rustc: Split Emscripten to a separate codegen backend

This commit introduces a separately compiled backend for Emscripten, avoiding
compiling the `JSBackend` target in the main LLVM codegen backend. This builds
on the foundation provided by #47671 to create a new codegen backend dedicated
solely to Emscripten, removing the `JSBackend` of the main codegen backend in
the process.

A new field was added to each target for this commit which specifies the backend
to use for translation, the default being `llvm` which is the main backend that
we use. The Emscripten targets specify an `emscripten` backend instead of the
main `llvm` one.

There's a whole bunch of consequences of this change, but I'll try to enumerate
them here:

* A *second* LLVM submodule was added in this commit. The main LLVM submodule
  will soon start to drift from the Emscripten submodule, but currently they're
  both at the same revision.
* Logic was added to rustbuild to *not* build the Emscripten backend by default.
  This is gated behind a `--enable-emscripten` flag to the configure script. By
  default users should neither check out the emscripten submodule nor compile
  it.
* The `init_repo.sh` script was updated to fetch the Emscripten submodule from
  GitHub the same way we do the main LLVM submodule (a tarball fetch).
* The Emscripten backend, turned off by default, is still turned on for a number
  of targets on CI. We'll only be shipping an Emscripten backend with Tier 1
  platforms, though. All cross-compiled platforms will not be receiving an
  Emscripten backend yet.

This commit means that when you download the `rustc` package in Rustup for Tier
1 platforms you'll be receiving two trans backends, one for Emscripten and one
that's the general LLVM backend. If you never compile for Emscripten you'll
never use the Emscripten backend, so we may update this one day to only download
the Emscripten backend when you add the Emscripten target. For now though it's
just an extra 10MB gzip'd.

Closes #46819

.gitmodules

@@ -51,3 +51,6 @@
 [submodule "src/doc/rust-by-example"]
 	path = src/doc/rust-by-example
 	url = https://github.com/rust-lang/rust-by-example
+[submodule "src/llvm-emscripten"]
+	path = src/llvm-emscripten
+	url = https://github.com/rust-lang/llvm

.travis.yml

@@ -81,7 +81,7 @@ matrix:
     # OSX 10.7 and `xcode7` is the latest Xcode able to compile LLVM for 10.7.
     - env: >
         RUST_CHECK_TARGET=dist
-        RUST_CONFIGURE_ARGS="--build=i686-apple-darwin --enable-extended --enable-profiler"
+        RUST_CONFIGURE_ARGS="--build=i686-apple-darwin --enable-extended --enable-profiler --enable-emscripten"
         SRC=.
         DEPLOY=1
         RUSTC_RETRY_LINKER_ON_SEGFAULT=1
@@ -95,7 +95,7 @@ matrix:
 
     - env: >
         RUST_CHECK_TARGET=dist
-        RUST_CONFIGURE_ARGS="--target=aarch64-apple-ios,armv7-apple-ios,armv7s-apple-ios,i386-apple-ios,x86_64-apple-ios --enable-extended --enable-sanitizers --enable-profiler"
+        RUST_CONFIGURE_ARGS="--target=aarch64-apple-ios,armv7-apple-ios,armv7s-apple-ios,i386-apple-ios,x86_64-apple-ios --enable-extended --enable-sanitizers --enable-profiler --enable-emscripten"
         SRC=.
         DEPLOY=1
         RUSTC_RETRY_LINKER_ON_SEGFAULT=1

appveyor.yml

@@ -63,6 +63,7 @@ environment:
       --build=x86_64-pc-windows-msvc
       --enable-extended
       --enable-profiler
+      --enable-emscripten
     SCRIPT: python x.py dist
     DEPLOY: 1
   - RUST_CONFIGURE_ARGS: >
@@ -70,10 +71,11 @@ environment:
       --target=i586-pc-windows-msvc
       --enable-extended
       --enable-profiler
+      --enable-emscripten
     SCRIPT: python x.py dist
     DEPLOY: 1
   - MSYS_BITS: 32
-    RUST_CONFIGURE_ARGS: --build=i686-pc-windows-gnu --enable-extended
+    RUST_CONFIGURE_ARGS: --build=i686-pc-windows-gnu --enable-extended --enable-emscripten
     SCRIPT: python x.py dist
     MINGW_URL: https://s3-us-west-1.amazonaws.com/rust-lang-ci2/rust-ci-mirror
     MINGW_ARCHIVE: i686-6.3.0-release-posix-dwarf-rt_v5-rev2.7z
@@ -81,7 +83,7 @@ environment:
     DEPLOY: 1
   - MSYS_BITS: 64
     SCRIPT: python x.py dist
-    RUST_CONFIGURE_ARGS: --build=x86_64-pc-windows-gnu --enable-extended
+    RUST_CONFIGURE_ARGS: --build=x86_64-pc-windows-gnu --enable-extended --enable-emscripten
     MINGW_URL: https://s3-us-west-1.amazonaws.com/rust-lang-ci2/rust-ci-mirror
     MINGW_ARCHIVE: x86_64-6.3.0-release-posix-seh-rt_v5-rev2.7z
     MINGW_DIR: mingw64

config.toml.example

@@ -305,6 +305,13 @@
 # result (broken, compiling, testing) into this JSON file.
 #save-toolstates = "/path/to/toolstates.json"
 
