Auto merge of #55349 - bjorn3:rustc_mir_collect_and_partition_mono_items, r=oli-obk

Move collect_and_partition_mono_items to rustc_mir

Most of the logic of it is inside rustc_mir anyway.

Also removes the single function crate rustc_metadata_utils. Based on #55225

6 files changed, 1722 insertions, 4 deletions

diff --git a/src/librustc_codegen_utils/Cargo.toml b/src/librustc_codegen_utils/Cargo.toml
index a1f4a323f84..4c57e978414 100644
--- a/src/librustc_codegen_utils/Cargo.toml
+++ b/src/librustc_codegen_utils/Cargo.toml
@@ -13,11 +13,13 @@ test = false
 flate2 = "1.0"
 log = "0.4"
 
+serialize = { path = "../libserialize" }
 syntax = { path = "../libsyntax" }
 syntax_pos = { path = "../libsyntax_pos" }
 rustc = { path = "../librustc" }
+rustc_allocator = { path = "../librustc_allocator" }
 rustc_target = { path = "../librustc_target" }
 rustc_data_structures = { path = "../librustc_data_structures" }
+rustc_metadata = { path = "../librustc_metadata" }
 rustc_mir = { path = "../librustc_mir" }
 rustc_incremental = { path = "../librustc_incremental" }
-rustc_metadata_utils = { path = "../librustc_metadata_utils" }
diff --git a/src/librustc_codegen_utils/command.rs b/src/librustc_codegen_utils/command.rs
new file mode 100644
index 00000000000..9ebbdd7c3c9
--- /dev/null
+++ b/src/librustc_codegen_utils/command.rs
@@ -0,0 +1,175 @@
+// 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.
+
+//! A thin wrapper around `Command` in the standard library which allows us to
+//! read the arguments that are built up.
+
+use std::ffi::{OsStr, OsString};
+use std::fmt;
+use std::io;
+use std::mem;
+use std::process::{self, Output};
+
+use rustc_target::spec::LldFlavor;
+
+#[derive(Clone)]
+pub struct Command {
+    program: Program,
+    args: Vec<OsString>,
+    env: Vec<(OsString, OsString)>,
+}
+
+#[derive(Clone)]
+enum Program {
+    Normal(OsString),
+    CmdBatScript(OsString),
+    Lld(OsString, LldFlavor)
+}
+
+impl Command {
+    pub fn new<P: AsRef<OsStr>>(program: P) -> Command {
+        Command::_new(Program::Normal(program.as_ref().to_owned()))
+    }
+
+    pub fn bat_script<P: AsRef<OsStr>>(program: P) -> Command {
+        Command::_new(Program::CmdBatScript(program.as_ref().to_owned()))
+    }
+
+    pub fn lld<P: AsRef<OsStr>>(program: P, flavor: LldFlavor) -> Command {
+        Command::_new(Program::Lld(program.as_ref().to_owned(), flavor))
+    }
+
+    fn _new(program: Program) -> Command {
+        Command {
+            program,
+            args: Vec::new(),
+            env: Vec::new(),
+        }
+    }
+
+    pub fn arg<P: AsRef<OsStr>>(&mut self, arg: P) -> &mut Command {
+        self._arg(arg.as_ref());
+        self
+    }
+
+    pub fn args<I>(&mut self, args: I) -> &mut Command
+        where I: IntoIterator,
+              I::Item: AsRef<OsStr>,
+    {
+        for arg in args {
+            self._arg(arg.as_ref());
+        }
+        self
+    }
+
+    fn _arg(&mut self, arg: &OsStr) {
+        self.args.push(arg.to_owned());
+    }
+
+    pub fn env<K, V>(&mut self, key: K, value: V) -> &mut Command
+        where K: AsRef<OsStr>,
+              V: AsRef<OsStr>
+    {
+        self._env(key.as_ref(), value.as_ref());
+        self
+    }
+
+    fn _env(&mut self, key: &OsStr, value: &OsStr) {
+        self.env.push((key.to_owned(), value.to_owned()));
+    }
+
+    pub fn output(&mut self) -> io::Result<Output> {
+        self.command().output()
+    }
+
+    pub fn command(&self) -> process::Command {
+        let mut ret = match self.program {
+            Program::Normal(ref p) => process::Command::new(p),
+            Program::CmdBatScript(ref p) => {
+                let mut c = process::Command::new("cmd");
+                c.arg("/c").arg(p);
+                c
+            }
+            Program::Lld(ref p, flavor) => {
+                let mut c = process::Command::new(p);
+                c.arg("-flavor").arg(match flavor {
+                    LldFlavor::Wasm => "wasm",
+                    LldFlavor::Ld => "gnu",
+                    LldFlavor::Link => "link",
+                    LldFlavor::Ld64 => "darwin",
+                });
+                c
+            }
+        };
+        ret.args(&self.args);
+        ret.envs(self.env.clone());
+        return ret
+    }
+
+    // extensions
+
+    pub fn get_args(&self) -> &[OsString] {
+        &self.args
+    }
+
+    pub fn take_args(&mut self) -> Vec<OsString> {
+        mem::replace(&mut self.args, Vec::new())
+    }
+
+    /// Returns a `true` if we're pretty sure that this'll blow OS spawn limits,
+    /// or `false` if we should attempt to spawn and see what the OS says.
+    pub fn very_likely_to_exceed_some_spawn_limit(&self) -> bool {
+        // We mostly only care about Windows in this method, on Unix the limits
+        // can be gargantuan anyway so we're pretty unlikely to hit them
+        if cfg!(unix) {
+            return false
+        }
+
+        // Right now LLD doesn't support the `@` syntax of passing an argument
+        // through files, so regardless of the platform we try to go to the OS
+        // on this one.
+        if let Program::Lld(..) = self.program {
+            return false
+        }
+
+        // Ok so on Windows to spawn a process is 32,768 characters in its
+        // command line [1]. Unfortunately we don't actually have access to that
+        // as it's calculated just before spawning. Instead we perform a
+        // poor-man's guess as to how long our command line will be. We're
+        // assuming here that we don't have to escape every character...
+        //
+        // Turns out though that `cmd.exe` has even smaller limits, 8192
+        // characters [2]. Linkers can often be batch scripts (for example
+        // Emscripten, Gecko's current build system) which means that we're
+        // running through batch scripts. These linkers often just forward
+        // arguments elsewhere (and maybe tack on more), so if we blow 8192
+        // bytes we'll typically cause them to blow as well.
+        //
+        // Basically as a result just perform an inflated estimate of what our
+        // command line will look like and test if it's > 8192 (we actually
+        // test against 6k to artificially inflate our estimate). If all else
+        // fails we'll fall back to the normal unix logic of testing the OS
+        // error code if we fail to spawn and automatically re-spawning the
+        // linker with smaller arguments.
