[libFuzzer] warn if the corpus is empty

llvm-svn: 266670

llvm/lib/Fuzzer/FuzzerDriver.cpp

@@ -376,8 +376,11 @@ static int FuzzerDriver(const std::vector<std::string> &Args,
     F.SetMaxLen(
         std::min(std::max(kMinDefaultLen, F.MaxUnitSizeInCorpus()), kMaxSaneLen));
 
-  if (F.CorpusSize() == 0)
+  if (F.CorpusSize() == 0) {
     F.AddToCorpus(Unit());  // Can't fuzz empty corpus, so add an empty input.
+    if (Options.Verbosity)
+      Printf("INFO: A corpus is not provided, starting from an empty corpus\n");
+  }
   F.ShuffleAndMinimize();
   if (Flags.drill)
     F.Drill();

llvm/lib/Fuzzer/test/fuzzer.test

@@ -118,3 +118,4 @@ FD_MASK_3: INITED
 
 RUN: LLVMFuzzer-SimpleTest -runs=10 --max_len=100 2>&1 | FileCheck %s --check-prefix=DASH_DASH
 DASH_DASH: WARNING: did you mean '-max_len=100' (single dash)?
+DASH_DASH: INFO: A corpus is not provided, starting from an empty corpus