+# This is an array of the codegen backends that will be compiled for the rustc
+# that's being compiled. The default is to only build the LLVM codegen backend,
+# but you can also optionally enable the "emscripten" backend for asm.js or
+# make this an empty array (but that probably won't get too far in the
+# bootstrap)
+#codegen-backends = ["llvm"]
+
 # =============================================================================
 # Options for specific targets
 #

src/bootstrap/bootstrap.py

@@ -640,14 +640,23 @@ class RustBuild(object):
              os.path.join(self.rust_root, ".gitmodules"),
              "--get-regexp", "path"]
         ).decode(default_encoding).splitlines()]
-        submodules = [module for module in submodules
+        filtered_submodules = []
-                      if not ((module.endswith("llvm") and
+        for module in submodules:
-                               self.get_toml('llvm-config')) or
+            if module.endswith("llvm"):
-                              (module.endswith("jemalloc") and
+                if self.get_toml('llvm-config'):
-                               (self.get_toml('use-jemalloc') == "false" or
+                    continue
-                                self.get_toml('jemalloc'))))]
+            if module.endswith("llvm-emscripten"):
+                backends = self.get_toml('codegen-backends')
+                if backends is None or not 'emscripten' in backends:
+                    continue
+            if module.endswith("jemalloc"):
+                if self.get_toml('use-jemalloc') == 'false':
+                    continue
+                if self.get_toml('jemalloc'):
+                    continue
+            filtered_submodules.append(module)
         run(["git", "submodule", "update",
-             "--init", "--recursive"] + submodules,
+             "--init", "--recursive"] + filtered_submodules,
             cwd=self.rust_root, verbose=self.verbose)
         run(["git", "submodule", "-q", "foreach", "git",
              "reset", "-q", "--hard"],

src/bootstrap/compile.rs

@@ -581,24 +581,30 @@ impl Step for RustcLink {
 }
 
 #[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
-pub struct RustcTrans {
+pub struct CodegenBackend {
     pub compiler: Compiler,
     pub target: Interned<String>,
+    pub backend: Interned<String>,
 }
 
-impl Step for RustcTrans {
+impl Step for CodegenBackend {
     type Output = ();
     const ONLY_HOSTS: bool = true;
     const DEFAULT: bool = true;
 
     fn should_run(run: ShouldRun) -> ShouldRun {
-        run.path("src/librustc_trans").krate("rustc_trans")
+        run.path("src/librustc_trans")
     }
 
     fn make_run(run: RunConfig) {
-        run.builder.ensure(RustcTrans {
+        let backend = run.builder.config.rust_codegen_backends.get(0);
+        let backend = backend.cloned().unwrap_or_else(|| {
+            INTERNER.intern_str("llvm")
+        });
+        run.builder.ensure(CodegenBackend {
             compiler: run.builder.compiler(run.builder.top_stage, run.host),
             target: run.target,
+            backend
         });
     }
 
@@ -609,58 +615,92 @@ impl Step for RustcTrans {
 
         builder.ensure(Rustc { compiler, target });
 
-        // Build LLVM for our target. This will implicitly build the host LLVM
-        // if necessary.
-        builder.ensure(native::Llvm { target });
-
         if build.force_use_stage1(compiler, target) {
-            builder.ensure(RustcTrans {
+            builder.ensure(CodegenBackend {
                 compiler: builder.compiler(1, build.build),
                 target,
+                backend: self.backend,
             });
             return;
         }
 
-        let _folder = build.fold_output(|| format!("stage{}-rustc_trans", compiler.stage));
-        println!("Building stage{} trans artifacts ({} -> {})",
-                 compiler.stage, &compiler.host, target);
-
         let mut cargo = builder.cargo(compiler, Mode::Librustc, target, "build");
+        let mut features = build.rustc_features().to_string();
         cargo.arg("--manifest-path")
-            .arg(build.src.join("src/librustc_trans/Cargo.toml"))
+            .arg(build.src.join("src/librustc_trans/Cargo.toml"));
-            .arg("--features").arg(build.rustc_features());
         rustc_cargo_env(build, &mut cargo);
 
