostream OS(Buf);
+    WriteBitcodeToFile(M.get(), OS);
+  }
+  if (Buf.size() > MaxSize)
+    return 0;
+  
+  // There are some invariants which are not checked by the verifier in favor
+  // of having them checked by the parser. They may be considered as bugs in the
+  // verifier and should be fixed there. However until all of those are covered
+  // we want to check for them explicitly. Otherwise we will add incorrect input
+  // to the corpus and this is going to confuse the fuzzer which will start 
+  // exploration of the bitcode reader error handling code.
+  auto NewM = parseModule(
+      reinterpret_cast<const uint8_t*>(Buf.data()), Buf.size(), Context);
+  if (!NewM || verifyModule(*NewM, &errs())) {
+    errs() << "mutator failed to re-read the module\n";
+    M->dump();
+    return 0;
   }
-#endif
 
-  return writeModule(*M, Data, MaxSize);
+  memcpy(Data, Buf.data(), Buf.size());
+  return Buf.size();
 }
 
 extern "C" int LLVMFuzzerTestOneInput(const uint8_t *Data, size_t Size) {