benchtests: Add random() benchmark

Add a simple benchmark to measure the overhead of internal libc locks in
the random() implementation on both single- and multi-threaded cases.
This relies on the implementation of random using internal locks to
access shared global data, and that the runtime uses multi-threaded
locking once a thread has been created (even after it finishes).

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

2 files changed, 107 insertions, 0 deletions

diff --git a/benchtests/Makefile b/benchtests/Makefile
index 05b8751..7e73b85 100644
--- a/benchtests/Makefile
+++ b/benchtests/Makefile
@@ -257,6 +257,7 @@ hash-benchset := \
 
 stdlib-benchset := \
   arc4random \
+  random-lock \
   strtod \
   # stdlib-benchset
 
diff --git a/benchtests/bench-random-lock.c b/benchtests/bench-random-lock.c
new file mode 100644
index 0000000..0fbe918
--- /dev/null
+++ b/benchtests/bench-random-lock.c
@@ -0,0 +1,106 @@
+/* Benchmark internal libc locking functions used in random.
+   Copyright (C) 2022-2024 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#define TEST_MAIN
+#define TEST_NAME "random-lock"
+#define TEST_FUNCTION test_main
+#include <pthread.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include "bench-timing.h"
+#include "json-lib.h"
+
+/* Modern cores run 20M iterations in about 1 second.  */
+#define NUM_ITERS 50000000
+
+
+/* Measure the overhead of __libc_lock_lock and __libc_lock_unlock by
+   calling random ().  */
+static void
+bench_random_lock (json_ctx_t *json_ctx, size_t iters)
+{
+  timing_t start, stop, total;
+
+  srandom (0);
+
+  /* Warmup to reduce variations due to frequency scaling.  */
+  for (int i = 0; i < iters / 4; i++)
+    (void) random ();
+
+  TIMING_NOW (start);
+
+  for (int i = 0; i < iters; i++)
+    (void) random ();
+
+  TIMING_NOW (stop);
+
+  TIMING_DIFF (total, start, stop);
+
+  json_element_double (json_ctx, (double) total / (double) iters);
+}
+
+static void *
+thread_start (void *p)
+{
+  return p;
+}
+
+int
+test_main (void)
+{
+  json_ctx_t json_ctx;
+
+  json_init (&json_ctx, 0, stdout);
+
+  json_document_begin (&json_ctx);
+
+  json_attr_string (&json_ctx, "timing_type", TIMING_TYPE);
+  json_attr_object_begin (&json_ctx, "functions");
+  json_attr_object_begin (&json_ctx, "random");
+  json_attr_string (&json_ctx, "bench-variant", "single-threaded");
+  json_array_begin (&json_ctx, "results");
+
+  /* Run benchmark single threaded.  */
+  bench_random_lock (&json_ctx, NUM_ITERS);
+
+  json_array_end (&json_ctx);
+  json_attr_object_end (&json_ctx);
+
+  json_attr_object_begin (&json_ctx, "random");
+  json_attr_string (&json_ctx, "bench-variant", "multi-threaded");
+  json_array_begin (&json_ctx, "results");
+
+  /* Start a short thread to force SINGLE_THREAD_P == false.  This relies on
+     the runtime disabling single-threaded optimizations when multiple
+     threads are used, even after they finish.  */
+
+  pthread_t t;
+  pthread_create (&t, NULL, thread_start, NULL);
+  pthread_join (t, NULL);
+
+  /* Repeat benchmark with single-threaded optimizations disabled.  */
+  bench_random_lock (&json_ctx, NUM_ITERS);
+
+  json_array_end (&json_ctx);
+  json_attr_object_end (&json_ctx);
+  json_attr_object_end (&json_ctx);
+  json_document_end (&json_ctx);
+  return 0;
+}
+
+#include "support/test-driver.c"