+        //
+        // [1]: https://msdn.microsoft.com/en-us/library/windows/desktop/ms682425(v=vs.85).aspx
+        // [2]: https://blogs.msdn.microsoft.com/oldnewthing/20031210-00/?p=41553
+
+        let estimated_command_line_len =
+            self.args.iter().map(|a| a.len()).sum::<usize>();
+        estimated_command_line_len > 1024 * 6
+    }
+}
+
+impl fmt::Debug for Command {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        self.command().fmt(f)
+    }
+}
diff --git a/src/librustc_codegen_utils/lib.rs b/src/librustc_codegen_utils/lib.rs
index 03b3b20a4e7..f0ce1e9b0ef 100644
--- a/src/librustc_codegen_utils/lib.rs
+++ b/src/librustc_codegen_utils/lib.rs
@@ -30,20 +30,28 @@ extern crate flate2;
 #[macro_use]
 extern crate log;
 
+extern crate serialize;
 #[macro_use]
 extern crate rustc;
+extern crate rustc_allocator;
 extern crate rustc_target;
+extern crate rustc_metadata;
 extern crate rustc_mir;
 extern crate rustc_incremental;
 extern crate syntax;
 extern crate syntax_pos;
 #[macro_use] extern crate rustc_data_structures;
-extern crate rustc_metadata_utils;
 
+use std::path::PathBuf;
+
+use rustc::session::Session;
 use rustc::ty::TyCtxt;
 
+pub mod command;
 pub mod link;
+pub mod linker;
 pub mod codegen_backend;
+pub mod symbol_export;
 pub mod symbol_names;
 pub mod symbol_names_test;
 
@@ -61,4 +69,43 @@ pub fn check_for_rustc_errors_attr(tcx: TyCtxt) {
     }
 }
 
+#[derive(Copy, Clone, Debug, PartialEq)]
+pub enum ModuleKind {
+    Regular,
+    Metadata,
+    Allocator,
+}
+
+#[derive(Debug)]
+pub struct CompiledModule {
+    pub name: String,
+    pub kind: ModuleKind,
+    pub object: Option<PathBuf>,
+    pub bytecode: Option<PathBuf>,
+    pub bytecode_compressed: Option<PathBuf>,
+}
+
+pub fn find_library(name: &str, search_paths: &[PathBuf], sess: &Session)
+                    -> PathBuf {
+    // On Windows, static libraries sometimes show up as libfoo.a and other
+    // times show up as foo.lib
+    let oslibname = format!("{}{}{}",
+                            sess.target.target.options.staticlib_prefix,
+                            name,
+                            sess.target.target.options.staticlib_suffix);
+    let unixlibname = format!("lib{}.a", name);
+
+    for path in search_paths {
+        debug!("looking for {} inside {:?}", name, path);
+        let test = path.join(&oslibname);
+        if test.exists() { return test }
+        if oslibname != unixlibname {
+            let test = path.join(&unixlibname);
+            if test.exists() { return test }
+        }
+    }
+    sess.fatal(&format!("could not find native static library `{}`, \
+                         perhaps an -L flag is missing?", name));
+}
+
 __build_diagnostic_array! { librustc_codegen_utils, DIAGNOSTICS }
diff --git a/src/librustc_codegen_utils/link.rs b/src/librustc_codegen_utils/link.rs
index 66e98793f42..b11aa687326 100644
--- a/src/librustc_codegen_utils/link.rs
+++ b/src/librustc_codegen_utils/link.rs
@@ -13,7 +13,6 @@ use rustc::session::Session;
 use std::path::{Path, PathBuf};
 use syntax::{ast, attr};
 use syntax_pos::Span;
-use rustc_metadata_utils::validate_crate_name;
 
 pub fn out_filename(sess: &Session,
                 crate_type: config::CrateType,
@@ -52,7 +51,7 @@ pub fn find_crate_name(sess: Option<&Session>,
                        attrs: &[ast::Attribute],
                        input: &Input) -> String {
     let validate = |s: String, span: Option<Span>| {
-        validate_crate_name(sess, &s, span);
+        ::rustc_metadata::validate_crate_name(sess, &s, span);
         s
     };
 
diff --git a/src/librustc_codegen_utils/linker.rs b/src/librustc_codegen_utils/linker.rs
new file mode 100644
index 00000000000..c1f41fd509a
--- /dev/null
+++ b/src/librustc_codegen_utils/linker.rs
@@ -0,0 +1,1100 @@
+// 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 rustc_data_structures::fx::FxHashMap;
+use std::ffi::{OsStr, OsString};
+use std::fs::{self, File};
+use std::io::prelude::*;
+use std::io::{self, BufWriter};
+use std::path::{Path, PathBuf};
+
+use command::Command;
+use rustc::hir::def_id::{LOCAL_CRATE, CrateNum};
+use rustc::middle::dependency_format::Linkage;
+use rustc::session::Session;
+use rustc::session::config::{self, CrateType, OptLevel, DebugInfo,
+                             CrossLangLto};
+use rustc::ty::TyCtxt;
+use rustc_target::spec::{LinkerFlavor, LldFlavor};
+use serialize::{json, Encoder};
+
+/// For all the linkers we support, and information they might
+/// need out of the shared crate context before we get rid of it.
+pub struct LinkerInfo {
+    exports: FxHashMap<CrateType, Vec<String>>,
+}
+
+impl LinkerInfo {
+    pub fn new(tcx: TyCtxt) -> LinkerInfo {
+        LinkerInfo {
+            exports: tcx.sess.crate_types.borrow().iter().map(|&c| {
+                (c, exported_symbols(tcx, c))
+            }).collect(),
+        }
+    }
+
+    pub fn to_linker<'a>(
+        &'a self,
+        cmd: Command,
+        sess: &'a Session,
+        flavor: LinkerFlavor,
+        target_cpu: &'a str,
+    ) -> Box<dyn Linker+'a> {
+        match flavor {
+            LinkerFlavor::Lld(LldFlavor::Link) |
+            LinkerFlavor::Msvc => {
+                Box::new(MsvcLinker {
+                    cmd,
+                    sess,
+                    info: self
+                }) as Box<dyn Linker>
+            }
+            LinkerFlavor::Em =>  {
+                Box::new(EmLinker {
+                    cmd,
+                    sess,
+                    info: self
+                }) as Box<dyn Linker>
+            }
+            LinkerFlavor::Gcc =>  {
+                Box::new(GccLinker {
+                    cmd,
+                    sess,
+                    info: self,
+                    hinted_static: false,
+                    is_ld: false,
+                    target_cpu,
+                }) as Box<dyn Linker>
+            }
+
+            LinkerFlavor::Lld(LldFlavor::Ld) |
+            LinkerFlavor::Lld(LldFlavor::Ld64) |
+            LinkerFlavor::Ld => {
+                Box::new(GccLinker {
+                    cmd,
+                    sess,
+                    info: self,
+                    hinted_static: false,
+                    is_ld: true,
+                    target_cpu,
+                }) as Box<dyn Linker>
+            }
+
+            LinkerFlavor::Lld(LldFlavor::Wasm) => {
+                Box::new(WasmLd {
+                    cmd,
+                    sess,
+                    info: self
+                }) as Box<dyn Linker>
+            }
+        }
+    }
+}
+
+/// Linker abstraction used by back::link to build up the command to invoke a
+/// linker.
+///
+/// This trait is the total list of requirements needed by `back::link` and
+/// represents the meaning of each option being passed down. This trait is then
+/// used to dispatch on whether a GNU-like linker (generally `ld.exe`) or an
+/// MSVC linker (e.g. `link.exe`) is being used.
+pub trait Linker {
+    fn link_dylib(&mut self, lib: &str);
+    fn link_rust_dylib(&mut self, lib: &str, path: &Path);
+    fn link_framework(&mut self, framework: &str);
+    fn link_staticlib(&mut self, lib: &str);
+    fn link_rlib(&mut self, lib: &Path);
+    fn link_whole_rlib(&mut self, lib: &Path);
+    fn link_whole_staticlib(&mut self, lib: &str, search_path: &[PathBuf]);
+    fn include_path(&mut self, path: &Path);
+    fn framework_path(&mut self, path: &Path);
+    fn output_filename(&mut self, path: &Path);
+    fn add_object(&mut self, path: &Path);
+    fn gc_sections(&mut self, keep_metadata: bool);
+    fn position_independent_executable(&mut self);
+    fn no_position_independent_executable(&mut self);
+    fn full_relro(&mut self);
+    fn partial_relro(&mut self);
+    fn no_relro(&mut self);
+    fn optimize(&mut self);
+    fn pgo_gen(&mut self);
+    fn debuginfo(&mut self);
+    fn no_default_libraries(&mut self);
+    fn build_dylib(&mut self, out_filename: &Path);
+    fn build_static_executable(&mut self);
+    fn args(&mut self, args: &[String]);
+    fn export_symbols(&mut self, tmpdir: &Path, crate_type: CrateType);
+    fn subsystem(&mut self, subsystem: &str);
+    fn group_start(&mut self);
+    fn group_end(&mut self);
+    fn cross_lang_lto(&mut self);
+    // Should have been finalize(self), but we don't support self-by-value on trait objects (yet?).
+    fn finalize(&mut self) -> Command;
+}
+
+pub struct GccLinker<'a> {
+    cmd: Command,
+    sess: &'a Session,
+    info: &'a LinkerInfo,
+    hinted_static: bool, // Keeps track of the current hinting mode.
