Add target to use LLVM wasm backend

The new target is wasm32-experimental-emscripten. Adds a new
configuration option to opt in to building experimental LLVM backends
such as the WebAssembly backend. The target name was chosen to be
similar to the existing wasm32-unknown-emscripten target so that the
build and tests would work with minimal other code changes. When/if the
new target replaces the old target, simply renaming it should just work.

src/bootstrap/config.rs

@@ -60,6 +60,7 @@ pub struct Config {
     pub llvm_static_stdcpp: bool,
     pub llvm_link_shared: bool,
     pub llvm_targets: Option<String>,
+    pub llvm_experimental_targets: Option<String>,
     pub llvm_link_jobs: Option<u32>,
     pub llvm_clean_rebuild: bool,
 
@@ -189,6 +190,7 @@ struct Llvm {
     version_check: Option<bool>,
     static_libstdcpp: Option<bool>,
     targets: Option<String>,
+    experimental_targets: Option<String>,
     link_jobs: Option<u32>,
     clean_rebuild: Option<bool>,
 }
@@ -350,6 +352,7 @@ impl Config {
             set(&mut config.llvm_static_stdcpp, llvm.static_libstdcpp);
             set(&mut config.llvm_clean_rebuild, llvm.clean_rebuild);
             config.llvm_targets = llvm.targets.clone();
+            config.llvm_experimental_targets = llvm.experimental_targets.clone();
             config.llvm_link_jobs = llvm.link_jobs;
         }
 

src/bootstrap/config.toml.example

@@ -53,6 +53,14 @@
 # Rust team and file an issue if you need assistance in porting!
 #targets = "X86;ARM;AArch64;Mips;PowerPC;SystemZ;JSBackend;MSP430;Sparc;NVPTX;Hexagon"
 
+# LLVM experimental targets to build support for. These targets are specified in
+# the same format as above, but since these targets are experimental, they are
+# not built by default and the experimental Rust compilation targets that depend
+# on them will not work unless the user opts in to building them. Possible
+# experimental LLVM targets include WebAssembly for the
+# wasm32-experimental-emscripten Rust target.
+#experimental-targets = ""
+
 # Cap the number of parallel linker invocations when compiling LLVM.
 # This can be useful when building LLVM with debug info, which significantly
 # increases the size of binaries and consequently the memory required by

src/bootstrap/native.rs

@@ -86,6 +86,11 @@ pub fn llvm(build: &Build, target: &str) {
         None => "X86;ARM;AArch64;Mips;PowerPC;SystemZ;JSBackend;MSP430;Sparc;NVPTX;Hexagon",
     };
 
+    let llvm_exp_targets = match build.config.llvm_experimental_targets {
+        Some(ref s) => s,
+        None => "",
+    };
+
     let assertions = if build.config.llvm_assertions {"ON"} else {"OFF"};
 
     cfg.target(target)
@@ -94,6 +99,7 @@ pub fn llvm(build: &Build, target: &str) {
        .profile(profile)
        .define("LLVM_ENABLE_ASSERTIONS", assertions)
        .define("LLVM_TARGETS_TO_BUILD", llvm_targets)
+       .define("LLVM_EXPERIMENTAL_TARGETS_TO_BUILD", llvm_exp_targets)
        .define("LLVM_INCLUDE_EXAMPLES", "OFF")
        .define("LLVM_INCLUDE_TESTS", "OFF")
        .define("LLVM_INCLUDE_DOCS", "OFF")

src/librustc_back/target/mod.rs

@@ -214,6 +214,7 @@ supported_targets! {
     ("le32-unknown-nacl", le32_unknown_nacl),
     ("asmjs-unknown-emscripten", asmjs_unknown_emscripten),
     ("wasm32-unknown-emscripten", wasm32_unknown_emscripten),
+    ("wasm32-experimental-emscripten", wasm32_experimental_emscripten),
 
     ("thumbv6m-none-eabi", thumbv6m_none_eabi),
     ("thumbv7m-none-eabi", thumbv7m_none_eabi),

src/librustc_back/target/wasm32_experimental_emscripten.rs

@@ -0,0 +1,53 @@
+// Copyright 2015 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.
+
+use LinkerFlavor;
+use super::{LinkArgs, Target, TargetOptions};
+use super::emscripten_base::{cmd};
+
+pub fn target() -> Result<Target, String> {
+    let mut post_link_args = LinkArgs::new();
+    post_link_args.insert(LinkerFlavor::Em,
+                          vec!["-s".to_string(),
+                               "WASM=1".to_string(),
+                               "-s".to_string(),
+                               "ERROR_ON_UNDEFINED_SYMBOLS=1".to_string()]);
+
+    let opts = TargetOptions {
+        linker: cmd("emcc"),
+        ar: cmd("emar"),
+
+        dynamic_linking: false,
+        executables: true,
+        // Today emcc emits two files - a .js file to bootstrap and
+        // possibly interpret the wasm, and a .wasm file
+        exe_suffix: ".js".to_string(),
+        linker_is_gnu: true,
+        allow_asm: false,
+        obj_is_bitcode: true,
+        is_like_emscripten: true,
+        max_atomic_width: Some(32),
+        post_link_args: post_link_args,
+        target_family: Some("unix".to_string()),
+        .. Default::default()
+    };
+    Ok(Target {
+        llvm_target: "wasm32-unknown-unknown".to_string(),
+        target_endian: "little".to_string(),
+        target_pointer_width: "32".to_string(),
+        target_os: "emscripten".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(),
+        linker_flavor: LinkerFlavor::Em,
+        options: opts,
+    })
+}

src/librustc_llvm/build.rs

@@ -93,7 +93,7 @@ fn main() {
 
     let mut optional_components =
         vec!["x86", "arm", "aarch64", "mips", "powerpc", "pnacl",
-             "systemz", "jsbackend", "msp430", "sparc", "nvptx"];
+             "systemz", "jsbackend", "webassembly", "msp430", "sparc", "nvptx"];
 
     let mut version_cmd = Command::new(&llvm_config);
     version_cmd.arg("--version");

src/librustc_llvm/lib.rs

@@ -389,6 +389,11 @@ pub fn initialize_available_targets() {
                  LLVMInitializeHexagonTargetMC,
                  LLVMInitializeHexagonAsmPrinter,
                  LLVMInitializeHexagonAsmParser);
+    init_target!(llvm_component = "webassembly",
+                 LLVMInitializeWebAssemblyTargetInfo,
+                 LLVMInitializeWebAssemblyTarget,
+                 LLVMInitializeWebAssemblyTargetMC,
+                 LLVMInitializeWebAssemblyAsmPrinter);
 }
 
 pub fn last_error() -> Option<String> {