$ cc -O2 -g -Wall -pthread -o eqfuzz eqfuzz.c
In file included from eqfuzz.c:18:
feeder_eq.c:499:16: warning: 'feeder_eq_class' defined but not used [-Wunused-variable]
 FEEDER_DECLARE(feeder_eq, NULL);
                ^~~~~~~~~~~~~~
eqshim.h:185:29: note: in definition of macro 'FEEDER_DECLARE'
  static struct feeder_class feeder ## _class = {   \
                             ^~~~~~~
In file included from eqfuzz.c:21:
feeder_eq.c:491:22: warning: 'feeder_eq_methods' defined but not used [-Wunused-variable]
feeder_eq.c:486:30: warning: 'feeder_eq_desc' defined but not used [-Wunused-variable]
(unmodified feeder_eq.c compiles clean of errors; only shim-noop warnings)

$ cc -O2 -g -o eqfuzz_fixed eqfuzz_fixed.c && cc -O2 -pthread -o eqchurn eqchurn.c && cc -O2 -o ospace ospace.c && cc -O0 -g -o dbg4_0 dbg4.c
(ok)

$ cc -O0 -o dbg2 dbg2.c   # same sources, gcc 8.3 [DragonFly] Release/2019-02-22
feeder_eq.c:314:22: warning: iteration 8 invokes undefined behavior [-Waggressive-loop-optimizations]
  info->treble.i1[i] = 0;
  ~~~~~~~~~~~~~~~~~^~~
(only with >8-channel init — gcc itself flags the DF-4711 OOB)