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+% Benchmark tests
+
+Rust supports benchmark tests, which can test the performance of your
+code. Let's make our `src/lib.rs` look like this (comments elided):
+
+```{rust,ignore}
+#![feature(test)]
+
+extern crate test;
+
+pub fn add_two(a: i32) -> i32 {
+    a + 2
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use test::Bencher;
+
+    #[test]
+    fn it_works() {
+        assert_eq!(4, add_two(2));
+    }
+
+    #[bench]
+    fn bench_add_two(b: &mut Bencher) {
+        b.iter(|| add_two(2));
+    }
+}
+```
+
+Note the `test` feature gate, which enables this unstable feature.
+
+We've imported the `test` crate, which contains our benchmarking support.
+We have a new function as well, with the `bench` attribute. Unlike regular
+tests, which take no arguments, benchmark tests take a `&mut Bencher`. This
+`Bencher` provides an `iter` method, which takes a closure. This closure
+contains the code we'd like to benchmark.
+
+We can run benchmark tests with `cargo bench`:
+
+```bash
+$ cargo bench
+   Compiling adder v0.0.1 (file:///home/steve/tmp/adder)
+     Running target/release/adder-91b3e234d4ed382a
+
+running 2 tests
+test tests::it_works ... ignored
+test tests::bench_add_two ... bench:         1 ns/iter (+/- 0)
+
+test result: ok. 0 passed; 0 failed; 1 ignored; 1 measured
+```
+
+Our non-benchmark test was ignored. You may have noticed that `cargo bench`
+takes a bit longer than `cargo test`. This is because Rust runs our benchmark
+a number of times, and then takes the average. Because we're doing so little
+work in this example, we have a `1 ns/iter (+/- 0)`, but this would show
+the variance if there was one.
+
+Advice on writing benchmarks:
+
+
+* Move setup code outside the `iter` loop; only put the part you want to measure inside
+* Make the code do "the same thing" on each iteration; do not accumulate or change state
+* Make the outer function idempotent too; the benchmark runner is likely to run
+  it many times
+*  Make the inner `iter` loop short and fast so benchmark runs are fast and the
+   calibrator can adjust the run-length at fine resolution
+* Make the code in the `iter` loop do something simple, to assist in pinpointing
+  performance improvements (or regressions)
+
+## Gotcha: optimizations
+
+There's another tricky part to writing benchmarks: benchmarks compiled with
+optimizations activated can be dramatically changed by the optimizer so that
+the benchmark is no longer benchmarking what one expects. For example, the
+compiler might recognize that some calculation has no external effects and
+remove it entirely.
+
+```{rust,ignore}
+#![feature(test)]
+
+extern crate test;
+use test::Bencher;
+
+#[bench]
+fn bench_xor_1000_ints(b: &mut Bencher) {
+    b.iter(|| {
+        (0..1000).fold(0, |old, new| old ^ new);
+    });
+}
+```
+
+gives the following results
+
+```text
+running 1 test
+test bench_xor_1000_ints ... bench:         0 ns/iter (+/- 0)
+
+test result: ok. 0 passed; 0 failed; 0 ignored; 1 measured
+```
+
+The benchmarking runner offers two ways to avoid this. Either, the closure that
+the `iter` method receives can return an arbitrary value which forces the
+optimizer to consider the result used and ensures it cannot remove the
+computation entirely. This could be done for the example above by adjusting the
+`b.iter` call to
+
+```rust
+# struct X;
+# impl X { fn iter<T, F>(&self, _: F) where F: FnMut() -> T {} } let b = X;
+b.iter(|| {
+    // note lack of `;` (could also use an explicit `return`).
+    (0..1000).fold(0, |old, new| old ^ new)
+});
+```
+
+Or, the other option is to call the generic `test::black_box` function, which
+is an opaque "black box" to the optimizer and so forces it to consider any
+argument as used.
+
+```rust
+#![feature(test)]
+
+extern crate test;
+
+# fn main() {
+# struct X;
+# impl X { fn iter<T, F>(&self, _: F) where F: FnMut() -> T {} } let b = X;
+b.iter(|| {
+    let n = test::black_box(1000);
+
+    (0..n).fold(0, |a, b| a ^ b)
+})
+# }
+```
+
+Neither of these read or modify the value, and are very cheap for small values.
+Larger values can be passed indirectly to reduce overhead (e.g.
+`black_box(&huge_struct)`).
+
+Performing either of the above changes gives the following benchmarking results
+
+```text
+running 1 test
+test bench_xor_1000_ints ... bench:       131 ns/iter (+/- 3)
+
+test result: ok. 0 passed; 0 failed; 0 ignored; 1 measured
+```
+
+However, the optimizer can still modify a testcase in an undesirable manner
+even when using either of the above.