link.rs

// Copyright 2012-2014 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 super::archive::{ArchiveBuilder, ArchiveConfig};
use super::linker::{Linker, GnuLinker, MsvcLinker};
use super::rpath::RPathConfig;
use super::rpath;
use super::msvc;
use super::svh::Svh;
use session::config;
use session::config::NoDebugInfo;
use session::config::{OutputFilenames, Input, OutputType};
use session::filesearch;
use session::search_paths::PathKind;
use session::Session;
use middle::cstore::{self, CrateStore, LinkMeta};
use middle::cstore::{LinkagePreference, NativeLibraryKind};
use middle::dependency_format::Linkage;
use middle::ty::{self, Ty};
use rustc::front::map::DefPath;
use trans::{CrateContext, CrateTranslation, gensym_name};
use util::common::time;
use util::sha2::{Digest, Sha256};
use util::fs::fix_windows_verbatim_for_gcc;
use rustc_back::tempdir::TempDir;

use std::ascii;
use std::char;
use std::env;
use std::ffi::OsString;
use std::fs;
use std::io::{self, Read, Write};
use std::iter::once;
use std::mem;
use std::path::{Path, PathBuf};
use std::process::Command;
use std::str;
use flate;
use serialize::hex::ToHex;
use syntax::ast;
use syntax::codemap::Span;
use syntax::parse::token::{self, InternedString};
use syntax::attr::AttrMetaMethods;

use rustc_front::hir;

// RLIB LLVM-BYTECODE OBJECT LAYOUT
// Version 1
// Bytes    Data
// 0..10    "RUST_OBJECT" encoded in ASCII
// 11..14   format version as little-endian u32
// 15..22   size in bytes of deflate compressed LLVM bitcode as
//          little-endian u64
// 23..     compressed LLVM bitcode

// This is the "magic number" expected at the beginning of a LLVM bytecode
// object in an rlib.
pub const RLIB_BYTECODE_OBJECT_MAGIC: &'static [u8] = b"RUST_OBJECT";

// The version number this compiler will write to bytecode objects in rlibs
pub const RLIB_BYTECODE_OBJECT_VERSION: u32 = 1;

// The offset in bytes the bytecode object format version number can be found at
pub const RLIB_BYTECODE_OBJECT_VERSION_OFFSET: usize = 11;

// The offset in bytes the size of the compressed bytecode can be found at in
// format version 1
pub const RLIB_BYTECODE_OBJECT_V1_DATASIZE_OFFSET: usize =
    RLIB_BYTECODE_OBJECT_VERSION_OFFSET + 4;

// The offset in bytes the compressed LLVM bytecode can be found at in format
// version 1
pub const RLIB_BYTECODE_OBJECT_V1_DATA_OFFSET: usize =
    RLIB_BYTECODE_OBJECT_V1_DATASIZE_OFFSET + 8;


/*
 * Name mangling and its relationship to metadata. This is complex. Read
 * carefully.
 *
 * The semantic model of Rust linkage is, broadly, that "there's no global
 * namespace" between crates. Our aim is to preserve the illusion of this
 * model despite the fact that it's not *quite* possible to implement on
 * modern linkers. We initially didn't use system linkers at all, but have
 * been convinced of their utility.
 *
 * There are a few issues to handle:
 *
 *  - Linkers operate on a flat namespace, so we have to flatten names.
 *    We do this using the C++ namespace-mangling technique. Foo::bar
 *    symbols and such.
 *
 *  - Symbols with the same name but different types need to get different
 *    linkage-names. We do this by hashing a string-encoding of the type into
 *    a fixed-size (currently 16-byte hex) cryptographic hash function (CHF:
 *    we use SHA256) to "prevent collisions". This is not airtight but 16 hex
 *    digits on uniform probability means you're going to need 2**32 same-name
 *    symbols in the same process before you're even hitting birthday-paradox
 *    collision probability.
 *
 *  - Symbols in different crates but with same names "within" the crate need
 *    to get different linkage-names.
 *
 *  - The hash shown in the filename needs to be predictable and stable for
 *    build tooling integration. It also needs to be using a hash function
 *    which is easy to use from Python, make, etc.
 *
 * So here is what we do:
 *
 *  - Consider the package id; every crate has one (specified with crate_id
 *    attribute).  If a package id isn't provided explicitly, we infer a
 *    versionless one from the output name. The version will end up being 0.0
 *    in this case. CNAME and CVERS are taken from this package id. For
 *    example, github.com/mozilla/CNAME#CVERS.
 *
 *  - Define CMH as SHA256(crateid).
 *
 *  - Define CMH8 as the first 8 characters of CMH.
 *
 *  - Compile our crate to lib CNAME-CMH8-CVERS.so
 *
 *  - Define STH(sym) as SHA256(CMH, type_str(sym))
 *
 *  - Suffix a mangled sym with ::STH@CVERS, so that it is unique in the
 *    name, non-name metadata, and type sense, and versioned in the way
 *    system linkers understand.
 */

