Add basic lookup benchmark

Based on the benchmark in the thread_local crate.
Also benchmarks thread_local + Arc overhead.
On my M1 Mac, Arc::clone dwarfs the overhead of all lookups.

This is unfortunate, because Arc::clone is required for our crate.

Author: Techcable

Cargo.toml

@@ -18,11 +18,19 @@ foldhash = "0.1"
 # persistent collections
 imbl = "5"
 
+[dev-dependencies]
+thread_local = "1"
+criterion = { version = "0.7" }
+
 [features]
 nightly = [
     "parking_lot/nightly",
 ]
 
+[[bench]]
+name = "lookup"
+harness = false
+
 [lints.rust]
 missing-docs = "deny"
 rust-2024-compatibility = "warn"

benches/lookup.rs

@@ -0,0 +1,50 @@
+#![allow(missing_docs)]
+use std::hint::black_box;
+use std::sync::Arc;
+
+use dropping_thread_local::DroppingThreadLocal;
+use thread_local::ThreadLocal;
+
+fn main() {
+    let mut c = criterion::Criterion::default().configure_from_args();
+
+    c.bench_function("std::get", |b| {
+        std::thread_local!(static LOCAL: Box<i32> = Box::new(0));
+        LOCAL.with(|x| black_box(**x));
+        b.iter(|| {
+            black_box(LOCAL.with(|x| **x));
+        });
+    });
+
+    c.bench_function("std::get_arc", |b| {
+        std::thread_local!(static LOCAL: Arc<i32> = Arc::new(0));
+        LOCAL.with(|x| black_box(**x));
+        b.iter_with_large_drop(|| {
+            black_box(LOCAL.with(|x| Arc::clone(x)));
+        });
+    });
+
+    c.bench_function("thread_local::get", |b| {
+        let local = ThreadLocal::new();
+        local.get_or(|| Box::new(0));
+        b.iter(|| {
+            black_box(local.get());
+        });
+    });
+
+    c.bench_function("thread_local::get_arc", |b| {
+        let local = ThreadLocal::new();
+        local.get_or(|| Arc::new(0));
+        b.iter_with_large_drop(|| {
+            black_box(Arc::clone(local.get().unwrap()));
+        });
+    });
+
+    c.bench_function("dropping_thread_local::get", |b| {
+        let local = DroppingThreadLocal::new();
+        local.get_or_init(|| Box::new(0));
+        b.iter_with_large_drop(|| {
+            black_box(local.get());
+        });
+    });
+}