wasmtime: add build-time option for parallel compilation (#1903)

When running in embedded environments, threads creation is sometimes
undesirable. This adds a feature to toggle wasmtime's internal thread
creation for parallel compilation.

Cargo.lock

@@ -2417,6 +2417,7 @@ dependencies = [
  "anyhow",
  "base64 0.12.1",
  "bincode",
+ "cfg-if",
  "cranelift-codegen",
  "cranelift-entity",
  "cranelift-frontend",

Cargo.toml

@@ -72,7 +72,7 @@ members = [
 ]
 
 [features]
-default = ["jitdump", "wasmtime/wat"]
+default = ["jitdump", "wasmtime/wat", "wasmtime/parallel-compilation"]
 lightbeam = [
     "wasmtime-environ/lightbeam",
     "wasmtime-jit/lightbeam",

crates/environ/Cargo.toml

@@ -20,7 +20,7 @@ cranelift-wasm = { path = "../../cranelift/wasm", version = "0.65.0", features =
 wasmparser = "0.58.0"
 lightbeam = { path = "../lightbeam", optional = true, version = "0.18.0" }
 indexmap = "1.0.2"
-rayon = "1.2.1"
+rayon = { version = "1.2.1", optional = true }
 thiserror = "1.0.4"
 directories = "2.0.1"
 sha2 = "0.8.0"
@@ -32,6 +32,7 @@ zstd = "0.5"
 toml = "0.5.5"
 file-per-thread-logger = "0.1.1"
 more-asserts = "0.2.1"
+cfg-if = "0.1.9"
 
 [target.'cfg(target_os = "windows")'.dependencies]
 winapi = "0.3.7"
@@ -46,5 +47,8 @@ pretty_env_logger = "0.4.0"
 filetime = "0.2.7"
 lazy_static = "1.3.0"
 
+[features]
+parallel-compilation = ["rayon"]
+
 [badges]
 maintenance = { status = "actively-developed" }

crates/environ/src/cranelift.rs

@@ -99,6 +99,7 @@ use cranelift_codegen::print_errors::pretty_error;
 use cranelift_codegen::{binemit, isa, Context};
 use cranelift_entity::PrimaryMap;
 use cranelift_wasm::{DefinedFuncIndex, FuncIndex, FuncTranslator, ModuleTranslationState};
+#[cfg(feature = "parallel-compilation")]
 use rayon::prelude::{IntoParallelRefIterator, ParallelIterator};
 use std::convert::TryFrom;
 use std::hash::{Hash, Hasher};
@@ -318,11 +319,10 @@ fn compile(env: CompileEnv<'_>) -> Result<ModuleCacheDataTupleType, CompileError
     let mut traps = PrimaryMap::with_capacity(env.function_body_inputs.len());
     let mut stack_maps = PrimaryMap::with_capacity(env.function_body_inputs.len());
 
-    env.function_body_inputs
-        .into_iter()
-        .collect::<Vec<(DefinedFuncIndex, &FunctionBodyData<'_>)>>()
-        .par_iter()
-        .map_init(FuncTranslator::new, |func_translator, (i, input)| {
+    type FunctionBodyInput<'a> = (DefinedFuncIndex, &'a FunctionBodyData<'a>);
+
+    let compile_function = |func_translator: &mut FuncTranslator,
+                            (i, input): &FunctionBodyInput| {
         let func_index = env.local.func_index(*i);
         let mut context = Context::new();
         context.func.name = get_func_name(func_index);
@@ -416,10 +416,28 @@ fn compile(env: CompileEnv<'_>) -> Result<ModuleCacheDataTupleType, CompileError
             unwind_info,
             stack_map_sink.finish(),
         ))
-        })
-        .collect::<Result<Vec<_>, CompileError>>()?
-        .into_iter()
-        .for_each(
+    };
+
+    let inputs: Vec<FunctionBodyInput> = env.function_body_inputs.into_iter().collect();
+
+    let results: Result<Vec<_>, CompileError> = {
+        cfg_if::cfg_if! {
+            if #[cfg(feature = "parallel-compilation")] {
+                inputs
+                    .par_iter()
+                    .map_init(FuncTranslator::new, compile_function)
+                    .collect()
+            } else {
+                let mut func_translator = FuncTranslator::new();
+                inputs
+                    .iter()
+                    .map(|input| compile_function(&mut func_translator, input))
+                    .collect()
+            }
+        }
+    };
+
+    results?.into_iter().for_each(
         |(
             function,
             func_jt_offsets,

crates/wasmtime/Cargo.toml

@@ -38,7 +38,7 @@ wasmtime-wasi = { path = "../wasi" }
 maintenance = { status = "actively-developed" }
 
 [features]
-default = ['wat', 'jitdump']
+default = ['wat', 'jitdump', 'parallel-compilation']
 
 # Enables experimental support for the lightbeam codegen backend, an alternative
 # to cranelift. Requires Nightly Rust currently, and this is not enabled by
@@ -50,3 +50,6 @@ jitdump = ["wasmtime-jit/jitdump"]
 
 # Enables support for the `VTune` profiler
 vtune = ["wasmtime-jit/vtune"]
+
+# Enables parallel compilation of WebAssembly code
+parallel-compilation = ["wasmtime-environ/parallel-compilation"]

crates/wiggle/wasmtime/Cargo.toml

@@ -11,7 +11,7 @@ repository = "https://github.com/bytecodealliance/wasmtime"
 include = ["src/**/*", "LICENSE"]
 
 [dependencies]
-wasmtime = { path = "../../wasmtime", version = "0.18.0" }
+wasmtime = { path = "../../wasmtime", version = "0.18.0", default-features = false }
 wasmtime-wiggle-macro = { path = "./macro", version = "0.18.0" }
 witx = { path = "../../wasi-common/WASI/tools/witx", version = "0.8.5", optional = true }
 wiggle = { path = "..", version = "0.18.0" }