pub fn find_crate_name(sess: Option<&Session>,
                       attrs: &[ast::Attribute],
                       input: &Input) -> String {
    let validate = |s: String, span: Option<Span>| {
        cstore::validate_crate_name(sess, &s[..], span);
        s
    };

    // Look in attributes 100% of the time to make sure the attribute is marked
    // as used. After doing this, however, we still prioritize a crate name from
    // the command line over one found in the #[crate_name] attribute. If we
    // find both we ensure that they're the same later on as well.
    let attr_crate_name = attrs.iter().find(|at| at.check_name("crate_name"))
                               .and_then(|at| at.value_str().map(|s| (at, s)));

    if let Some(sess) = sess {
        if let Some(ref s) = sess.opts.crate_name {
            if let Some((attr, ref name)) = attr_crate_name {
                if *s != &name[..] {
                    let msg = format!("--crate-name and #[crate_name] are \
                                       required to match, but `{}` != `{}`",
                                      s, name);
                    sess.span_err(attr.span, &msg[..]);
                }
            }
            return validate(s.clone(), None);
        }
    }

    if let Some((attr, s)) = attr_crate_name {
        return validate(s.to_string(), Some(attr.span));
    }
    if let Input::File(ref path) = *input {
        if let Some(s) = path.file_stem().and_then(|s| s.to_str()) {
            if s.starts_with("-") {
                let msg = format!("crate names cannot start with a `-`, but \
                                   `{}` has a leading hyphen", s);
                if let Some(sess) = sess {
                    sess.err(&msg);
                }
            } else {
                return validate(s.replace("-", "_"), None);
            }
        }
    }

    "rust_out".to_string()
}

pub fn build_link_meta(sess: &Session,
                       krate: &hir::Crate,
                       name: &str)
                       -> LinkMeta {
    let r = LinkMeta {
        crate_name: name.to_owned(),
        crate_hash: Svh::calculate(&sess.opts.cg.metadata, krate),
    };
    info!("{:?}", r);
    return r;
}

fn truncated_hash_result(symbol_hasher: &mut Sha256) -> String {
    let output = symbol_hasher.result_bytes();
    // 64 bits should be enough to avoid collisions.
    output[.. 8].to_hex().to_string()
}


// This calculates STH for a symbol, as defined above
fn symbol_hash<'tcx>(tcx: &ty::ctxt<'tcx>,
                     symbol_hasher: &mut Sha256,
                     t: Ty<'tcx>,
                     link_meta: &LinkMeta)
                     -> String {
    // NB: do *not* use abbrevs here as we want the symbol names
    // to be independent of one another in the crate.

