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// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.
// The wasm32-unknown-unknown target is currently a highly experimental version
// of a wasm-based target which does *not* use the Emscripten toolchain. Instead
// this is a pretty flavorful (aka hacked up) target right now. The definition
// and semantics of this target are likely to change and so this shouldn't be
// relied on just yet.
//
// In general everyone is currently waiting on a linker for wasm code. In the
// meantime we have no means of actually making use of the traditional separate
// compilation model. At a high level this means that assembling Rust programs
// into a WebAssembly program looks like:
//
// 1. All intermediate artifacts are LLVM bytecode. We'll be using LLVM as
// a linker later on.
// 2. For the final artifact we emit one giant assembly file (WebAssembly
// doesn't have an object file format). To do this we force LTO to be turned
// on (`requires_lto` below) to ensure all Rust code is in one module. Any
// "linked" C library is basically just ignored.
// 3. Using LLVM we emit a `foo.s` file (assembly) with some... what I can only
// describe as arcane syntax. From there we need to actually change this
// into a wasm module. For this step we use the `binaryen` project. This
// project is mostly intended as a WebAssembly code generator, but for now
// we're just using its LLVM-assembly-to-wasm-module conversion utilities.
//
// And voila, out comes a web assembly module! There's some various tweaks here
// and there, but that's the high level at least. Note that this will be
// rethought from the ground up once a linker (lld) is available, so this is all
// temporary and should improve in the future.
use LinkerFlavor;
use super::{Target, TargetOptions, PanicStrategy};
pub fn target() -> Result<Target, String> {
let opts = TargetOptions {
linker: "not-used".to_string(),
// we allow dynamic linking, but only cdylibs. Basically we allow a
// final library artifact that exports some symbols (a wasm module) but
// we don't allow intermediate `dylib` crate types
dynamic_linking: true,
only_cdylib: true,
// This means we'll just embed a `start` function in the wasm module
executables: true,
// relatively self-explanatory!
exe_suffix: ".wasm".to_string(),
dll_prefix: "".to_string(),
dll_suffix: ".wasm".to_string(),
linker_is_gnu: false,
// We're storing bitcode for now in all the rlibs
obj_is_bitcode: true,
// A bit of a lie, but "eh"
max_atomic_width: Some(32),
// Unwinding doesn't work right now, so the whole target unconditionally
// defaults to panic=abort. Note that this is guaranteed to change in
// the future once unwinding is implemented. Don't rely on this.
panic_strategy: PanicStrategy::Abort,
// There's no linker yet so we're forced to use LLVM as a linker. This
// means that we must always enable LTO for final artifacts.
requires_lto: true,
// Wasm doesn't have atomics yet, so tell LLVM that we're in a single
// threaded model which will legalize atomics to normal operations.
singlethread: true,
// Because we're always enabling LTO we can't enable builtin lowering as
// otherwise we'll lower the definition of the `memcpy` function to
// memcpy itself. Note that this is specifically because we're
// performing LTO with compiler-builtins.
no_builtins: true,
// no dynamic linking, no need for default visibility!
default_hidden_visibility: true,
.. Default::default()
};
Ok(Target {
llvm_target: "wasm32-unknown-unknown".to_string(),
target_endian: "little".to_string(),
target_pointer_width: "32".to_string(),
target_c_int_width: "32".to_string(),
// This is basically guaranteed to change in the future, don't rely on
// this. Use `not(target_os = "emscripten")` for now.
target_os: "unknown".to_string(),
target_env: "".to_string(),
target_vendor: "unknown".to_string(),
data_layout: "e-m:e-p:32:32-i64:64-n32:64-S128".to_string(),
arch: "wasm32".to_string(),
// A bit of a lie, but it gets the job done
linker_flavor: LinkerFlavor::Binaryen,
options: opts,
})
}
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