Bluegill

#include <benchmark/benchmark.h>
static inline constexpr int FIBONACCI_N = 10;

int fib(int n) { return n < 2 ? n : fib(n - 1) + fib(n - 2); }
static void BM_fib(benchmark::State &state) {
    for (auto _ : state) {
        int res = fib(FIBONACCI_N);
        benchmark::DoNotOptimize(res);
    }
}
BENCHMARK(BM_fib);

int x;  // Magic line.
        // clang does not use DP anymore: much slower.
        // gcc chooses a different optimizing approach: much faster.

int fib_x(int n) { return n < 2 ? n : fib_x(n - 1) + fib_x(n - 2) + x; }
static void BM_fib_x(benchmark::State &state) {
    for (auto _ : state) {
        int res = fib_x(FIBONACCI_N);
        benchmark::DoNotOptimize(res);
    }
}
BENCHMARK(BM_fib_x);

BENCHMARK_MAIN();