+    // Link as ld
+    is_ld: bool,
+    target_cpu: &'a str,
+}
+
+impl<'a> GccLinker<'a> {
+    /// Argument that must be passed *directly* to the linker
+    ///
+    /// These arguments need to be prepended with '-Wl,' when a gcc-style linker is used
+    fn linker_arg<S>(&mut self, arg: S) -> &mut Self
+        where S: AsRef<OsStr>
+    {
+        if !self.is_ld {
+            let mut os = OsString::from("-Wl,");
+            os.push(arg.as_ref());
+            self.cmd.arg(os);
+        } else {
+            self.cmd.arg(arg);
+        }
+        self
+    }
+
+    fn takes_hints(&self) -> bool {
+        !self.sess.target.target.options.is_like_osx
+    }
+
+    // Some platforms take hints about whether a library is static or dynamic.
+    // For those that support this, we ensure we pass the option if the library
+    // was flagged "static" (most defaults are dynamic) to ensure that if
+    // libfoo.a and libfoo.so both exist that the right one is chosen.
+    fn hint_static(&mut self) {
+        if !self.takes_hints() { return }
+        if !self.hinted_static {
+            self.linker_arg("-Bstatic");
+            self.hinted_static = true;
+        }
+    }
+
+    fn hint_dynamic(&mut self) {
+        if !self.takes_hints() { return }
+        if self.hinted_static {
+            self.linker_arg("-Bdynamic");
+            self.hinted_static = false;
+        }
+    }
+
+    fn push_cross_lang_lto_args(&mut self, plugin_path: Option<&OsStr>) {
+        if let Some(plugin_path) = plugin_path {
+            let mut arg = OsString::from("-plugin=");
+            arg.push(plugin_path);
+            self.linker_arg(&arg);
+        }
+
+        let opt_level = match self.sess.opts.optimize {
+            config::OptLevel::No => "O0",
+            config::OptLevel::Less => "O1",
+            config::OptLevel::Default => "O2",
+            config::OptLevel::Aggressive => "O3",
+            config::OptLevel::Size => "Os",
+            config::OptLevel::SizeMin => "Oz",
+        };
+
+        self.linker_arg(&format!("-plugin-opt={}", opt_level));
+        let target_cpu = self.target_cpu;
+        self.linker_arg(&format!("-plugin-opt=mcpu={}", target_cpu));
+
+        match self.sess.lto() {
+            config::Lto::Thin |
+            config::Lto::ThinLocal => {
+                self.linker_arg("-plugin-opt=thin");
+            }
+            config::Lto::Fat |
+            config::Lto::No => {
+                // default to regular LTO
+            }
+        }
+    }
+}
+
+impl<'a> Linker for GccLinker<'a> {
+    fn link_dylib(&mut self, lib: &str) { self.hint_dynamic(); self.cmd.arg(format!("-l{}",lib)); }
+    fn link_staticlib(&mut self, lib: &str) {
+        self.hint_static(); self.cmd.arg(format!("-l{}",lib));
+    }
+    fn link_rlib(&mut self, lib: &Path) { self.hint_static(); self.cmd.arg(lib); }
+    fn include_path(&mut self, path: &Path) { self.cmd.arg("-L").arg(path); }
+    fn framework_path(&mut self, path: &Path) { self.cmd.arg("-F").arg(path); }
+    fn output_filename(&mut self, path: &Path) { self.cmd.arg("-o").arg(path); }
+    fn add_object(&mut self, path: &Path) { self.cmd.arg(path); }
+    fn position_independent_executable(&mut self) { self.cmd.arg("-pie"); }
+    fn no_position_independent_executable(&mut self) { self.cmd.arg("-no-pie"); }
+    fn full_relro(&mut self) { self.linker_arg("-zrelro"); self.linker_arg("-znow"); }
+    fn partial_relro(&mut self) { self.linker_arg("-zrelro"); }
+    fn no_relro(&mut self) { self.linker_arg("-znorelro"); }
+    fn build_static_executable(&mut self) { self.cmd.arg("-static"); }
+    fn args(&mut self, args: &[String]) { self.cmd.args(args); }
+
+    fn link_rust_dylib(&mut self, lib: &str, _path: &Path) {
+        self.hint_dynamic();
+        self.cmd.arg(format!("-l{}",lib));
+    }
+
+    fn link_framework(&mut self, framework: &str) {
+        self.hint_dynamic();
+        self.cmd.arg("-framework").arg(framework);
+    }
+
+    // Here we explicitly ask that the entire archive is included into the
+    // result artifact. For more details see #15460, but the gist is that
+    // the linker will strip away any unused objects in the archive if we
+    // don't otherwise explicitly reference them. This can occur for
+    // libraries which are just providing bindings, libraries with generic
+    // functions, etc.
+    fn link_whole_staticlib(&mut self, lib: &str, search_path: &[PathBuf]) {
+        self.hint_static();
+        let target = &self.sess.target.target;
+        if !target.options.is_like_osx {
+            self.linker_arg("--whole-archive").cmd.arg(format!("-l{}",lib));
+            self.linker_arg("--no-whole-archive");
+        } else {
+            // -force_load is the macOS equivalent of --whole-archive, but it
+            // involves passing the full path to the library to link.
+            self.linker_arg("-force_load");
+            let lib = ::find_library(lib, search_path, &self.sess);
+            self.linker_arg(&lib);
+        }
+    }
+
+    fn link_whole_rlib(&mut self, lib: &Path) {
+        self.hint_static();
+        if self.sess.target.target.options.is_like_osx {
+            self.linker_arg("-force_load");
+            self.linker_arg(&lib);
+        } else {
+            self.linker_arg("--whole-archive").cmd.arg(lib);
+            self.linker_arg("--no-whole-archive");
+        }
+    }
+
+    fn gc_sections(&mut self, keep_metadata: bool) {
+        // The dead_strip option to the linker specifies that functions and data
+        // unreachable by the entry point will be removed. This is quite useful
+        // with Rust's compilation model of compiling libraries at a time into
+        // one object file. For example, this brings hello world from 1.7MB to
+        // 458K.
+        //
+        // Note that this is done for both executables and dynamic libraries. We
+        // won't get much benefit from dylibs because LLVM will have already
+        // stripped away as much as it could. This has not been seen to impact
+        // link times negatively.
+        //
+        // -dead_strip can't be part of the pre_link_args because it's also used
+        // for partial linking when using multiple codegen units (-r).  So we
+        // insert it here.
+        if self.sess.target.target.options.is_like_osx {
+            self.linker_arg("-dead_strip");
+        } else if self.sess.target.target.options.is_like_solaris {
+            self.linker_arg("-zignore");
+
+        // If we're building a dylib, we don't use --gc-sections because LLVM
+        // has already done the best it can do, and we also don't want to
+        // eliminate the metadata. If we're building an executable, however,
+        // --gc-sections drops the size of hello world from 1.8MB to 597K, a 67%
+        // reduction.
+        } else if !keep_metadata {
+            self.linker_arg("--gc-sections");
+        }
+    }
+
+    fn optimize(&mut self) {
+        if !self.sess.target.target.options.linker_is_gnu { return }
+
+        // GNU-style linkers support optimization with -O. GNU ld doesn't
+        // need a numeric argument, but other linkers do.
+        if self.sess.opts.optimize == config::OptLevel::Default ||
+           self.sess.opts.optimize == config::OptLevel::Aggressive {
+            self.linker_arg("-O1");
+        }
+    }
+
+    fn pgo_gen(&mut self) {
+        if !self.sess.target.target.options.linker_is_gnu { return }
+
+        // If we're doing PGO generation stuff and on a GNU-like linker, use the
+        // "-u" flag to properly pull in the profiler runtime bits.
+        //
+        // This is because LLVM otherwise won't add the needed initialization
+        // for us on Linux (though the extra flag should be harmless if it
+        // does).
+        //
+        // See https://reviews.llvm.org/D14033 and https://reviews.llvm.org/D14030.
+        //
+        // Though it may be worth to try to revert those changes upstream, since
+        // the overhead of the initialization should be minor.
+        self.cmd.arg("-u");
+        self.cmd.arg("__llvm_profile_runtime");
+    }
+
+    fn debuginfo(&mut self) {
+        match self.sess.opts.debuginfo {
+            DebugInfo::None => {
+                // If we are building without debuginfo enabled and we were called with
+                // `-Zstrip-debuginfo-if-disabled=yes`, tell the linker to strip any debuginfo
+                // found when linking to get rid of symbols from libstd.