    symbol_hasher.reset();
    symbol_hasher.input_str(&link_meta.crate_name);
    symbol_hasher.input_str("-");
    symbol_hasher.input_str(link_meta.crate_hash.as_str());
    for meta in tcx.sess.crate_metadata.borrow().iter() {
        symbol_hasher.input_str(&meta[..]);
    }
    symbol_hasher.input_str("-");
    symbol_hasher.input(&tcx.sess.cstore.encode_type(tcx, t));
    // Prefix with 'h' so that it never blends into adjacent digits
    let mut hash = String::from("h");
    hash.push_str(&truncated_hash_result(symbol_hasher));
    hash
}

fn get_symbol_hash<'a, 'tcx>(ccx: &CrateContext<'a, 'tcx>, t: Ty<'tcx>) -> String {
    match ccx.type_hashcodes().borrow().get(&t) {
        Some(h) => return h.to_string(),
        None => {}
    }

    let mut symbol_hasher = ccx.symbol_hasher().borrow_mut();
    let hash = symbol_hash(ccx.tcx(), &mut *symbol_hasher, t, ccx.link_meta());
    ccx.type_hashcodes().borrow_mut().insert(t, hash.clone());
    hash
}


// Name sanitation. LLVM will happily accept identifiers with weird names, but
// gas doesn't!
// gas accepts the following characters in symbols: a-z, A-Z, 0-9, ., _, $
pub fn sanitize(s: &str) -> String {
    let mut result = String::new();
    for c in s.chars() {
        match c {
            // Escape these with $ sequences
            '@' => result.push_str("$SP$"),
            '*' => result.push_str("$BP$"),
            '&' => result.push_str("$RF$"),
            '<' => result.push_str("$LT$"),
            '>' => result.push_str("$GT$"),
            '(' => result.push_str("$LP$"),
            ')' => result.push_str("$RP$"),
            ',' => result.push_str("$C$"),

            // '.' doesn't occur in types and functions, so reuse it
            // for ':' and '-'
            '-' | ':' => result.push('.'),

            // These are legal symbols
            'a' ... 'z'
            | 'A' ... 'Z'
            | '0' ... '9'
            | '_' | '.' | '$' => result.push(c),

            _ => {
                result.push('$');
                for c in c.escape_unicode().skip(1) {
                    match c {
                        '{' => {},
                        '}' => result.push('$'),
                        c => result.push(c),
                    }
                }
            }
        }
    }

    // Underscore-qualify anything that didn't start as an ident.
    if !result.is_empty() &&
        result.as_bytes()[0] != '_' as u8 &&
        ! (result.as_bytes()[0] as char).is_xid_start() {
        return format!("_{}", &result[..]);
    }

    return result;
}

pub fn mangle<PI: Iterator<Item=InternedString>>(path: PI, hash: Option<&str>) -> String {
    // Follow C++ namespace-mangling style, see
    // http://en.wikipedia.org/wiki/Name_mangling for more info.
    //
    // It turns out that on OSX you can actually have arbitrary symbols in
    // function names (at least when given to LLVM), but this is not possible
    // when using unix's linker. Perhaps one day when we just use a linker from LLVM
    // we won't need to do this name mangling. The problem with name mangling is
    // that it seriously limits the available characters. For example we can't
    // have things like &T in symbol names when one would theoretically
    // want them for things like impls of traits on that type.
    //
    // To be able to work on all platforms and get *some* reasonable output, we
    // use C++ name-mangling.

    let mut n = String::from("_ZN"); // _Z == Begin name-sequence, N == nested

    fn push(n: &mut String, s: &str) {
        let sani = sanitize(s);
        n.push_str(&format!("{}{}", sani.len(), sani));
    }

    // First, connect each component with <len, name> pairs.
    for data in path {
        push(&mut n, &data);
    }

    match hash {
        Some(s) => push(&mut n, s),
        None => {}
    }

    n.push('E'); // End name-sequence.
    n
}

pub fn exported_name(path: DefPath, hash: &str) -> String {
    let path = path.into_iter()
                   .map(|e| e.data.as_interned_str());
    mangle(path, Some(hash))
}

pub fn mangle_exported_name<'a, 'tcx>(ccx: &CrateContext<'a, 'tcx>, path: DefPath,
                                      t: Ty<'tcx>, id: ast::NodeId) -> String {
    let mut hash = get_symbol_hash(ccx, t);

