Introduce benchmarking API

The new crate introduced here, `wasmtime-bench-api`, creates a shared library, e.g. `wasmtime_bench_api.so`, for executing Wasm benchmarks using Wasmtime. It allows us to measure several phases separately by exposing `engine_compile_module`, `engine_instantiate_module`, and `engine_execute_module`, which pass around an opaque pointer to the internally initialized state. This state is initialized and freed by `engine_create` and `engine_free`, respectively. The API also introduces a way of passing in functions to satisfy the `"bench" "start"` and `"bench" "end"` symbols that we expect Wasm benchmarks to import. The API is exposed in a C-compatible way so that we can dynamically load it (carefully) in our benchmark runner.

Cargo.toml

@@ -64,6 +64,7 @@ opt-level = 0
 [workspace]
 members = [
   "cranelift",
+  "crates/bench-api",
   "crates/c-api",
   "crates/fuzzing",
   "crates/misc/run-examples",

crates/bench-api/Cargo.toml

@@ -0,0 +1,25 @@
+[package]
+name = "wasmtime-bench-api"
+version = "0.19.0"
+authors = ["The Wasmtime Project Developers"]
+description = "Exposes a benchmarking API for the Wasmtime runtime"
+license = "Apache-2.0 WITH LLVM-exception"
+repository = "https://github.com/bytecodealliance/wasmtime"
+readme = "README.md"
+edition = "2018"
+publish = false
+
+[lib]
+name = "wasmtime_bench_api"
+crate-type = ["rlib", "cdylib"]
+# The rlib is only included here so that `cargo test` will run.
+
+[dependencies]
+anyhow = "1.0"
+wasmtime = { path = "../wasmtime", default-features = false }
+wasmtime-wasi = { path = "../wasi" }
+wasi-common = { path = "../wasi-common" }
+
+
+[dev-dependencies]
+wat = "1.0"