-        // Pass down configuration from the LLVM build into the build of
+        match &*self.backend {
-        // librustc_llvm and librustc_trans.
+            "llvm" | "emscripten" => {
-        if build.is_rust_llvm(target) {
+                // Build LLVM for our target. This will implicitly build the
-            cargo.env("LLVM_RUSTLLVM", "1");
+                // host LLVM if necessary.
-        }
+                let llvm_config = builder.ensure(native::Llvm {
-        cargo.env("LLVM_CONFIG", build.llvm_config(target));
+                    target,
-        let target_config = build.config.target_config.get(&target);
+                    emscripten: self.backend == "emscripten",
-        if let Some(s) = target_config.and_then(|c| c.llvm_config.as_ref()) {
+                });
-            cargo.env("CFG_LLVM_ROOT", s);
+
-        }
+                if self.backend == "emscripten" {
-        // Building with a static libstdc++ is only supported on linux right now,
+                    features.push_str(" emscripten");
-        // not for MSVC or macOS
+                }
-        if build.config.llvm_static_stdcpp &&
+
-           !target.contains("freebsd") &&
+                let _folder = build.fold_output(|| format!("stage{}-rustc_trans", compiler.stage));
-           !target.contains("windows") &&
+                println!("Building stage{} codegen artifacts ({} -> {}, {})",
-           !target.contains("apple") {
+                         compiler.stage, &compiler.host, target, self.backend);
-            let file = compiler_file(build,
+
-                                     build.cxx(target).unwrap(),
+                // Pass down configuration from the LLVM build into the build of
-                                     target,
+                // librustc_llvm and librustc_trans.
-                                     "libstdc++.a");
+                if build.is_rust_llvm(target) {
-            cargo.env("LLVM_STATIC_STDCPP", file);
+                    cargo.env("LLVM_RUSTLLVM", "1");
-        }
+                }
-        if build.config.llvm_link_shared {
+                cargo.env("LLVM_CONFIG", &llvm_config);
-            cargo.env("LLVM_LINK_SHARED", "1");
+                if self.backend != "emscripten" {
+                    let target_config = build.config.target_config.get(&target);
+                    if let Some(s) = target_config.and_then(|c| c.llvm_config.as_ref()) {
+                        cargo.env("CFG_LLVM_ROOT", s);
+                    }
+                }
+                // Building with a static libstdc++ is only supported on linux right now,
+                // not for MSVC or macOS
+                if build.config.llvm_static_stdcpp &&
+                   !target.contains("freebsd") &&
+                   !target.contains("windows") &&
+                   !target.contains("apple") {
+                    let file = compiler_file(build,
+                                             build.cxx(target).unwrap(),
+                                             target,
+                                             "libstdc++.a");
+                    cargo.env("LLVM_STATIC_STDCPP", file);
+                }
+                if build.config.llvm_link_shared {
+                    cargo.env("LLVM_LINK_SHARED", "1");
+                }
+            }
+            _ => panic!("unknown backend: {}", self.backend),
         }
 
-        run_cargo(build,
+        let tmp_stamp = build.cargo_out(compiler, Mode::Librustc, target)
-                  &mut cargo,
+            .join(".tmp.stamp");
-                  &librustc_trans_stamp(build, compiler, target),
+        let files = run_cargo(build,
-                  false);
+                              cargo.arg("--features").arg(features),
+                              &tmp_stamp,
+                              false);
+        let mut files = files.into_iter()
+            .filter(|f| {
+                let filename = f.file_name().unwrap().to_str().unwrap();
+                is_dylib(filename) && filename.contains("rustc_trans-")
+            });
+        let codegen_backend = match files.next() {
+            Some(f) => f,
+            None => panic!("no dylibs built for codegen backend?"),
+        };
+        if let Some(f) = files.next() {
+            panic!("codegen backend built two dylibs:\n{}\n{}",
+                   codegen_backend.display(),
+                   f.display());
+        }
+        let stamp = codegen_backend_stamp(build, compiler, target, self.backend);
+        let codegen_backend = codegen_backend.to_str().unwrap();
+        t!(t!(File::create(&stamp)).write_all(codegen_backend.as_bytes()));
     }
 }
 
@@ -682,33 +722,29 @@ fn copy_codegen_backends_to_sysroot(builder: &Builder,
     // not linked into the main compiler by default but is rather dynamically
     // selected at runtime for inclusion.
     //
-    // Here we're looking for the output dylib of the `RustcTrans` step and
+    // Here we're looking for the output dylib of the `CodegenBackend` step and
     // we're copying that into the `codegen-backends` folder.
     let libdir = builder.sysroot_libdir(target_compiler, target);
     let dst = libdir.join("codegen-backends");
     t!(fs::create_dir_all(&dst));
-    let stamp = librustc_trans_stamp(build, compiler, target);
 