    // Paths can be completely identical for different nodes,
    // e.g. `fn foo() { { fn a() {} } { fn a() {} } }`, so we
    // generate unique characters from the node id. For now
    // hopefully 3 characters is enough to avoid collisions.
    const EXTRA_CHARS: &'static str =
        "abcdefghijklmnopqrstuvwxyz\
         ABCDEFGHIJKLMNOPQRSTUVWXYZ\
         0123456789";
    let id = id as usize;
    let extra1 = id % EXTRA_CHARS.len();
    let id = id / EXTRA_CHARS.len();
    let extra2 = id % EXTRA_CHARS.len();
    let id = id / EXTRA_CHARS.len();
    let extra3 = id % EXTRA_CHARS.len();
    hash.push(EXTRA_CHARS.as_bytes()[extra1] as char);
    hash.push(EXTRA_CHARS.as_bytes()[extra2] as char);
    hash.push(EXTRA_CHARS.as_bytes()[extra3] as char);

    exported_name(path, &hash[..])
}

pub fn mangle_internal_name_by_type_and_seq<'a, 'tcx>(ccx: &CrateContext<'a, 'tcx>,
                                                      t: Ty<'tcx>,
                                                      name: &str) -> String {
    let path = [token::intern(&t.to_string()).as_str(), gensym_name(name).as_str()];
    let hash = get_symbol_hash(ccx, t);
    mangle(path.iter().cloned(), Some(&hash[..]))
}

pub fn mangle_internal_name_by_path_and_seq(path: DefPath, flav: &str) -> String {
    let names =
        path.into_iter()
            .map(|e| e.data.as_interned_str())
            .chain(once(gensym_name(flav).as_str())); // append unique version of "flav"
    mangle(names, None)
}

// The third parameter is for an extra path to add to PATH for MSVC
// cross linkers for host toolchain DLL dependencies
pub fn get_linker(sess: &Session) -> (String, Command, Option<PathBuf>) {
    if let Some(ref linker) = sess.opts.cg.linker {
        (linker.clone(), Command::new(linker), None)
    } else if sess.target.target.options.is_like_msvc {
        let (cmd, host) = msvc::link_exe_cmd(sess);
        ("link.exe".to_string(), cmd, host)
    } else {
        (sess.target.target.options.linker.clone(),
         Command::new(&sess.target.target.options.linker), None)
    }
}

pub fn get_ar_prog(sess: &Session) -> String {
    sess.opts.cg.ar.clone().unwrap_or_else(|| {
        sess.target.target.options.ar.clone()
    })
}

fn command_path(sess: &Session, extra: Option<PathBuf>) -> OsString {
    // The compiler's sysroot often has some bundled tools, so add it to the
    // PATH for the child.
    let mut new_path = sess.host_filesearch(PathKind::All)
                           .get_tools_search_paths();
    if let Some(path) = env::var_os("PATH") {
        new_path.extend(env::split_paths(&path));
    }
    new_path.extend(extra);
    env::join_paths(new_path).unwrap()
}

pub fn remove(sess: &Session, path: &Path) {
    match fs::remove_file(path) {
        Ok(..) => {}
        Err(e) => {
            sess.err(&format!("failed to remove {}: {}",
                             path.display(),
                             e));
        }
    }
}

/// Perform the linkage portion of the compilation phase. This will generate all
/// of the requested outputs for this compilation session.
pub fn link_binary(sess: &Session,
                   trans: &CrateTranslation,
                   outputs: &OutputFilenames,
                   crate_name: &str) -> Vec<PathBuf> {
    let mut out_filenames = Vec::new();
    for &crate_type in sess.crate_types.borrow().iter() {
        if invalid_output_for_target(sess, crate_type) {
            sess.bug(&format!("invalid output type `{:?}` for target os `{}`",
                             crate_type, sess.opts.target_triple));
        }
        let out_file = link_binary_output(sess, trans, crate_type, outputs,
                                          crate_name);
        out_filenames.push(out_file);
    }

