modernize llvm_mode readmes

1 files changed, 24 insertions, 0 deletions

diff --git a/llvm_mode/README.neverzero.md b/llvm_mode/README.neverzero.md
new file mode 100644
index 00000000..5fcf7b47
--- /dev/null
+++ b/llvm_mode/README.neverzero.md
@@ -0,0 +1,24 @@
+# NeverZero counters for LLVM instrumentation
+
+## Usage
+
+In larger, complex or reiterative programs the map that collects the edge pairs
+can easily fill up and wrap.
+This is not that much of an issue - unless by chance it wraps just to a 0
+when the program execution ends.
+In this case afl-fuzz is not able to see that the pair has been accessed and
+will ignore it.
+
+NeverZero prevents this behaviour. If a counter wraps, it jumps over the 0
+directly to a 1. This improves path discovery (by a very little amount)
+at a very little cost (one instruction per edge).
+
+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 < 9 then set
+
+```
+export AFL_LLVM_NOT_ZERO=1
+```