-    let mut copied = None;
+    for backend in builder.config.rust_codegen_backends.iter() {
-    for file in read_stamp_file(&stamp) {
+        let stamp = codegen_backend_stamp(build, compiler, target, *backend);
-        let filename = match file.file_name().and_then(|s| s.to_str()) {
+        let mut dylib = String::new();
-            Some(s) => s,
+        t!(t!(File::open(&stamp)).read_to_string(&mut dylib));
-            None => continue,
+        let file = Path::new(&dylib);
+        let filename = file.file_name().unwrap().to_str().unwrap();
+        // change `librustc_trans-xxxxxx.so` to `librustc_trans-llvm.so`
+        let target_filename = {
+            let dash = filename.find("-").unwrap();
+            let dot = filename.find(".").unwrap();
+            format!("{}-{}{}",
+                    &filename[..dash],
+                    backend,
+                    &filename[dot..])
         };
-        if !is_dylib(filename) || !filename.contains("rustc_trans-") {
+        copy(&file, &dst.join(target_filename));
-            continue
-        }
-        match copied {
-            None => copied = Some(file.clone()),
-            Some(ref s) => {
-                panic!("copied two codegen backends:\n{}\n{}",
-                       s.display(),
-                       file.display());
-            }
-        }
-        copy(&file, &dst.join(filename));
     }
-    assert!(copied.is_some(), "failed to find a codegen backend to copy");
 }
 
 /// Cargo's output path for the standard library in a given stage, compiled
@@ -729,10 +765,12 @@ pub fn librustc_stamp(build: &Build, compiler: Compiler, target: Interned<String
     build.cargo_out(compiler, Mode::Librustc, target).join(".librustc.stamp")
 }
 
-pub fn librustc_trans_stamp(build: &Build,
+fn codegen_backend_stamp(build: &Build,
-                            compiler: Compiler,
+                         compiler: Compiler,
-                            target: Interned<String>) -> PathBuf {
+                         target: Interned<String>,
-    build.cargo_out(compiler, Mode::Librustc, target).join(".librustc_trans.stamp")
+                         backend: Interned<String>) -> PathBuf {
+    build.cargo_out(compiler, Mode::Librustc, target)
+        .join(format!(".librustc_trans-{}.stamp", backend))
 }
 
 fn compiler_file(build: &Build,
@@ -849,10 +887,13 @@ impl Step for Assemble {
                 compiler: build_compiler,
                 target: target_compiler.host,
             });
-            builder.ensure(RustcTrans {
+            for &backend in build.config.rust_codegen_backends.iter() {
-                compiler: build_compiler,
+                builder.ensure(CodegenBackend {
-                target: target_compiler.host,
+                    compiler: build_compiler,
-            });
+                    target: target_compiler.host,
+                    backend,
+                });
+            }
         }
 
         let stage = target_compiler.stage;
@@ -922,7 +963,9 @@ fn stderr_isatty() -> bool {
     }
 }
 
-pub fn run_cargo(build: &Build, cargo: &mut Command, stamp: &Path, is_check: bool) {
+pub fn run_cargo(build: &Build, cargo: &mut Command, stamp: &Path, is_check: bool)
+    -> Vec<PathBuf>
+{
     // Instruct Cargo to give us json messages on stdout, critically leaving
     // stderr as piped so we can get those pretty colors.
     cargo.arg("--message-format").arg("json")
@@ -1066,8 +1109,8 @@ pub fn run_cargo(build: &Build, cargo: &mut Command, stamp: &Path, is_check: boo
     let mut new_contents = Vec::new();
     let mut max = None;
     let mut max_path = None;
-    for dep in deps {
+    for dep in deps.iter() {
-        let mtime = mtime(&dep);
+        let mtime = mtime(dep);
         if Some(mtime) > max {
             max = Some(mtime);
             max_path = Some(dep.clone());
@@ -1080,7 +1123,7 @@ pub fn run_cargo(build: &Build, cargo: &mut Command, stamp: &Path, is_check: boo
     if stamp_contents == new_contents && max <= stamp_mtime {
         build.verbose(&format!("not updating {:?}; contents equal and {} <= {}",
                 stamp, max, stamp_mtime));
-        return
+        return deps
     }
     if max > stamp_mtime {
         build.verbose(&format!("updating {:?} as {:?} changed", stamp, max_path));
@@ -1088,4 +1131,5 @@ pub fn run_cargo(build: &Build, cargo: &mut Command, stamp: &Path, is_check: boo
         build.verbose(&format!("updating {:?} as deps changed", stamp));
     }
     t!(t!(File::create(stamp)).write_all(&new_contents));
+    deps
 }

src/bootstrap/config.rs

@@ -91,6 +91,7 @@ pub struct Config {
     pub rust_optimize_tests: bool,
     pub rust_debuginfo_tests: bool,
     pub rust_dist_src: bool,
+    pub rust_codegen_backends: Vec<Interned<String>>,
 
     pub build: Interned<String>,
     pub hosts: Vec<Interned<String>>,
@@ -280,6 +281,7 @@ struct Rust {
     quiet_tests: Option<bool>,
     test_miri: Option<bool>,
     save_toolstates: Option<String>,
+    codegen_backends: Option<Vec<String>>,
 }
 