+                match self.sess.opts.debugging_opts.strip_debuginfo_if_disabled {
+                    Some(true) => { self.linker_arg("-S"); },
+                    _ => {},
+                }
+            },
+            _ => {},
+        };
+    }
+
+    fn no_default_libraries(&mut self) {
+        if !self.is_ld {
+            self.cmd.arg("-nodefaultlibs");
+        }
+    }
+
+    fn build_dylib(&mut self, out_filename: &Path) {
+        // On mac we need to tell the linker to let this library be rpathed
+        if self.sess.target.target.options.is_like_osx {
+            self.cmd.arg("-dynamiclib");
+            self.linker_arg("-dylib");
+
+            // Note that the `osx_rpath_install_name` option here is a hack
+            // purely to support rustbuild right now, we should get a more
+            // principled solution at some point to force the compiler to pass
+            // the right `-Wl,-install_name` with an `@rpath` in it.
+            if self.sess.opts.cg.rpath ||
+               self.sess.opts.debugging_opts.osx_rpath_install_name {
+                self.linker_arg("-install_name");
+                let mut v = OsString::from("@rpath/");
+                v.push(out_filename.file_name().unwrap());
+                self.linker_arg(&v);
+            }
+        } else {
+            self.cmd.arg("-shared");
+        }
+    }
+
+    fn export_symbols(&mut self, tmpdir: &Path, crate_type: CrateType) {
+        // If we're compiling a dylib, then we let symbol visibility in object
+        // files to take care of whether they're exported or not.
+        //
+        // If we're compiling a cdylib, however, we manually create a list of
+        // exported symbols to ensure we don't expose any more. The object files
+        // have far more public symbols than we actually want to export, so we
+        // hide them all here.
+        if crate_type == CrateType::Dylib ||
+           crate_type == CrateType::ProcMacro {
+            return
+        }
+
+        let mut arg = OsString::new();
+        let path = tmpdir.join("list");
+
+        debug!("EXPORTED SYMBOLS:");
+
+        if self.sess.target.target.options.is_like_osx {
+            // Write a plain, newline-separated list of symbols
+            let res = (|| -> io::Result<()> {
+                let mut f = BufWriter::new(File::create(&path)?);
+                for sym in self.info.exports[&crate_type].iter() {
+                    debug!("  _{}", sym);
+                    writeln!(f, "_{}", sym)?;
+                }
+                Ok(())
+            })();
+            if let Err(e) = res {
+                self.sess.fatal(&format!("failed to write lib.def file: {}", e));
+            }
+        } else {
+            // Write an LD version script
+            let res = (|| -> io::Result<()> {
+                let mut f = BufWriter::new(File::create(&path)?);
+                writeln!(f, "{{\n  global:")?;
+                for sym in self.info.exports[&crate_type].iter() {
+                    debug!("    {};", sym);
+                    writeln!(f, "    {};", sym)?;
+                }
+                writeln!(f, "\n  local:\n    *;\n}};")?;
+                Ok(())
+            })();
+            if let Err(e) = res {
+                self.sess.fatal(&format!("failed to write version script: {}", e));
+            }
+        }
+
+        if self.sess.target.target.options.is_like_osx {
+            if !self.is_ld {
+                arg.push("-Wl,")
+            }
+            arg.push("-exported_symbols_list,");
+        } else if self.sess.target.target.options.is_like_solaris {
+            if !self.is_ld {
+                arg.push("-Wl,")
+            }
+            arg.push("-M,");
+        } else {
+            if !self.is_ld {
+                arg.push("-Wl,")
+            }
+            arg.push("--version-script=");
+        }
+
+        arg.push(&path);
+        self.cmd.arg(arg);
+    }
+
+    fn subsystem(&mut self, subsystem: &str) {
+        self.linker_arg("--subsystem");
+        self.linker_arg(&subsystem);
+    }
+
+    fn finalize(&mut self) -> Command {
+        self.hint_dynamic(); // Reset to default before returning the composed command line.
+        let mut cmd = Command::new("");
+        ::std::mem::swap(&mut cmd, &mut self.cmd);
+        cmd
+    }
+
+    fn group_start(&mut self) {
+        if !self.sess.target.target.options.is_like_osx {
+            self.linker_arg("--start-group");
+        }
+    }
+
+    fn group_end(&mut self) {
+        if !self.sess.target.target.options.is_like_osx {
+            self.linker_arg("--end-group");
+        }
+    }
+
+    fn cross_lang_lto(&mut self) {
+        match self.sess.opts.debugging_opts.cross_lang_lto {
+            CrossLangLto::Disabled => {
+                // Nothing to do
+            }
+            CrossLangLto::LinkerPluginAuto => {
+                self.push_cross_lang_lto_args(None);
+            }
+            CrossLangLto::LinkerPlugin(ref path) => {
+                self.push_cross_lang_lto_args(Some(path.as_os_str()));
+            }
+        }
+    }
+}
+
+pub struct MsvcLinker<'a> {
+    cmd: Command,
+    sess: &'a Session,
+    info: &'a LinkerInfo
+}
+
+impl<'a> Linker for MsvcLinker<'a> {
+    fn link_rlib(&mut self, lib: &Path) { self.cmd.arg(lib); }
+    fn add_object(&mut self, path: &Path) { self.cmd.arg(path); }
+    fn args(&mut self, args: &[String]) { self.cmd.args(args); }
+
+    fn build_dylib(&mut self, out_filename: &Path) {
+        self.cmd.arg("/DLL");
+        let mut arg: OsString = "/IMPLIB:".into();
+        arg.push(out_filename.with_extension("dll.lib"));
+        self.cmd.arg(arg);
+    }
+
+    fn build_static_executable(&mut self) {
+        // noop
+    }
+
+    fn gc_sections(&mut self, _keep_metadata: bool) {
+        // MSVC's ICF (Identical COMDAT Folding) link optimization is
+        // slow for Rust and thus we disable it by default when not in
+        // optimization build.
+        if self.sess.opts.optimize != config::OptLevel::No {
+            self.cmd.arg("/OPT:REF,ICF");
+        } else {
+            // It is necessary to specify NOICF here, because /OPT:REF
+            // implies ICF by default.
+            self.cmd.arg("/OPT:REF,NOICF");
+        }
+    }
+
+    fn link_dylib(&mut self, lib: &str) {
+        self.cmd.arg(&format!("{}.lib", lib));
+    }
+
+    fn link_rust_dylib(&mut self, lib: &str, path: &Path) {
+        // When producing a dll, the MSVC linker may not actually emit a
+        // `foo.lib` file if the dll doesn't actually export any symbols, so we
+        // check to see if the file is there and just omit linking to it if it's
+        // not present.
+        let name = format!("{}.dll.lib", lib);
+        if fs::metadata(&path.join(&name)).is_ok() {
+            self.cmd.arg(name);
+        }
+    }
+
+    fn link_staticlib(&mut self, lib: &str) {
+        self.cmd.arg(&format!("{}.lib", lib));
+    }
+
+    fn position_independent_executable(&mut self) {
+        // noop
+    }
+
+    fn no_position_independent_executable(&mut self) {
+        // noop
+    }
+
+    fn full_relro(&mut self) {
+        // noop
+    }
+
+    fn partial_relro(&mut self) {
+        // noop
+    }
+
+    fn no_relro(&mut self) {
+        // noop
+    }
+
+    fn no_default_libraries(&mut self) {
+        // Currently we don't pass the /NODEFAULTLIB flag to the linker on MSVC
+        // as there's been trouble in the past of linking the C++ standard
+        // library required by LLVM. This likely needs to happen one day, but
+        // in general Windows is also a more controlled environment than
+        // Unix, so it's not necessarily as critical that this be implemented.
+        //
+        // Note that there are also some licensing worries about statically
+        // linking some libraries which require a specific agreement, so it may
+        // not ever be possible for us to pass this flag.