    // Remove the temporary object file and metadata if we aren't saving temps
    if !sess.opts.cg.save_temps {
        for obj in object_filenames(sess, outputs) {
            remove(sess, &obj);
        }
        remove(sess, &outputs.with_extension("metadata.o"));
    }

    out_filenames
}


/// Returns default crate type for target
///
/// Default crate type is used when crate type isn't provided neither
/// through cmd line arguments nor through crate attributes
///
/// It is CrateTypeExecutable for all platforms but iOS as there is no
/// way to run iOS binaries anyway without jailbreaking and
/// interaction with Rust code through static library is the only
/// option for now
pub fn default_output_for_target(sess: &Session) -> config::CrateType {
    if !sess.target.target.options.executables {
        config::CrateTypeStaticlib
    } else {
        config::CrateTypeExecutable
    }
}

/// Checks if target supports crate_type as output
pub fn invalid_output_for_target(sess: &Session,
                                 crate_type: config::CrateType) -> bool {
    match (sess.target.target.options.dynamic_linking,
           sess.target.target.options.executables, crate_type) {
        (false, _, config::CrateTypeDylib) => true,
        (_, false, config::CrateTypeExecutable) => true,
        _ => false
    }
}

fn is_writeable(p: &Path) -> bool {
    match p.metadata() {
        Err(..) => true,
        Ok(m) => !m.permissions().readonly()
    }
}

pub fn filename_for_input(sess: &Session,
                          crate_type: config::CrateType,
                          crate_name: &str,
                          outputs: &OutputFilenames) -> PathBuf {
    let libname = format!("{}{}", crate_name, sess.opts.cg.extra_filename);
    match crate_type {
        config::CrateTypeRlib => {
            outputs.out_directory.join(&format!("lib{}.rlib", libname))
        }
        config::CrateTypeDylib => {
            let (prefix, suffix) = (&sess.target.target.options.dll_prefix,
                                    &sess.target.target.options.dll_suffix);
            outputs.out_directory.join(&format!("{}{}{}", prefix, libname,
                                                suffix))
        }
        config::CrateTypeStaticlib => {
            let (prefix, suffix) = (&sess.target.target.options.staticlib_prefix,
                                    &sess.target.target.options.staticlib_suffix);
            outputs.out_directory.join(&format!("{}{}{}", prefix, libname,
                                                suffix))
        }
        config::CrateTypeExecutable => {
            let suffix = &sess.target.target.options.exe_suffix;
            let out_filename = outputs.path(OutputType::Exe);
            if suffix.is_empty() {
                out_filename.to_path_buf()
            } else {
                out_filename.with_extension(&suffix[1..])
            }
        }
    }
}

pub fn each_linked_rlib(sess: &Session,
                        f: &mut FnMut(ast::CrateNum, &Path)) {
    let crates = sess.cstore.used_crates(LinkagePreference::RequireStatic).into_iter();
    let fmts = sess.dependency_formats.borrow();
    let fmts = fmts.get(&config::CrateTypeExecutable).or_else(|| {
        fmts.get(&config::CrateTypeStaticlib)
    }).unwrap_or_else(|| {
        sess.bug("could not find formats for rlibs")
    });
    for (cnum, path) in crates {
        match fmts[cnum as usize - 1] {
            Linkage::NotLinked | Linkage::IncludedFromDylib => continue,
            _ => {}
        }
        let name = sess.cstore.crate_name(cnum).clone();
        let path = match path {
            Some(p) => p,
            None => {
                sess.fatal(&format!("could not find rlib for: `{}`", name));
            }
        };
        f(cnum, &path);
    }
}

fn link_binary_output(sess: &Session,
                      trans: &CrateTranslation,
                      crate_type: config::CrateType,
                      outputs: &OutputFilenames,
                      crate_name: &str) -> PathBuf {
    let objects = object_filenames(sess, outputs);
    let default_filename = filename_for_input(sess, crate_type, crate_name,
                                              outputs);
    let out_filename = outputs.outputs.get(&OutputType::Exe)
                              .and_then(|s| s.to_owned())
                              .or_else(|| outputs.single_output_file.clone())
                              .unwrap_or(default_filename);