crates/bench-api/src/lib.rs

@@ -0,0 +1,183 @@
+//! Expose a C-compatible API for controlling the Wasmtime engine during benchmarking. The API expects very sequential
+//! use:
+//!  - `engine_create`
+//!  - `engine_compile_module`
+//!  - `engine_instantiate_module`
+//!  - `engine_execute_module`
+//!  - `engine_free`
+//!
+//! An example of this C-style usage, without error checking, is shown below:
+//!
+//! ```
+//! use wasmtime_bench_api::*;
+//! let module = wat::parse_bytes(br#"(module
+//!    (func $bench_start (import "bench" "start"))
+//!    (func $bench_end (import "bench" "end"))
+//!    (func $start (export "_start")
+//!      (call $bench_start) (i32.const 2) (i32.const 2) (i32.add) (drop) (call $bench_end))
+//! )"#).unwrap();
+//! let engine = unsafe { engine_create(module.as_ptr(), module.len()) };
+//!
+//! // Start compilation timer.
+//! unsafe { engine_compile_module(engine) };
+//! // End compilation timer.
+//!
+//! // The Wasm benchmark will expect us to provide functions to start ("bench" "start") and stop ("bench" "stop") the
+//! // measurement counters/timers during execution; here we provide a no-op implementation.
+//! extern "C" fn noop() {}
+//!
+//! // Start instantiation timer.
+//! unsafe { engine_instantiate_module(engine, noop, noop) };
+//! // End instantiation timer.
+//!
+//! // No need to start timers for the execution since, by convention, the timer functions we passed during
+//! // instantiation will be called by the benchmark at the appropriate time (before and after the benchmarked section).
+//! unsafe { engine_execute_module(engine) };
+//!
+//! unsafe { engine_free(engine) }
+//! ```
+use anyhow::Result;
+use core::slice;
+use std::os::raw::c_int;
+use wasi_common::WasiCtxBuilder;
+use wasmtime::{Config, Engine, Instance, Linker, Module, Store};
+use wasmtime_wasi::Wasi;
+
+/// Exposes a C-compatible way of creating the engine from the bytes of a single Wasm module. This function returns a
+/// pointer to an opaque structure that contains the engine's initialized state.
+#[no_mangle]
+pub extern "C" fn engine_create(
+    wasm_bytes: *const u8,
+    wasm_bytes_length: usize,
+) -> *mut OpaqueEngineState {
+    let wasm_bytes = unsafe { slice::from_raw_parts(wasm_bytes, wasm_bytes_length) };
+    let state = Box::new(EngineState::new(wasm_bytes));
+    Box::into_raw(state) as *mut _
+}
+
+/// Free the engine state allocated by this library.
+#[no_mangle]
+pub extern "C" fn engine_free(state: *mut OpaqueEngineState) {
+    unsafe {
+        Box::from_raw(state);
+    }
+}
+
+/// Compile the Wasm benchmark module.
+#[no_mangle]
+pub extern "C" fn engine_compile_module(state: *mut OpaqueEngineState) -> c_int {
+    let result = unsafe { OpaqueEngineState::convert(state) }.compile();
+    to_c_error(result, "failed to compile")
+}
+
+/// Instantiate the Wasm benchmark module.
+#[no_mangle]
+pub extern "C" fn engine_instantiate_module(
+    state: *mut OpaqueEngineState,
+    bench_start: extern "C" fn(),
+    bench_end: extern "C" fn(),
+) -> c_int {
+    let result = unsafe { OpaqueEngineState::convert(state) }.instantiate(bench_start, bench_end);
+    to_c_error(result, "failed to instantiate")
+}
+
+/// Execute the Wasm benchmark module.
+#[no_mangle]
+pub extern "C" fn engine_execute_module(state: *mut OpaqueEngineState) -> c_int {
+    let result = unsafe { OpaqueEngineState::convert(state) }.execute();
+    to_c_error(result, "failed to execute")
+}
+
+/// Helper function for converting a Rust result to a C error code (0 == success). Additionally, this will print an
+/// error indicating some information regarding the failure.
+fn to_c_error<T>(result: Result<T>, message: &str) -> c_int {
+    match result {
+        Ok(_) => 0,
+        Err(error) => {
+            println!("{}: {:?}", message, error);
+            1
+        }
+    }
+}
+
+/// Opaque pointer type for hiding the engine state details.
+#[repr(C)]
+pub struct OpaqueEngineState {
+    _private: [u8; 0],
+}
+impl OpaqueEngineState {
+    unsafe fn convert(ptr: *mut OpaqueEngineState) -> &'static mut EngineState<'static> {
+        assert!(!ptr.is_null());
+        &mut *(ptr as *mut EngineState)
+    }
+}
+
+/// This structure contains the actual Rust implementation of the state required to manage the Wasmtime engine between
+/// calls.
+struct EngineState<'a> {
+    bytes: &'a [u8],
+    engine: Engine,
+    store: Store,
+    module: Option<Module>,
+    instance: Option<Instance>,
+}
+
+impl<'a> EngineState<'a> {
+    fn new(bytes: &'a [u8]) -> Self {
+        // TODO turn off caching?
+        let engine = Engine::new(&Config::new());
+        let store = Store::new(&engine);
+        Self {
+            bytes,
+            engine,
+            store,
+            module: None,
+            instance: None,
+        }
+    }
+
+    fn compile(&mut self) -> Result<()> {
+        self.module = Some(Module::from_binary(&self.engine, self.bytes)?);
+        Ok(())
+    }
+
+    fn instantiate(
+        &mut self,
+        bench_start: extern "C" fn(),
+        bench_end: extern "C" fn(),
+    ) -> Result<()> {
+        // TODO instantiate WASI modules?
+        match &self.module {
+            Some(module) => {
+                let mut linker = Linker::new(&self.store);
+
+                // Import a very restricted WASI environment.
+                let mut cx = WasiCtxBuilder::new();
+                cx.inherit_stdio();
+                let cx = cx.build()?;
+                let wasi = Wasi::new(linker.store(), cx);
+                wasi.add_to_linker(&mut linker)?;
+
+                // Import the specialized benchmarking functions.
+                linker.func("bench", "start", move || bench_start())?;
+                linker.func("bench", "end", move || bench_end())?;
+
+                self.instance = Some(linker.instantiate(module)?);
+            }
+            None => panic!("compile the module before instantiating it"),
+        }
+        Ok(())
+    }
+
+    fn execute(&self) -> Result<()> {
+        match &self.instance {
+            Some(instance) => {
+                let start_func = instance.get_func("_start").expect("a _start function");
+                let runnable_func = start_func.get0::<()>()?;
+                runnable_func()?;
+            }
+            None => panic!("instantiate the module before executing it"),
+        }
+        Ok(())
+    }
+}