Add benchmark

Author: winner245

libcxx/test/benchmarks/bitset.bench.cpp

@@ -103,4 +103,14 @@ BENCHMARK(BM_BitsetToString<262144>)->Arg(50)->Name("BM_BitsetToString<262144>/U
 BENCHMARK(BM_BitsetToString<524288>)->Arg(50)->Name("BM_BitsetToString<524288>/Uniform (50%)");
 BENCHMARK(BM_BitsetToString<1048576>)->Arg(50)->Name("BM_BitsetToString<1048576>/Uniform (50%)"); // 1 << 20
 
+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();