benchtests: Add pthread_once common-case test.

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.  This gives us, on average, the
best-case latency of pthread_once (the initialization is the
exactly-once slow path, and we're not looking at initialization-related
synchronization overheads in this case).

1 files changed, 2 insertions, 0 deletions

diff --git a/benchtests/README b/benchtests/README
index 2a940fab7f..52a3cc2062 100644
--- a/benchtests/README
+++ b/benchtests/README
@@ -60,6 +60,8 @@ one to add `foo' to the bench tests:
   - include-sources: This should be assigned a comma-separated list of source
     files that need to be included to provide definitions of global variables
     and functions (specifically, this includes using "#include "source").
+    See pthread_once-inputs and pthreads_once-source.c for an example of how
+    to use this to benchmark a function that needs state across several calls.
   - name: See following section for instructions on how to use this directive.
 
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