+    }
+
+    fn include_path(&mut self, path: &Path) {
+        let mut arg = OsString::from("/LIBPATH:");
+        arg.push(path);
+        self.cmd.arg(&arg);
+    }
+
+    fn output_filename(&mut self, path: &Path) {
+        let mut arg = OsString::from("/OUT:");
+        arg.push(path);
+        self.cmd.arg(&arg);
+    }
+
+    fn framework_path(&mut self, _path: &Path) {
+        bug!("frameworks are not supported on windows")
+    }
+    fn link_framework(&mut self, _framework: &str) {
+        bug!("frameworks are not supported on windows")
+    }
+
+    fn link_whole_staticlib(&mut self, lib: &str, _search_path: &[PathBuf]) {
+        // not supported?
+        self.link_staticlib(lib);
+    }
+    fn link_whole_rlib(&mut self, path: &Path) {
+        // not supported?
+        self.link_rlib(path);
+    }
+    fn optimize(&mut self) {
+        // Needs more investigation of `/OPT` arguments
+    }
+
+    fn pgo_gen(&mut self) {
+        // Nothing needed here.
+    }
+
+    fn debuginfo(&mut self) {
+        // This will cause the Microsoft linker to generate a PDB file
+        // from the CodeView line tables in the object files.
+        self.cmd.arg("/DEBUG");
+
+        // This will cause the Microsoft linker to embed .natvis info into the the PDB file
+        let sysroot = self.sess.sysroot();
+        let natvis_dir_path = sysroot.join("lib\\rustlib\\etc");
+        if let Ok(natvis_dir) = fs::read_dir(&natvis_dir_path) {
+            // LLVM 5.0.0's lld-link frontend doesn't yet recognize, and chokes
+            // on, the /NATVIS:... flags.  LLVM 6 (or earlier) should at worst ignore
+            // them, eventually mooting this workaround, per this landed patch:
+            // https://github.com/llvm-mirror/lld/commit/27b9c4285364d8d76bb43839daa100
+            if let Some(ref linker_path) = self.sess.opts.cg.linker {
+                if let Some(linker_name) = Path::new(&linker_path).file_stem() {
+                    if linker_name.to_str().unwrap().to_lowercase() == "lld-link" {
+                        self.sess.warn("not embedding natvis: lld-link may not support the flag");
+                        return;
+                    }
+                }
+            }
+            for entry in natvis_dir {
+                match entry {
+                    Ok(entry) => {
+                        let path = entry.path();
+                        if path.extension() == Some("natvis".as_ref()) {
+                            let mut arg = OsString::from("/NATVIS:");
+                            arg.push(path);
+                            self.cmd.arg(arg);
+                        }
+                    },
+                    Err(err) => {
+                        self.sess.warn(&format!("error enumerating natvis directory: {}", err));
+                    },
+                }
+            }
+        }
+    }
+
+    // Currently the compiler doesn't use `dllexport` (an LLVM attribute) to
+    // export symbols from a dynamic library. When building a dynamic library,
+    // however, we're going to want some symbols exported, so this function
+    // generates a DEF file which lists all the symbols.
+    //
+    // The linker will read this `*.def` file and export all the symbols from
+    // the dynamic library. Note that this is not as simple as just exporting
+    // all the symbols in the current crate (as specified by `codegen.reachable`)
+    // but rather we also need to possibly export the symbols of upstream
+    // crates. Upstream rlibs may be linked statically to this dynamic library,
+    // in which case they may continue to transitively be used and hence need
+    // their symbols exported.
+    fn export_symbols(&mut self,
+                      tmpdir: &Path,
+                      crate_type: CrateType) {
+        let path = tmpdir.join("lib.def");
+        let res = (|| -> io::Result<()> {
+            let mut f = BufWriter::new(File::create(&path)?);
+
+            // Start off with the standard module name header and then go
+            // straight to exports.
+            writeln!(f, "LIBRARY")?;
+            writeln!(f, "EXPORTS")?;
+            for symbol in self.info.exports[&crate_type].iter() {
+                debug!("  _{}", symbol);
+                writeln!(f, "  {}", symbol)?;
+            }
+            Ok(())
+        })();
+        if let Err(e) = res {
+            self.sess.fatal(&format!("failed to write lib.def file: {}", e));
+        }
+        let mut arg = OsString::from("/DEF:");
+        arg.push(path);
+        self.cmd.arg(&arg);
+    }
+
+    fn subsystem(&mut self, subsystem: &str) {
+        // Note that previous passes of the compiler validated this subsystem,
+        // so we just blindly pass it to the linker.
+        self.cmd.arg(&format!("/SUBSYSTEM:{}", subsystem));
+
+        // Windows has two subsystems we're interested in right now, the console
+        // and windows subsystems. These both implicitly have different entry
+        // points (starting symbols). The console entry point starts with
+        // `mainCRTStartup` and the windows entry point starts with
+        // `WinMainCRTStartup`. These entry points, defined in system libraries,
+        // will then later probe for either `main` or `WinMain`, respectively to
+        // start the application.
+        //
+        // In Rust we just always generate a `main` function so we want control
+        // to always start there, so we force the entry point on the windows
+        // subsystem to be `mainCRTStartup` to get everything booted up
+        // correctly.
+        //
+        // For more information see RFC #1665
+        if subsystem == "windows" {
+            self.cmd.arg("/ENTRY:mainCRTStartup");
+        }
+    }
+
+    fn finalize(&mut self) -> Command {
+        let mut cmd = Command::new("");
+        ::std::mem::swap(&mut cmd, &mut self.cmd);
+        cmd
+    }
+
+    // MSVC doesn't need group indicators
+    fn group_start(&mut self) {}
+    fn group_end(&mut self) {}
+
+    fn cross_lang_lto(&mut self) {
+        // Do nothing
+    }
+}
+
+pub struct EmLinker<'a> {
+    cmd: Command,
+    sess: &'a Session,
+    info: &'a LinkerInfo
+}
+
+impl<'a> Linker for EmLinker<'a> {
+    fn include_path(&mut self, path: &Path) {
+        self.cmd.arg("-L").arg(path);
+    }
+
+    fn link_staticlib(&mut self, lib: &str) {
+        self.cmd.arg("-l").arg(lib);
+    }
+
+    fn output_filename(&mut self, path: &Path) {
+        self.cmd.arg("-o").arg(path);
+    }
+
+    fn add_object(&mut self, path: &Path) {
+        self.cmd.arg(path);
+    }
+
+    fn link_dylib(&mut self, lib: &str) {
+        // Emscripten always links statically
+        self.link_staticlib(lib);
+    }
+
+    fn link_whole_staticlib(&mut self, lib: &str, _search_path: &[PathBuf]) {
+        // not supported?
+        self.link_staticlib(lib);
+    }
+
+    fn link_whole_rlib(&mut self, lib: &Path) {
+        // not supported?
+        self.link_rlib(lib);
+    }
+
+    fn link_rust_dylib(&mut self, lib: &str, _path: &Path) {
+        self.link_dylib(lib);
+    }
+
+    fn link_rlib(&mut self, lib: &Path) {
+        self.add_object(lib);
+    }
+
+    fn position_independent_executable(&mut self) {
+        // noop
+    }
+
+    fn no_position_independent_executable(&mut self) {
+        // noop
+    }
+
+    fn full_relro(&mut self) {
+        // noop
+    }
+
+    fn partial_relro(&mut self) {
+        // noop
+    }
+
+    fn no_relro(&mut self) {
+        // noop
+    }
+
+    fn args(&mut self, args: &[String]) {
+        self.cmd.args(args);
+    }
+
+    fn framework_path(&mut self, _path: &Path) {
+        bug!("frameworks are not supported on Emscripten")
+    }
+
+    fn link_framework(&mut self, _framework: &str) {
+        bug!("frameworks are not supported on Emscripten")
+    }
+
+    fn gc_sections(&mut self, _keep_metadata: bool) {
+        // noop
+    }
+
+    fn optimize(&mut self) {
+        // Emscripten performs own optimizations
+        self.cmd.arg(match self.sess.opts.optimize {
+            OptLevel::No => "-O0",
+            OptLevel::Less => "-O1",
+            OptLevel::Default => "-O2",
+            OptLevel::Aggressive => "-O3",
+            OptLevel::Size => "-Os",
+            OptLevel::SizeMin => "-Oz"
+        });
+        // Unusable until https://github.com/rust-lang/rust/issues/38454 is resolved
+        self.cmd.args(&["--memory-init-file", "0"]);
+    }
+
+    fn pgo_gen(&mut self) {
+        // noop, but maybe we need something like the gnu linker?