 /// TOML representation of how each build target is configured.
@@ -318,6 +320,7 @@ impl Config {
         config.ignore_git = false;
         config.rust_dist_src = true;
         config.test_miri = false;
+        config.rust_codegen_backends = vec![INTERNER.intern_str("llvm")];
 
         config.on_fail = flags.on_fail;
         config.stage = flags.stage;
@@ -465,6 +468,12 @@ impl Config {
             config.musl_root = rust.musl_root.clone().map(PathBuf::from);
             config.save_toolstates = rust.save_toolstates.clone().map(PathBuf::from);
 
+            if let Some(ref backends) = rust.codegen_backends {
+                config.rust_codegen_backends = backends.iter()
+                    .map(|s| INTERNER.intern_str(s))
+                    .collect();
+            }
+
             match rust.codegen_units {
                 Some(0) => config.rust_codegen_units = Some(num_cpus::get() as u32),
                 Some(n) => config.rust_codegen_units = Some(n),

src/bootstrap/configure.py

@@ -65,6 +65,7 @@ o("sanitizers", "build.sanitizers", "build the sanitizer runtimes (asan, lsan, m
 o("dist-src", "rust.dist-src", "when building tarballs enables building a source tarball")
 o("cargo-openssl-static", "build.openssl-static", "static openssl in cargo")
 o("profiler", "build.profiler", "build the profiler runtime")
+o("emscripten", None, "compile the emscripten backend as well as LLVM")
 
 # Optimization and debugging options. These may be overridden by the release
 # channel, etc.
@@ -321,6 +322,8 @@ for key in known_args:
         set('build.host', value.split(','))
     elif option.name == 'target':
         set('build.target', value.split(','))
+    elif option.name == 'emscripten':
+        set('rust.codegen-backends', ['llvm', 'emscripten'])
     elif option.name == 'option-checking':
         # this was handled above
         pass

src/bootstrap/lib.rs

@@ -480,6 +480,10 @@ impl Build {
         self.out.join(&*target).join("llvm")
     }
 
+    fn emscripten_llvm_out(&self, target: Interned<String>) -> PathBuf {
+        self.out.join(&*target).join("llvm-emscripten")
+    }
+
     /// Output directory for all documentation for a target
     fn doc_out(&self, target: Interned<String>) -> PathBuf {
         self.out.join(&*target).join("doc")

src/bootstrap/native.rs

@@ -22,7 +22,7 @@ use std::env;
 use std::ffi::OsString;
 use std::fs::{self, File};
 use std::io::{Read, Write};
-use std::path::Path;
+use std::path::{Path, PathBuf};
 use std::process::Command;
 
 use build_helper::output;
@@ -30,7 +30,7 @@ use cmake;
 use cc;
 
 use Build;
-use util;
+use util::{self, exe};
 use build_helper::up_to_date;
 use builder::{Builder, RunConfig, ShouldRun, Step};
 use cache::Interned;
@@ -38,30 +38,42 @@ use cache::Interned;
 #[derive(Debug, Copy, Clone, Hash, PartialEq, Eq)]
 pub struct Llvm {
     pub target: Interned<String>,
+    pub emscripten: bool,
 }
 
 impl Step for Llvm {
-    type Output = ();
+    type Output = PathBuf; // path to llvm-config
+
     const ONLY_HOSTS: bool = true;
 
     fn should_run(run: ShouldRun) -> ShouldRun {
-        run.path("src/llvm")
+        run.path("src/llvm").path("src/llvm-emscripten")
     }
 
     fn make_run(run: RunConfig) {
-        run.builder.ensure(Llvm { target: run.target })
+        let emscripten = run.path.map(|p| {
+            p.ends_with("llvm-emscripten")
+        }).unwrap_or(false);
+        run.builder.ensure(Llvm {
+            target: run.target,
+            emscripten,
+        });
     }
 
     /// Compile LLVM for `target`.
-    fn run(self, builder: &Builder) {
+    fn run(self, builder: &Builder) -> PathBuf {
         let build = builder.build;
         let target = self.target;
+        let emscripten = self.emscripten;
 
         // If we're using a custom LLVM bail out here, but we can only use a
         // custom LLVM for the build triple.
-        if let Some(config) = build.config.target_config.get(&target) {
+        if !self.emscripten {
-            if let Some(ref s) = config.llvm_config {
+            if let Some(config) = build.config.target_config.get(&target) {
-                return check_llvm_version(build, s);
+                if let Some(ref s) = config.llvm_config {
+                    check_llvm_version(build, s);
+                    return s.to_path_buf()
+                }
             }
         }
 
@@ -69,8 +81,17 @@ impl Step for Llvm {
         let mut rebuild_trigger_contents = String::new();
         t!(t!(File::open(&rebuild_trigger)).read_to_string(&mut rebuild_trigger_contents));
 
