Add benchmark

Author: winner245

libcxx/test/benchmarks/bitset.bench.cpp

@@ -0,0 +1,22 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include <benchmark/benchmark.h>
+#include <bitset>
+
+static void BM_ctor_ull(benchmark::State& state) {
+  unsigned long long val = (1ULL << state.range(0)) - 1;
+  for (auto _ : state) {
+    std::bitset<128> b(val);
+    benchmark::DoNotOptimize(b);
+  }
+}
+
+BENCHMARK(BM_ctor_ull)->DenseRange(1, 63);
+
+BENCHMARK_MAIN();