+    }
+
+    fn debuginfo(&mut self) {
+        // Preserve names or generate source maps depending on debug info
+        self.cmd.arg(match self.sess.opts.debuginfo {
+            DebugInfo::None => "-g0",
+            DebugInfo::Limited => "-g3",
+            DebugInfo::Full => "-g4"
+        });
+    }
+
+    fn no_default_libraries(&mut self) {
+        self.cmd.args(&["-s", "DEFAULT_LIBRARY_FUNCS_TO_INCLUDE=[]"]);
+    }
+
+    fn build_dylib(&mut self, _out_filename: &Path) {
+        bug!("building dynamic library is unsupported on Emscripten")
+    }
+
+    fn build_static_executable(&mut self) {
+        // noop
+    }
+
+    fn export_symbols(&mut self, _tmpdir: &Path, crate_type: CrateType) {
+        let symbols = &self.info.exports[&crate_type];
+
+        debug!("EXPORTED SYMBOLS:");
+
+        self.cmd.arg("-s");
+
+        let mut arg = OsString::from("EXPORTED_FUNCTIONS=");
+        let mut encoded = String::new();
+
+        {
+            let mut encoder = json::Encoder::new(&mut encoded);
+            let res = encoder.emit_seq(symbols.len(), |encoder| {
+                for (i, sym) in symbols.iter().enumerate() {
+                    encoder.emit_seq_elt(i, |encoder| {
+                        encoder.emit_str(&("_".to_string() + sym))
+                    })?;
+                }
+                Ok(())
+            });
+            if let Err(e) = res {
+                self.sess.fatal(&format!("failed to encode exported symbols: {}", e));
+            }
+        }
+        debug!("{}", encoded);
+        arg.push(encoded);
+
+        self.cmd.arg(arg);
+    }
+
+    fn subsystem(&mut self, _subsystem: &str) {
+        // noop
+    }
+
+    fn finalize(&mut self) -> Command {
+        let mut cmd = Command::new("");
+        ::std::mem::swap(&mut cmd, &mut self.cmd);
+        cmd
+    }
+
+    // Appears not necessary on Emscripten
+    fn group_start(&mut self) {}
+    fn group_end(&mut self) {}
+
+    fn cross_lang_lto(&mut self) {
+        // Do nothing
+    }
+}
+
+fn exported_symbols(tcx: TyCtxt, crate_type: CrateType) -> Vec<String> {
+    let mut symbols = Vec::new();
+
+    let export_threshold =
+        ::symbol_export::crates_export_threshold(&[crate_type]);
+    for &(symbol, level) in tcx.exported_symbols(LOCAL_CRATE).iter() {
+        if level.is_below_threshold(export_threshold) {
+            symbols.push(symbol.symbol_name(tcx).to_string());
+        }
+    }
+
+    let formats = tcx.sess.dependency_formats.borrow();
+    let deps = formats[&crate_type].iter();
+
+    for (index, dep_format) in deps.enumerate() {
+        let cnum = CrateNum::new(index + 1);
+        // For each dependency that we are linking to statically ...
+        if *dep_format == Linkage::Static {
+            // ... we add its symbol list to our export list.
+            for &(symbol, level) in tcx.exported_symbols(cnum).iter() {
+                if level.is_below_threshold(export_threshold) {
+                    symbols.push(symbol.symbol_name(tcx).to_string());
+                }
+            }
+        }
+    }
+
+    symbols
+}
+
+pub struct WasmLd<'a> {
+    cmd: Command,
+    sess: &'a Session,
+    info: &'a LinkerInfo,
+}
+
+impl<'a> Linker for WasmLd<'a> {
+    fn link_dylib(&mut self, lib: &str) {
+        self.cmd.arg("-l").arg(lib);
+    }
+
+    fn link_staticlib(&mut self, lib: &str) {
+        self.cmd.arg("-l").arg(lib);
+    }
+
+    fn link_rlib(&mut self, lib: &Path) {
+        self.cmd.arg(lib);
+    }
+
+    fn include_path(&mut self, path: &Path) {
+        self.cmd.arg("-L").arg(path);
+    }
+
+    fn framework_path(&mut self, _path: &Path) {
+        panic!("frameworks not supported")
+    }
+
+    fn output_filename(&mut self, path: &Path) {
+        self.cmd.arg("-o").arg(path);
+    }
+
+    fn add_object(&mut self, path: &Path) {
+        self.cmd.arg(path);
+    }
+
+    fn position_independent_executable(&mut self) {
+    }
+
+    fn full_relro(&mut self) {
+    }
+
+    fn partial_relro(&mut self) {
+    }
+
+    fn no_relro(&mut self) {
+    }
+
+    fn build_static_executable(&mut self) {
+    }
+
+    fn args(&mut self, args: &[String]) {
+        self.cmd.args(args);
+    }
+
+    fn link_rust_dylib(&mut self, lib: &str, _path: &Path) {
+        self.cmd.arg("-l").arg(lib);
+    }
+
+    fn link_framework(&mut self, _framework: &str) {
+        panic!("frameworks not supported")
+    }
+
+    fn link_whole_staticlib(&mut self, lib: &str, _search_path: &[PathBuf]) {
+        self.cmd.arg("-l").arg(lib);
+    }
+
+    fn link_whole_rlib(&mut self, lib: &Path) {
+        self.cmd.arg(lib);
+    }
+
+    fn gc_sections(&mut self, _keep_metadata: bool) {
+        self.cmd.arg("--gc-sections");
+    }
+
+    fn optimize(&mut self) {
+        self.cmd.arg(match self.sess.opts.optimize {
+            OptLevel::No => "-O0",
+            OptLevel::Less => "-O1",
+            OptLevel::Default => "-O2",
+            OptLevel::Aggressive => "-O3",
+            // Currently LLD doesn't support `Os` and `Oz`, so pass through `O2`
+            // instead.
+            OptLevel::Size => "-O2",
+            OptLevel::SizeMin => "-O2"
+        });
+    }
+
+    fn pgo_gen(&mut self) {
+    }
+
+    fn debuginfo(&mut self) {
+    }
+
+    fn no_default_libraries(&mut self) {
+    }
+
+    fn build_dylib(&mut self, _out_filename: &Path) {
+    }
+
+    fn export_symbols(&mut self, _tmpdir: &Path, crate_type: CrateType) {
+        for sym in self.info.exports[&crate_type].iter() {
+            self.cmd.arg("--export").arg(&sym);
+        }
+    }
+
+    fn subsystem(&mut self, _subsystem: &str) {
+    }
+
+    fn no_position_independent_executable(&mut self) {
+    }
+
+    fn finalize(&mut self) -> Command {
+        // There have been reports in the wild (rustwasm/wasm-bindgen#119) of
+        // using threads causing weird hangs and bugs. Disable it entirely as
+        // this isn't yet the bottleneck of compilation at all anyway.
+        self.cmd.arg("--no-threads");
+
+        // By default LLD only gives us one page of stack (64k) which is a
+        // little small. Default to a larger stack closer to other PC platforms
+        // (1MB) and users can always inject their own link-args to override this.
+        self.cmd.arg("-z").arg("stack-size=1048576");
+
+        // By default LLD's memory layout is:
+        //
+        // 1. First, a blank page
+        // 2. Next, all static data
+        // 3. Finally, the main stack (which grows down)
+        //
+        // This has the unfortunate consequence that on stack overflows you
+        // corrupt static data and can cause some exceedingly weird bugs. To
+        // help detect this a little sooner we instead request that the stack is
+        // placed before static data.
+        //
+        // This means that we'll generate slightly larger binaries as references
+        // to static data will take more bytes in the ULEB128 encoding, but
+        // stack overflow will be guaranteed to trap as it underflows instead of
+        // corrupting static data.
+        self.cmd.arg("--stack-first");
+
+        // FIXME we probably shouldn't pass this but instead pass an explicit
+        // whitelist of symbols we'll allow to be undefined. Unfortunately
+        // though we can't handle symbols like `log10` that LLVM injects at a
+        // super late date without actually parsing object files. For now let's
+        // stick to this and hopefully fix it before stabilization happens.
