[sanitizer/coverage] Add AFL-style coverage counters (search heuristic for fuzzing).

Introduce -mllvm -sanitizer-coverage-8bit-counters=1
which adds imprecise thread-unfriendly 8-bit coverage counters.

The run-time library maps these 8-bit counters to 8-bit bitsets in the same way
AFL (http://lcamtuf.coredump.cx/afl/technical_details.txt) does:
counter values are divided into 8 ranges and based on the counter
value one of the bits in the bitset is set.
The AFL ranges are used here: 1, 2, 3, 4-7, 8-15, 16-31, 32-127, 128+.

These counters provide a search heuristic for single-threaded
coverage-guided fuzzers, we do not expect them to be useful for other purposes.

Depending on the value of -fsanitize-coverage=[123] flag,
these counters will be added to the function entry blocks (=1),
every basic block (=2), or every edge (=3).

Use these counters as an optional search heuristic in the Fuzzer library.
Add a test where this heuristic is critical.

llvm-svn: 231166

1 files changed, 1 insertions, 0 deletions

diff --git a/llvm/lib/Fuzzer/FuzzerDriver.cpp b/llvm/lib/Fuzzer/FuzzerDriver.cpp
index 1746afd822d..9ccd744c28d 100644
--- a/llvm/lib/Fuzzer/FuzzerDriver.cpp
+++ b/llvm/lib/Fuzzer/FuzzerDriver.cpp
@@ -158,6 +158,7 @@ int FuzzerDriver(int argc, char **argv, UserCallback Callback) {
   Options.DoCrossOver = Flags.cross_over;
   Options.MutateDepth = Flags.mutate_depth;
   Options.ExitOnFirst = Flags.exit_on_first;
+  Options.UseCounters = Flags.use_counters;
   Options.UseFullCoverageSet = Flags.use_full_coverage_set;
   Options.UseCoveragePairs = Flags.use_coverage_pairs;
   Options.PreferSmallDuringInitialShuffle =