This is an LLVM-based implementation of the n-gram branch coverage proposed in the paper "Be Sensitive and Collaborative: Analzying Impact of Coverage Metrics in Greybox Fuzzing", by Jinghan Wang, et. al.
Note that the original implementation (available here) is built on top of AFL's QEMU mode. This is essentially a port that uses LLVM vectorized instructions to achieve the same results when compiling source code.
Simply copy the source files from either circular-buf or vectorized into
AFL's llvm_mode directory and build afl-clang-fast as normal.
The size of n (i.e., the number of branches to remember) is an option that can
be specified in the AFL_NGRAM_SIZE environment variable.