cranelift-wasm: support multi-value Wasm (#1049)

This commit introduces initial support for multi-value Wasm. Wasm blocks and
calls can now take and return an arbitrary number of values.

The encoding for multi-value blocks means that we need to keep the contents of
the "Types" section around when translating function bodies. To do this, we
introduce a `WasmTypesMap` type that maps the type indices to their parameters
and returns, construct it when parsing the "Types" section, and shepherd it
through a bunch of functions and methods when translating function bodies.

cranelift/wasm/src/environ/spec.rs

@@ -15,6 +15,7 @@ use cranelift_codegen::cursor::FuncCursor;
 use cranelift_codegen::ir::immediates::Offset32;
 use cranelift_codegen::ir::{self, InstBuilder};
 use cranelift_codegen::isa::TargetFrontendConfig;
+use cranelift_entity::PrimaryMap;
 use cranelift_frontend::FunctionBuilder;
 use failure_derive::Fail;
 use std::boxed::Box;
@@ -103,6 +104,24 @@ pub enum ReturnMode {
     FallthroughReturn,
 }
 
+/// A map containing a Wasm module's original, raw signatures.
+///
+/// This is used for translating multi-value Wasm blocks inside functions, which
+/// are encoded to refer to their type signature via index.
+#[derive(Debug)]
+pub struct WasmTypesMap {
+    pub(crate) inner:
+        PrimaryMap<SignatureIndex, (Box<[wasmparser::Type]>, Box<[wasmparser::Type]>)>,
+}
+
+impl WasmTypesMap {
+    pub(crate) fn new() -> Self {
+        WasmTypesMap {
+            inner: PrimaryMap::new(),
+        }
+    }
+}
+
 /// Environment affecting the translation of a single WebAssembly function.
 ///
 /// A `FuncEnvironment` trait object is required to translate a WebAssembly function to Cranelift
@@ -449,6 +468,7 @@ pub trait ModuleEnvironment<'data> {
     /// functions is already provided by `reserve_func_types`.
     fn define_function_body(
         &mut self,
+        wasm_types: &WasmTypesMap,
         body_bytes: &'data [u8],
         body_offset: usize,
     ) -> WasmResult<()>;