-        let out_dir = build.llvm_out(target);
+        let (out_dir, llvm_config_ret_dir) = if emscripten {
+            let dir = build.emscripten_llvm_out(target);
+            let config_dir = dir.join("bin");
+            (dir, config_dir)
+        } else {
+            (build.llvm_out(target),
+                build.llvm_out(build.config.build).join("bin"))
+        };
         let done_stamp = out_dir.join("llvm-finished-building");
+        let build_llvm_config = llvm_config_ret_dir
+            .join(exe("llvm-config", &*build.config.build));
         if done_stamp.exists() {
             let mut done_contents = String::new();
             t!(t!(File::open(&done_stamp)).read_to_string(&mut done_contents));
@@ -78,17 +99,19 @@ impl Step for Llvm {
             // If LLVM was already built previously and contents of the rebuild-trigger file
             // didn't change from the previous build, then no action is required.
             if done_contents == rebuild_trigger_contents {
-                return
+                return build_llvm_config
             }
         }
 
         let _folder = build.fold_output(|| "llvm");
-        println!("Building LLVM for {}", target);
+        let descriptor = if emscripten { "Emscripten " } else { "" };
+        println!("Building {}LLVM for {}", descriptor, target);
         let _time = util::timeit();
         t!(fs::create_dir_all(&out_dir));
 
         // http://llvm.org/docs/CMake.html
-        let mut cfg = cmake::Config::new(build.src.join("src/llvm"));
+        let root = if self.emscripten { "src/llvm-emscripten" } else { "src/llvm" };
+        let mut cfg = cmake::Config::new(build.src.join(root));
         if build.config.ninja {
             cfg.generator("Ninja");
         }
@@ -99,13 +122,22 @@ impl Step for Llvm {
             (true, true) => "RelWithDebInfo",
         };
 
-        // NOTE: remember to also update `config.toml.example` when changing the defaults!
+        // NOTE: remember to also update `config.toml.example` when changing the
-        let llvm_targets = match build.config.llvm_targets {
+        // defaults!
-            Some(ref s) => s,
+        let llvm_targets = if self.emscripten {
-            None => "X86;ARM;AArch64;Mips;PowerPC;SystemZ;JSBackend;MSP430;Sparc;NVPTX;Hexagon",
+            "JSBackend"
+        } else {
+            match build.config.llvm_targets {
+                Some(ref s) => s,
+                None => "X86;ARM;AArch64;Mips;PowerPC;SystemZ;MSP430;Sparc;NVPTX;Hexagon",
+            }
         };
 
-        let llvm_exp_targets = &build.config.llvm_experimental_targets;
+        let llvm_exp_targets = if self.emscripten {
+            ""
+        } else {
+            &build.config.llvm_experimental_targets[..]
+        };
 
         let assertions = if build.config.llvm_assertions {"ON"} else {"OFF"};
 
@@ -155,7 +187,10 @@ impl Step for Llvm {
 
         // http://llvm.org/docs/HowToCrossCompileLLVM.html
         if target != build.build {
-            builder.ensure(Llvm { target: build.build });
+            builder.ensure(Llvm {
+                target: build.build,
+                emscripten: false,
+            });
             // FIXME: if the llvm root for the build triple is overridden then we
             //        should use llvm-tblgen from there, also should verify that it
             //        actually exists most of the time in normal installs of LLVM.
@@ -241,6 +276,8 @@ impl Step for Llvm {
         cfg.build();
 
         t!(t!(File::create(&done_stamp)).write_all(rebuild_trigger_contents.as_bytes()));
+
+        build_llvm_config
     }
 }
 

src/ci/docker/asmjs/Dockerfile

@@ -29,6 +29,6 @@ ENV EM_CONFIG=/emsdk-portable/.emscripten
 
 ENV TARGETS=asmjs-unknown-emscripten
 
-ENV RUST_CONFIGURE_ARGS --target=$TARGETS
+ENV RUST_CONFIGURE_ARGS --target=$TARGETS --enable-emscripten
 
 ENV SCRIPT python2.7 ../x.py test --target $TARGETS

src/ci/docker/dist-i686-linux/Dockerfile

@@ -85,7 +85,8 @@ ENV RUST_CONFIGURE_ARGS \
       --host=$HOSTS \
       --enable-extended \
       --enable-sanitizers \
-      --enable-profiler
+      --enable-profiler \
+      --enable-emscripten
 ENV SCRIPT python2.7 ../x.py dist --host $HOSTS --target $HOSTS
 
 # This is the only builder which will create source tarballs

src/ci/docker/dist-various-1/Dockerfile

@@ -95,7 +95,8 @@ ENV RUST_CONFIGURE_ARGS \
       --musl-root-armv7=/musl-armv7 \
       --musl-root-aarch64=/musl-aarch64 \
       --musl-root-mips=/musl-mips \
-      --musl-root-mipsel=/musl-mipsel
+      --musl-root-mipsel=/musl-mipsel \
+      --enable-emscripten
 
