Roll AFL 2.31b:2.38b and switch coverage to use trace-pc-guard.

third_party/afl/src/docs/perf_tips.txt

 =================================
 Tips for performance optimization
 =================================
 
   This file provides tips for troubleshooting slow or wasteful fuzzing jobs.
   See README for the general instruction manual.
 
 1) Keep your test cases small
 -----------------------------
 
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     on a particular schedule and is known to underestimate the needs of
     short-lived processes spawned by afl-fuzz (or any other fuzzer). On Linux,
     this can be fixed with:
 
     cd /sys/devices/system/cpu
     echo performance | tee cpu*/cpufreq/scaling_governor
 
     On other systems, the impact of CPU scaling will be different; when fuzzing,
     use OS-specific tools to find out if all cores are running at full speed.
 
+  - Transparent huge pages. Some allocators, such as jemalloc, can incur a
+    heavy fuzzing penalty when transparent huge pages (THP) are enabled in the
+    kernel. You can disable this via:
+
+    echo never > /sys/kernel/mm/transparent_hugepage/enabled
+
   - Suboptimal scheduling strategies. The significance of this will vary from
     one target to another, but on Linux, you may want to make sure that the
     following options are set:
 
     echo 1 >/proc/sys/kernel/sched_child_runs_first
     echo 1 >/proc/sys/kernel/sched_autogroup_enabled
 
     Setting a different scheduling policy for the fuzzer process - say
     SCHED_RR - can usually speed things up, too, but needs to be done with
     care.
 
 9) If all other options fail, use -d
 ------------------------------------
 
 For programs that are genuinely slow, in cases where you really can't escape
 using huge input files, or when you simply want to get quick and dirty results
 early on, you can always resort to the -d mode.
 
 The mode causes afl-fuzz to skip all the deterministic fuzzing steps, which
 makes output a lot less neat and can ultimately make the testing a bit less
 in-depth, but it will give you an experience more familiar from other fuzzing
 tools.