In larger, complex or reiterative programs the counters that collect the edge coverage can easily fill up and wrap around. This is not that much of an issue - unless by chance it wraps just to a value of zero when the program execution ends. In this case afl-fuzz is not able to see that the edge has been accessed and will ignore it.
NeverZero prevents this behaviour. If a counter wraps, it jumps over the value 0 directly to a 1. This improves path discovery (by a very little amount) at a very little cost (one instruction per edge).
(The alternative of saturated counters has been tested also and proved to be inferior in terms of path discovery.)
This is implemented in afl-gcc, however for llvm_mode this is optional if the llvm version is below 9 - as there is a perfomance bug that is only fixed in version 9 and onwards.
If you want to enable this for llvm versions below 9 then set
export AFL_LLVM_NOT_ZERO=1
In case you are on llvm 9 or greater and you do not want this behaviour then you can set:
AFL_LLVM_SKIP_NEVERZERO=1
If the target does not have extensive loops or functions that are called a lot then this can give a small performance boost.