 ENV SCRIPT python2.7 ../x.py dist --target $TARGETS
 

src/ci/docker/dist-x86_64-linux/Dockerfile

@@ -85,7 +85,8 @@ ENV RUST_CONFIGURE_ARGS \
       --host=$HOSTS \
       --enable-extended \
       --enable-sanitizers \
-      --enable-profiler
+      --enable-profiler \
+      --enable-emscripten
 ENV SCRIPT python2.7 ../x.py dist --host $HOSTS --target $HOSTS
 
 # This is the only builder which will create source tarballs

src/ci/init_repo.sh

@@ -48,7 +48,12 @@ travis_time_start
 # Update the cache (a pristine copy of the rust source master)
 retry sh -c "rm -rf $cache_src_dir && mkdir -p $cache_src_dir && \
     git clone --depth 1 https://github.com/rust-lang/rust.git $cache_src_dir"
-(cd $cache_src_dir && git rm src/llvm)
+if [ -d $cache_src_dir/src/llvm ]; then
+  (cd $cache_src_dir && git rm src/llvm)
+fi
+if [ -d $cache_src_dir/src/llvm-emscripten ]; then
+  (cd $cache_src_dir && git rm src/llvm-emscripten)
+fi
 retry sh -c "cd $cache_src_dir && \
     git submodule deinit -f . && git submodule sync && git submodule update --init"
 
@@ -64,14 +69,14 @@ travis_time_start
 # http://stackoverflow.com/questions/12641469/list-submodules-in-a-git-repository
 modules="$(git config --file .gitmodules --get-regexp '\.path$' | cut -d' ' -f2)"
 for module in $modules; do
-    if [ "$module" = src/llvm ]; then
+    if [ "$module" = src/llvm ] || [ "$module" = src/llvm-emscripten ]; then
-        commit="$(git ls-tree HEAD src/llvm | awk '{print $3}')"
+        commit="$(git ls-tree HEAD $module | awk '{print $3}')"
-        git rm src/llvm
+        git rm $module
         retry sh -c "rm -f $commit.tar.gz && \
             curl -sSL -O https://github.com/rust-lang/llvm/archive/$commit.tar.gz"
         tar -C src/ -xf "$commit.tar.gz"
         rm "$commit.tar.gz"
-        mv "src/llvm-$commit" src/llvm
+        mv "src/llvm-$commit" $module
         continue
     fi
     if [ ! -e "$cache_src_dir/$module/.git" ]; then

src/librustc_back/target/asmjs_unknown_emscripten.rs

@@ -31,6 +31,7 @@ pub fn target() -> Result<Target, String> {
         max_atomic_width: Some(32),
         post_link_args: args,
         target_family: Some("unix".to_string()),
+        codegen_backend: "emscripten".to_string(),
         .. Default::default()
     };
     Ok(Target {

src/librustc_back/target/mod.rs

@@ -465,6 +465,9 @@ pub struct TargetOptions {
     /// Whether to lower 128-bit operations to compiler_builtins calls.  Use if
     /// your backend only supports 64-bit and smaller math.
     pub i128_lowering: bool,
+
+    /// The codegen backend to use for this target, typically "llvm"
+    pub codegen_backend: String,
 }
 
 impl Default for TargetOptions {
@@ -534,6 +537,7 @@ impl Default for TargetOptions {
             singlethread: false,
             no_builtins: false,
             i128_lowering: false,
+            codegen_backend: "llvm".to_string(),
         }
     }
 }
@@ -780,6 +784,7 @@ impl Target {
         key!(requires_lto, bool);
         key!(singlethread, bool);
         key!(no_builtins, bool);
+        key!(codegen_backend);
 
         if let Some(array) = obj.find("abi-blacklist").and_then(Json::as_array) {
             for name in array.iter().filter_map(|abi| abi.as_string()) {
@@ -976,6 +981,7 @@ impl ToJson for Target {
         target_option_val!(requires_lto);
         target_option_val!(singlethread);
         target_option_val!(no_builtins);
+        target_option_val!(codegen_backend);
 
         if default.abi_blacklist != self.options.abi_blacklist {
             d.insert("abi-blacklist".to_string(), self.options.abi_blacklist.iter()

src/librustc_back/target/wasm32_unknown_emscripten.rs

@@ -35,6 +35,7 @@ pub fn target() -> Result<Target, String> {
         max_atomic_width: Some(32),
         post_link_args,
         target_family: Some("unix".to_string()),
+        codegen_backend: "emscripten".to_string(),
         .. Default::default()
     };
     Ok(Target {

src/librustc_driver/lib.rs

@@ -218,19 +218,16 @@ pub fn get_trans(sess: &Session) -> Box<TransCrate> {
     static mut LOAD: fn() -> Box<TransCrate> = || unreachable!();
 
