bench: benchmark several common WASI scenarios (#5274)

In order to properly understand the impact of providing thread-safe
implmentations of WASI contexts (#5235), we need benchmarks that measure
the current performance of WASI calls using Wiggle. This change adds
several common WASI scenarios as WAT files (see `benches/wasi/*.wat`)
and benchmarks them with `criterion`. Using `criterion`'s `iter_custom`,
the WAT file runs the desired number of benchmark iterations internally
and the total duration of the runs is divided to get the average time
for each loop iteration.

Why WAT? When compiling these benchmarks from Rust to `wasm32-wasi`, the
output files are large, contain other WASI imports than the desired
ones, and overall it is difficult to tell if we are measuring what we
expect. By hand-writing the WAT, it is (slightly) more clear what each
benchmark is doing.

benches/wasi.rs

@@ -0,0 +1,71 @@
+//! Measure some common WASI call scenarios.
+
+use criterion::{criterion_group, criterion_main, Criterion};
+use std::time::Instant;
+use wasmtime::{Engine, Linker, Module, Store, TypedFunc};
+use wasmtime_wasi::{sync::WasiCtxBuilder, WasiCtx};
+
+criterion_group!(benches, bench_wasi);
+criterion_main!(benches);
+
+fn bench_wasi(c: &mut Criterion) {
+    // Benchmark each `*.wat` file in the `wasi` directory.
+    for file in std::fs::read_dir("benches/wasi").unwrap() {
+        let path = file.unwrap().path();
+        if path.extension().map(|e| e == "wat").unwrap_or(false) {
+            let wat = std::fs::read(&path).unwrap();
+            let (mut store, run_fn) = instantiate(&wat);
+            let bench_name = format!("wasi/{}", path.file_name().unwrap().to_string_lossy());
+            // To avoid overhead, the module itself must iterate the expected
+            // number of times in a specially-crafted `run` function (see
+            // `instantiate` for details).
+            c.bench_function(&bench_name, move |b| {
+                b.iter_custom(|iters| {
+                    let start = Instant::now();
+                    let result = run_fn.call(&mut store, iters).unwrap();
+                    assert_eq!(iters, result);
+                    start.elapsed()
+                })
+            });
+        }
+    }
+}
+
+/// Compile and instantiate the Wasm module, returning the exported `run`
+/// function. This function expects `run` to:
+/// - have a single `u64` parameter indicating the number of loop iterations to
+///   execute
+/// - execute the body of the function for that number of loop iterations
+/// - return a single `u64` indicating how many loop iterations were executed
+///   (to double-check)
+fn instantiate(wat: &[u8]) -> (Store<WasiCtx>, TypedFunc<u64, u64>) {
+    let engine = Engine::default();
+    let wasi = wasi_context();
+    let mut store = Store::new(&engine, wasi);
+    let module = Module::new(&engine, wat).unwrap();
+    let mut linker = Linker::new(&engine);
+    wasmtime_wasi::add_to_linker(&mut linker, |cx| cx).unwrap();
+    let instance = linker.instantiate(&mut store, &module).unwrap();
+    let run = instance.get_typed_func(&mut store, "run").unwrap();
+    (store, run)
+}
+
+/// Build a WASI context with some actual data to retrieve.
+fn wasi_context() -> WasiCtx {
+    let wasi = WasiCtxBuilder::new();
+    wasi.envs(&[
+        ("a".to_string(), "b".to_string()),
+        ("b".to_string(), "c".to_string()),
+        ("c".to_string(), "d".to_string()),
+    ])
+    .unwrap()
+    .args(&[
+        "exe".to_string(),
+        "--flag1".to_string(),
+        "--flag2".to_string(),
+        "--flag3".to_string(),
+        "--flag4".to_string(),
+    ])
+    .unwrap()
+    .build()
+}