chore: add vendor dependencies for kauma build

vendor/once_cell/examples/bench_acquire.rs

@@ -0,0 +1,39 @@
+//! Benchmark the overhead that the synchronization of `OnceCell::get` causes.
+//! We do some other operations that write to memory to get an imprecise but somewhat realistic
+//! measurement.
+
+use once_cell::sync::OnceCell;
+use std::sync::atomic::{AtomicUsize, Ordering};
+
+const N_THREADS: usize = 16;
+const N_ROUNDS: usize = 1_000_000;
+
+static CELL: OnceCell<usize> = OnceCell::new();
+static OTHER: AtomicUsize = AtomicUsize::new(0);
+
+fn main() {
+    let start = std::time::Instant::now();
+    let threads =
+        (0..N_THREADS).map(|i| std::thread::spawn(move || thread_main(i))).collect::<Vec<_>>();
+    for thread in threads {
+        thread.join().unwrap();
+    }
+    println!("{:?}", start.elapsed());
+    println!("{:?}", OTHER.load(Ordering::Relaxed));
+}
+
+#[inline(never)]
+fn thread_main(i: usize) {
+    // The operations we do here don't really matter, as long as we do multiple writes, and
+    // everything is messy enough to prevent the compiler from optimizing the loop away.
+    let mut data = [i; 128];
+    let mut accum = 0usize;
+    for _ in 0..N_ROUNDS {
+        let _value = CELL.get_or_init(|| i + 1);
+        let k = OTHER.fetch_add(data[accum & 0x7F] as usize, Ordering::Relaxed);
+        for j in data.iter_mut() {
+            *j = (*j).wrapping_add(accum);
+            accum = accum.wrapping_add(k);
+        }
+    }
+}