+        self.cmd.arg("--allow-undefined");
+
+        // For now we just never have an entry symbol
+        self.cmd.arg("--no-entry");
+
+        // Make the default table accessible
+        self.cmd.arg("--export-table");
+
+        // Rust code should never have warnings, and warnings are often
+        // indicative of bugs, let's prevent them.
+        self.cmd.arg("--fatal-warnings");
+
+        let mut cmd = Command::new("");
+        ::std::mem::swap(&mut cmd, &mut self.cmd);
+        cmd
+    }
+
+    // Not needed for now with LLD
+    fn group_start(&mut self) {}
+    fn group_end(&mut self) {}
+
+    fn cross_lang_lto(&mut self) {
+        // Do nothing for now
+    }
+}
diff --git a/src/librustc_codegen_utils/symbol_export.rs b/src/librustc_codegen_utils/symbol_export.rs
new file mode 100644
index 00000000000..2d650f7f18d
--- /dev/null
+++ b/src/librustc_codegen_utils/symbol_export.rs
@@ -0,0 +1,395 @@
+// Copyright 2016 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 rustc_data_structures::sync::Lrc;
+use std::sync::Arc;
+
+use rustc::ty::Instance;
+use rustc::hir;
+use rustc::hir::Node;
+use rustc::hir::CodegenFnAttrFlags;
+use rustc::hir::def_id::{CrateNum, DefId, LOCAL_CRATE, CRATE_DEF_INDEX};
+use rustc_data_structures::fingerprint::Fingerprint;
+use rustc::middle::exported_symbols::{SymbolExportLevel, ExportedSymbol, metadata_symbol_name};
+use rustc::session::config;
+use rustc::ty::{TyCtxt, SymbolName};
+use rustc::ty::query::Providers;
+use rustc::ty::subst::Substs;
+use rustc::util::nodemap::{FxHashMap, DefIdMap};
+use rustc_allocator::ALLOCATOR_METHODS;
+use rustc_data_structures::indexed_vec::IndexVec;
+use std::collections::hash_map::Entry::*;
+
+pub type ExportedSymbols = FxHashMap<
+    CrateNum,
+    Arc<Vec<(String, SymbolExportLevel)>>,
+>;
+
+pub fn threshold(tcx: TyCtxt) -> SymbolExportLevel {
+    crates_export_threshold(&tcx.sess.crate_types.borrow())
+}
+
+fn crate_export_threshold(crate_type: config::CrateType) -> SymbolExportLevel {
+    match crate_type {
+        config::CrateType::Executable |
+        config::CrateType::Staticlib  |
+        config::CrateType::ProcMacro  |
+        config::CrateType::Cdylib     => SymbolExportLevel::C,
+        config::CrateType::Rlib       |
+        config::CrateType::Dylib      => SymbolExportLevel::Rust,
+    }
+}
+
+pub fn crates_export_threshold(crate_types: &[config::CrateType])
+                                      -> SymbolExportLevel {
+    if crate_types.iter().any(|&crate_type| {
+        crate_export_threshold(crate_type) == SymbolExportLevel::Rust
+    }) {
+        SymbolExportLevel::Rust
+    } else {
+        SymbolExportLevel::C
+    }
+}
+
+fn reachable_non_generics_provider<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
+                                             cnum: CrateNum)
+                                             -> Lrc<DefIdMap<SymbolExportLevel>>
+{
+    assert_eq!(cnum, LOCAL_CRATE);
+
+    if !tcx.sess.opts.output_types.should_codegen() {
+        return Lrc::new(DefIdMap())
+    }
+
+    // Check to see if this crate is a "special runtime crate". These
+    // crates, implementation details of the standard library, typically
+    // have a bunch of `pub extern` and `#[no_mangle]` functions as the
+    // ABI between them. We don't want their symbols to have a `C`
+    // export level, however, as they're just implementation details.
+    // Down below we'll hardwire all of the symbols to the `Rust` export
+    // level instead.
+    let special_runtime_crate = tcx.is_panic_runtime(LOCAL_CRATE) ||
+        tcx.is_compiler_builtins(LOCAL_CRATE);
+
+    let mut reachable_non_generics: DefIdMap<_> = tcx.reachable_set(LOCAL_CRATE).0
+        .iter()
+        .filter_map(|&node_id| {
+            // We want to ignore some FFI functions that are not exposed from
+            // this crate. Reachable FFI functions can be lumped into two
+            // categories:
+            //
+            // 1. Those that are included statically via a static library
+            // 2. Those included otherwise (e.g. dynamically or via a framework)
+            //
+            // Although our LLVM module is not literally emitting code for the
+            // statically included symbols, it's an export of our library which
+            // needs to be passed on to the linker and encoded in the metadata.
+            //
+            // As a result, if this id is an FFI item (foreign item) then we only
+            // let it through if it's included statically.
+            match tcx.hir.get(node_id) {
+                Node::ForeignItem(..) => {
+                    let def_id = tcx.hir.local_def_id(node_id);
+                    if tcx.is_statically_included_foreign_item(def_id) {
+                        Some(def_id)
+                    } else {
+                        None
+                    }
+                }
+
+                // Only consider nodes that actually have exported symbols.
+                Node::Item(&hir::Item {
+                    node: hir::ItemKind::Static(..),
+                    ..
+                }) |
+                Node::Item(&hir::Item {
+                    node: hir::ItemKind::Fn(..), ..
+                }) |
+                Node::ImplItem(&hir::ImplItem {
+                    node: hir::ImplItemKind::Method(..),
+                    ..
+                }) => {
+                    let def_id = tcx.hir.local_def_id(node_id);
+                    let generics = tcx.generics_of(def_id);
+                    if !generics.requires_monomorphization(tcx) &&
+                        // Functions marked with #[inline] are only ever codegened
+                        // with "internal" linkage and are never exported.
+                        !Instance::mono(tcx, def_id).def.requires_local(tcx) {
+                        Some(def_id)
+                    } else {
+                        None
+                    }
+                }
+
+                _ => None
+            }
+        })
+        .map(|def_id| {
+            let export_level = if special_runtime_crate {
+                let name = tcx.symbol_name(Instance::mono(tcx, def_id)).as_str();
+                // We can probably do better here by just ensuring that
+                // it has hidden visibility rather than public
+                // visibility, as this is primarily here to ensure it's
+                // not stripped during LTO.
+                //
+                // In general though we won't link right if these
+                // symbols are stripped, and LTO currently strips them.
+                if &*name == "rust_eh_personality" ||
+                   &*name == "rust_eh_register_frames" ||
+                   &*name == "rust_eh_unregister_frames" {
+                    SymbolExportLevel::C
+                } else {
+                    SymbolExportLevel::Rust
+                }
+            } else {
+                symbol_export_level(tcx, def_id)
+            };
+            debug!("EXPORTED SYMBOL (local): {} ({:?})",
+                   tcx.symbol_name(Instance::mono(tcx, def_id)),
+                   export_level);
+            (def_id, export_level)
+        })
+        .collect();
+
+    if let Some(id) = *tcx.sess.derive_registrar_fn.get() {
+        let def_id = tcx.hir.local_def_id(id);
+        reachable_non_generics.insert(def_id, SymbolExportLevel::C);
+    }
+
+    if let Some(id) = *tcx.sess.plugin_registrar_fn.get() {
+        let def_id = tcx.hir.local_def_id(id);
+        reachable_non_generics.insert(def_id, SymbolExportLevel::C);
+    }
+
+    Lrc::new(reachable_non_generics)
+}
+
+fn is_reachable_non_generic_provider_local<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
+                                                     def_id: DefId)
+                                                     -> bool {
+    let export_threshold = threshold(tcx);
+
+    if let Some(&level) = tcx.reachable_non_generics(def_id.krate).get(&def_id) {
+        level.is_below_threshold(export_threshold)
+    } else {
+        false
+    }
+}
+
+fn is_reachable_non_generic_provider_extern<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
+                                                      def_id: DefId)
+                                                      -> bool {
+    tcx.reachable_non_generics(def_id.krate).contains_key(&def_id)
+}
+
+fn exported_symbols_provider_local<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
+                                             cnum: CrateNum)
+                                             -> Arc<Vec<(ExportedSymbol<'tcx>,
+                                                         SymbolExportLevel)>>
+{
+    assert_eq!(cnum, LOCAL_CRATE);
+
+    if !tcx.sess.opts.output_types.should_codegen() {
+        return Arc::new(vec![])
+    }
+
+    let mut symbols: Vec<_> = tcx.reachable_non_generics(LOCAL_CRATE)
+                                 .iter()
+                                 .map(|(&def_id, &level)| {
+                                    (ExportedSymbol::NonGeneric(def_id), level)
+                                 })
+                                 .collect();
+
+    if tcx.sess.entry_fn.borrow().is_some() {
+        let exported_symbol = ExportedSymbol::NoDefId(SymbolName::new("main"));
+
+        symbols.push((exported_symbol, SymbolExportLevel::C));
+    }
+
+    if tcx.sess.allocator_kind.get().is_some() {
+        for method in ALLOCATOR_METHODS {
+            let symbol_name = format!("__rust_{}", method.name);
+            let exported_symbol = ExportedSymbol::NoDefId(SymbolName::new(&symbol_name));
+
+            symbols.push((exported_symbol, SymbolExportLevel::Rust));
+        }
+    }
+
+    if tcx.sess.opts.debugging_opts.pgo_gen.is_some() {
+        // These are weak symbols that point to the profile version and the
+        // profile name, which need to be treated as exported so LTO doesn't nix
+        // them.
