final touches

docs/ChangeLog

@@ -17,6 +17,8 @@ sending a mail to <afl-users+subscribe@googlegroups.com>.
 Version ++2.52d (tbd):
 -----------------------------
 
+  - added never zero counters for afl-gcc and optional (because of an
+    optimization issue in llvm < 9) for llvm_mode (AFL_LLVM_NEVER_ZERO=1)
   - added whitelist support for llvm_mode via AFL_LLVM_WHITELIST to allow
     only to instrument what is actually interesting. Gives more speed and less
     map pollution (originally by choller@mozilla)

docs/env_variables.txt

@@ -108,8 +108,11 @@ Then there are a few specific features that are only available in llvm_mode:
 
   OTHER
   =====
-    - Setting AFL_NZERO_COUNTS=1 during compilation will use counters
-      that skip zero on overflow.
+    - Setting export AFL_LLVM_NOT_ZERO=1 during compilation will use counters
+      that skip zero on overflow. This is the default for llvm >= 9,
+      however for llvm versions below that this will increase an unnecessary
+      slowdown due a performance issue that is only fixed in llvm 9+.
+      This feature increases path discovery by a little bit.
 
 Note that AFL_INST_RATIO will behave a bit differently than for afl-gcc,
 because functions are *not* instrumented unconditionally - so low values