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diff --git a/instrumentation/README.neverzero.md b/instrumentation/README.neverzero.md deleted file mode 100644 index 9bcae324..00000000 --- a/instrumentation/README.neverzero.md +++ /dev/null @@ -1,41 +0,0 @@ -# NeverZero counters for LLVM instrumentation - -## Usage - -In larger, complex or reiterative programs the byte sized 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 and afl-gcc-fast, however for llvm_mode this is -optional if multithread safe counters are selected or the llvm version is below -9 - as there are severe performance costs in these cases. - -If you want to enable this for llvm versions below 9 or thread safe counters -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. - -Please note that the default counter implementations are not thread safe! - -Support for thread safe counters in mode LLVM CLASSIC can be activated with setting -`AFL_LLVM_THREADSAFE_INST=1`. \ No newline at end of file |