+        const PROFILER_WEAK_SYMBOLS: [&'static str; 2] = [
+            "__llvm_profile_raw_version",
+            "__llvm_profile_filename",
+        ];
+        for sym in &PROFILER_WEAK_SYMBOLS {
+            let exported_symbol = ExportedSymbol::NoDefId(SymbolName::new(sym));
+            symbols.push((exported_symbol, SymbolExportLevel::C));
+        }
+    }
+
+    if tcx.sess.crate_types.borrow().contains(&config::CrateType::Dylib) {
+        let symbol_name = metadata_symbol_name(tcx);
+        let exported_symbol = ExportedSymbol::NoDefId(SymbolName::new(&symbol_name));
+
+        symbols.push((exported_symbol, SymbolExportLevel::Rust));
+    }
+
+    if tcx.sess.opts.share_generics() && tcx.local_crate_exports_generics() {
+        use rustc::mir::mono::{Linkage, Visibility, MonoItem};
+        use rustc::ty::InstanceDef;
+
+        // Normally, we require that shared monomorphizations are not hidden,
+        // because if we want to re-use a monomorphization from a Rust dylib, it
+        // needs to be exported.
+        // However, on platforms that don't allow for Rust dylibs, having
+        // external linkage is enough for monomorphization to be linked to.
+        let need_visibility = tcx.sess.target.target.options.dynamic_linking &&
+                              !tcx.sess.target.target.options.only_cdylib;
+
+        let (_, cgus) = tcx.collect_and_partition_mono_items(LOCAL_CRATE);
+
+        for (mono_item, &(linkage, visibility)) in cgus.iter()
+                                                       .flat_map(|cgu| cgu.items().iter()) {
+            if linkage != Linkage::External {
+                // We can only re-use things with external linkage, otherwise
+                // we'll get a linker error
+                continue
+            }
+
+            if need_visibility && visibility == Visibility::Hidden {
+                // If we potentially share things from Rust dylibs, they must
+                // not be hidden
+                continue
+            }
+
+            if let &MonoItem::Fn(Instance {
+                def: InstanceDef::Item(def_id),
+                substs,
+            }) = mono_item {
+                if substs.types().next().is_some() {
+                    symbols.push((ExportedSymbol::Generic(def_id, substs),
+                                  SymbolExportLevel::Rust));
+                }
+            }
+        }
+    }
+
+    // Sort so we get a stable incr. comp. hash.
+    symbols.sort_unstable_by(|&(ref symbol1, ..), &(ref symbol2, ..)| {
+        symbol1.compare_stable(tcx, symbol2)
+    });
+
+    Arc::new(symbols)
+}
+
+fn upstream_monomorphizations_provider<'a, 'tcx>(
+    tcx: TyCtxt<'a, 'tcx, 'tcx>,
+    cnum: CrateNum)
+    -> Lrc<DefIdMap<Lrc<FxHashMap<&'tcx Substs<'tcx>, CrateNum>>>>
+{
+    debug_assert!(cnum == LOCAL_CRATE);
+
+    let cnums = tcx.all_crate_nums(LOCAL_CRATE);
+
+    let mut instances: DefIdMap<FxHashMap<_, _>> = DefIdMap();
+
+    let cnum_stable_ids: IndexVec<CrateNum, Fingerprint> = {
+        let mut cnum_stable_ids = IndexVec::from_elem_n(Fingerprint::ZERO,
+                                                        cnums.len() + 1);
+
+        for &cnum in cnums.iter() {
+            cnum_stable_ids[cnum] = tcx.def_path_hash(DefId {
+                krate: cnum,
+                index: CRATE_DEF_INDEX,
+            }).0;
+        }
+
+        cnum_stable_ids
+    };
+
+    for &cnum in cnums.iter() {
+        for &(ref exported_symbol, _) in tcx.exported_symbols(cnum).iter() {
+            if let &ExportedSymbol::Generic(def_id, substs) = exported_symbol {
+                let substs_map = instances.entry(def_id).or_default();
+
+                match substs_map.entry(substs) {
+                    Occupied(mut e) => {
+                        // If there are multiple monomorphizations available,
+                        // we select one deterministically.
+                        let other_cnum = *e.get();
+                        if cnum_stable_ids[other_cnum] > cnum_stable_ids[cnum] {
+                            e.insert(cnum);
+                        }
+                    }
+                    Vacant(e) => {
+                        e.insert(cnum);
+                    }
+                }
+            }
+        }
+    }
+
+    Lrc::new(instances.into_iter()
+                      .map(|(key, value)| (key, Lrc::new(value)))
+                      .collect())
+}
+
+fn upstream_monomorphizations_for_provider<'a, 'tcx>(
+    tcx: TyCtxt<'a, 'tcx, 'tcx>,
+    def_id: DefId)
+    -> Option<Lrc<FxHashMap<&'tcx Substs<'tcx>, CrateNum>>>
+{
+    debug_assert!(!def_id.is_local());
+    tcx.upstream_monomorphizations(LOCAL_CRATE)
+       .get(&def_id)
+       .cloned()
+}
+
+fn is_unreachable_local_definition_provider(tcx: TyCtxt, def_id: DefId) -> bool {
+    if let Some(node_id) = tcx.hir.as_local_node_id(def_id) {
+        !tcx.reachable_set(LOCAL_CRATE).0.contains(&node_id)
+    } else {
+        bug!("is_unreachable_local_definition called with non-local DefId: {:?}",
+              def_id)
+    }
+}
+
+pub fn provide(providers: &mut Providers) {
+    providers.reachable_non_generics = reachable_non_generics_provider;
+    providers.is_reachable_non_generic = is_reachable_non_generic_provider_local;
+    providers.exported_symbols = exported_symbols_provider_local;
+    providers.upstream_monomorphizations = upstream_monomorphizations_provider;
+    providers.is_unreachable_local_definition = is_unreachable_local_definition_provider;
+}
+
+pub fn provide_extern(providers: &mut Providers) {
+    providers.is_reachable_non_generic = is_reachable_non_generic_provider_extern;
+    providers.upstream_monomorphizations_for = upstream_monomorphizations_for_provider;
+}
+
+fn symbol_export_level(tcx: TyCtxt, sym_def_id: DefId) -> SymbolExportLevel {
+    // We export anything that's not mangled at the "C" layer as it probably has
+    // to do with ABI concerns. We do not, however, apply such treatment to
+    // special symbols in the standard library for various plumbing between
+    // core/std/allocators/etc. For example symbols used to hook up allocation
+    // are not considered for export
+    let codegen_fn_attrs = tcx.codegen_fn_attrs(sym_def_id);
+    let is_extern = codegen_fn_attrs.contains_extern_indicator();
+    let std_internal =
+        codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::RUSTC_STD_INTERNAL_SYMBOL);
+
+    if is_extern && !std_internal {
+        SymbolExportLevel::C
+    } else {
+        SymbolExportLevel::Rust
+    }
+}