     INIT.call_once(|| {
-        let trans_name = sess.opts.debugging_opts.codegen_backend.as_ref();
+        let trans_name = sess.opts.debugging_opts.codegen_backend.as_ref()
-        let backend = match trans_name.map(|s| &**s) {
+            .unwrap_or(&sess.target.target.options.codegen_backend);
-            None |
+        let backend = match &trans_name[..] {
-            Some("llvm") => get_trans_default(),
+            "metadata_only" => {
-            Some("metadata_only") => {
                 rustc_trans_utils::trans_crate::MetadataOnlyTransCrate::new
             }
-            Some(filename) if filename.contains(".") => {
+            filename if filename.contains(".") => {
                 load_backend_from_dylib(filename.as_ref())
             }
-            Some(trans_name) => {
+            trans_name => get_trans_sysroot(trans_name),
-                sess.fatal(&format!("unknown codegen backend {}", trans_name));
-            }
         };
 
         unsafe {
@@ -242,7 +239,7 @@ pub fn get_trans(sess: &Session) -> Box<TransCrate> {
     backend
 }
 
-fn get_trans_default() -> fn() -> Box<TransCrate> {
+fn get_trans_sysroot(backend_name: &str) -> fn() -> Box<TransCrate> {
     // For now we only allow this function to be called once as it'll dlopen a
     // few things, which seems to work best if we only do that once. In
     // general this assertion never trips due to the once guard in `get_trans`,
@@ -324,6 +321,7 @@ fn get_trans_default() -> fn() -> Box<TransCrate> {
 
     let mut file: Option<PathBuf> = None;
 
+    let expected_name = format!("rustc_trans-{}", backend_name);
     for entry in d.filter_map(|e| e.ok()) {
         let path = entry.path();
         let filename = match path.file_name().and_then(|s| s.to_str()) {
@@ -334,7 +332,7 @@ fn get_trans_default() -> fn() -> Box<TransCrate> {
             continue
         }
         let name = &filename[DLL_PREFIX.len() .. filename.len() - DLL_SUFFIX.len()];
-        if !name.starts_with("rustc_trans") {
+        if name != expected_name {
             continue
         }
         if let Some(ref prev) = file {
@@ -350,8 +348,9 @@ fn get_trans_default() -> fn() -> Box<TransCrate> {
     match file {
         Some(ref s) => return load_backend_from_dylib(s),
         None => {
-            let err = format!("failed to load default codegen backend, no appropriate \
+            let err = format!("failed to load default codegen backend for `{}`, \
-                               codegen dylib found in `{}`", sysroot.display());
+                               no appropriate codegen dylib found in `{}`",
+                               backend_name, sysroot.display());
             early_error(ErrorOutputType::default(), &err);
         }
     }
@@ -1072,7 +1071,7 @@ pub fn version(binary: &str, matches: &getopts::Matches) {
         println!("commit-date: {}", unw(commit_date_str()));
         println!("host: {}", config::host_triple());
         println!("release: {}", unw(release_str()));
-        get_trans_default()().print_version();
+        get_trans_sysroot("llvm")().print_version();
     }
 }
 
@@ -1369,7 +1368,7 @@ pub fn handle_options(args: &[String]) -> Option<getopts::Matches> {
     }
 
     if cg_flags.contains(&"passes=list".to_string()) {
-        get_trans_default()().print_passes();
+        get_trans_sysroot("llvm")().print_passes();
         return None;
     }
 

src/librustc_llvm/Cargo.toml

@@ -10,6 +10,7 @@ path = "lib.rs"
 
 [features]
 static-libstdcpp = []
+emscripten = []
 
 [dependencies]
 bitflags = "1.0"

src/librustc_trans/Cargo.toml

@@ -39,4 +39,11 @@ tempdir = "0.3"
 cc = "1.0.1"
 
 [features]
+# Used to communicate the feature to `rustc_back` in the same manner that the
+# `rustc` driver script communicate this.
 jemalloc = ["rustc_back/jemalloc"]
+
+# This is used to convince Cargo to separately cache builds of `rustc_trans`
+# when this option is enabled or not. That way we can build two, cache two
+# artifacts, and have nice speedy rebuilds.
+emscripten = ["rustc_llvm/emscripten"]

src/llvm-emscripten

@@ -0,0 +1 @@
+Subproject commit 2717444753318e461e0c3b30dacd03ffbac96903

src/tools/tidy/src/lib.rs

@@ -54,6 +54,7 @@ fn filter_dirs(path: &Path) -> bool {
         "src/dlmalloc",
         "src/jemalloc",
         "src/llvm",
+        "src/llvm-emscripten",
         "src/libbacktrace",
         "src/libcompiler_builtins",
         "src/librustc_data_structures/owning_ref",