1 files changed, 16 insertions, 0 deletions
diff --git a/llvm/lib/Fuzzer/FuzzerLoop.cpp b/llvm/lib/Fuzzer/FuzzerLoop.cpp
index 017ea03eaf7..c71d3022784 100644
--- a/llvm/lib/Fuzzer/FuzzerLoop.cpp
+++ b/llvm/lib/Fuzzer/FuzzerLoop.cpp
@@ -188,7 +188,22 @@ void Fuzzer::StaticDeathCallback() {
   F->DeathCallback();
 }
 
+static void WarnOnUnsuccessfullMerge(bool DoWarn) {
+  Printf(
+   "***\n"
+   "***\n"
+   "***\n"
+   "*** NOTE: merge did not succeed due to a failure on one of the inputs.\n"
+   "*** You will need to filter out crashes from the corpus, e.g. like this:\n"
+   "***   for f in WITH_CRASHES/*; do ./fuzzer $f && cp $f NO_CRASHES; done\n"
+   "*** Future versions may have crash-resistant merge, stay tuned.\n"
+   "***\n"
+   "***\n"
+   "***\n");
+}
+
 void Fuzzer::DumpCurrentUnit(const char *Prefix) {
+  WarnOnUnsuccessfullMerge(InMergeMode);
   if (!CurrentUnitData) return;  // Happens when running individual inputs.
   MD.PrintMutationSequence();
   Printf("; base unit: %s\n", Sha1ToString(BaseSha1).c_str());
@@ -612,6 +627,7 @@ void Fuzzer::Merge(const std::vector<std::string> &Corpora) {
     Printf("Merge requires two or more corpus dirs\n");
     return;
   }
+  InMergeMode = true;
   std::vector<std::string> ExtraCorpora(Corpora.begin() + 1, Corpora.end());
 
   assert(Options.MaxLen > 0);