[libFuzzer] Remove mutation stats and weighted mutation selection.

Summary:
This was an experimental feature. After evaluating it with:

1) https://github.com/google/fuzzer-test-suite/tree/master/engine-comparison

2) enabling on real world fuzz targets running at ClusterFuzz and OSS-Fuzz

The following conclusions were made:

1) With fuzz targets that have reached a code coverage plateau, the feature does
   not improve libFuzzer's ability to discover new coverage and may actually
   negatively impact it.

2) With fuzz targets that have not yet reached a code coverage plateau, the
   feature might speed up new units discovery in some cases, but it is quite
   rare and hard to confirm with a high level on confidence.

Revert of https://reviews.llvm.org/D48054 and https://reviews.llvm.org/D49621.

Reviewers: metzman, morehouse

Reviewed By: metzman, morehouse

Subscribers: delcypher, #sanitizers, llvm-commits, kcc

Differential Revision: https://reviews.llvm.org/D51455

llvm-svn: 340976

1 files changed, 0 insertions, 5 deletions

diff --git a/compiler-rt/lib/fuzzer/FuzzerLoop.cpp b/compiler-rt/lib/fuzzer/FuzzerLoop.cpp
index c7b13d1e507..7b98f55ae46 100644
--- a/compiler-rt/lib/fuzzer/FuzzerLoop.cpp
+++ b/compiler-rt/lib/fuzzer/FuzzerLoop.cpp
@@ -38,7 +38,6 @@
 
 namespace fuzzer {
 static const size_t kMaxUnitSizeToPrint = 256;
-static const size_t kUpdateMutationWeightRuns = 10000;
 
 thread_local bool Fuzzer::IsMyThread;
 
@@ -361,7 +360,6 @@ void Fuzzer::PrintFinalStats() {
     TPC.DumpCoverage();
   if (Options.PrintCorpusStats)
     Corpus.PrintStats();
-  if (Options.PrintMutationStats) MD.PrintMutationStats();
   if (!Options.PrintFinalStats)
     return;
   size_t ExecPerSec = execPerSec();
@@ -550,9 +548,6 @@ static bool LooseMemeq(const uint8_t *A, const uint8_t *B, size_t Size) {
 
 void Fuzzer::ExecuteCallback(const uint8_t *Data, size_t Size) {
   TPC.RecordInitialStack();
-  if (Options.UseWeightedMutations &&
-      TotalNumberOfRuns % kUpdateMutationWeightRuns == 0)
-    MD.UpdateDistribution();
   TotalNumberOfRuns++;
   assert(InFuzzingThread());
   if (SMR.IsClient())