glibc/benchtests/pthread_once-inputs

# This tests the common-case scenario for pthread_once.
# We have a single thread that runs a no-op initialization once and then
# repeatedly runs checks of the initialization (i.e., an acquire load and
# conditional jump) in a tight loop.
# scripts/bench.py doesn't handle function pointers, so we just use void *:
## args: pthread_once_t *:void *
## includes: pthread.h
## include-sources: pthread_once-source.c
&once, once_handler