Fix broken links - 1st run

1 files changed, 42 insertions, 1 deletions

diff --git a/instrumentation/README.llvm.md b/instrumentation/README.llvm.md
index fa025643..ca9ce933 100644
--- a/instrumentation/README.llvm.md
+++ b/instrumentation/README.llvm.md
@@ -234,4 +234,45 @@ are 2-16.
 
 It is highly recommended to increase the MAP_SIZE_POW2 definition in config.h to
 at least 18 and maybe up to 20 for this as otherwise too many map collisions
-occur.
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+occur.
+
+## 8) NeverZero counters
+
+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 behavior. If a counter wraps, it jumps over the value 0
+directly to a 1. This improves path discovery (by a very small amount) at a very
+low 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 behavior, 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`.
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