    // Make sure files are writeable.  Mac, FreeBSD, and Windows system linkers
    // check this already -- however, the Linux linker will happily overwrite a
    // read-only file.  We should be consistent.
    for file in objects.iter().chain(Some(&out_filename)) {
        if !is_writeable(file) {
            sess.fatal(&format!("output file {} is not writeable -- check its \
                                permissions", file.display()));
        }
    }

    let tmpdir = match TempDir::new("rustc") {
        Ok(tmpdir) => tmpdir,
        Err(err) => sess.fatal(&format!("couldn't create a temp dir: {}", err)),
    };

    match crate_type {
        config::CrateTypeRlib => {
            link_rlib(sess, Some(trans), &objects, &out_filename,
                      tmpdir.path()).build();
        }
        config::CrateTypeStaticlib => {
            link_staticlib(sess, &objects, &out_filename, tmpdir.path());
        }
        config::CrateTypeExecutable => {
            link_natively(sess, false, &objects, &out_filename, trans, outputs,
                          tmpdir.path());
        }
        config::CrateTypeDylib => {
            link_natively(sess, true, &objects, &out_filename, trans, outputs,
                          tmpdir.path());
        }
    }

    out_filename
}

fn object_filenames(sess: &Session, outputs: &OutputFilenames) -> Vec<PathBuf> {
    (0..sess.opts.cg.codegen_units).map(|i| {
        let ext = format!("{}.o", i);
        outputs.temp_path(OutputType::Object).with_extension(&ext)
    }).collect()
}

fn archive_search_paths(sess: &Session) -> Vec<PathBuf> {
    let mut search = Vec::new();
    sess.target_filesearch(PathKind::Native).for_each_lib_search_path(|path, _| {
        search.push(path.to_path_buf());
    });
    return search;
}

fn archive_config<'a>(sess: &'a Session,
                      output: &Path,
                      input: Option<&Path>) -> ArchiveConfig<'a> {
    ArchiveConfig {
        sess: sess,
        dst: output.to_path_buf(),
        src: input.map(|p| p.to_path_buf()),
        lib_search_paths: archive_search_paths(sess),
        ar_prog: get_ar_prog(sess),
        command_path: command_path(sess, None),
    }
}

// Create an 'rlib'
//
// An rlib in its current incarnation is essentially a renamed .a file. The
// rlib primarily contains the object file of the crate, but it also contains
// all of the object files from native libraries. This is done by unzipping
// native libraries and inserting all of the contents into this archive.
fn link_rlib<'a>(sess: &'a Session,
                 trans: Option<&CrateTranslation>, // None == no metadata/bytecode
                 objects: &[PathBuf],
                 out_filename: &Path,
                 tmpdir: &Path) -> ArchiveBuilder<'a> {
    info!("preparing rlib from {:?} to {:?}", objects, out_filename);
    let mut ab = ArchiveBuilder::new(archive_config(sess, out_filename, None));
    for obj in objects {
        ab.add_file(obj);
    }

    for (l, kind) in sess.cstore.used_libraries() {
        match kind {
            NativeLibraryKind::NativeStatic => ab.add_native_library(&l),
            NativeLibraryKind::NativeFramework |
            NativeLibraryKind::NativeUnknown => {}
        }
    }

    // After adding all files to the archive, we need to update the
    // symbol table of the archive.
    ab.update_symbols();

    // For OSX/iOS, we must be careful to update symbols only when adding
    // object files.  We're about to start adding non-object files, so run
    // `ar` now to process the object files.
    if sess.target.target.options.is_like_osx && !ab.using_llvm() {
        ab.build();
    }

    // Note that it is important that we add all of our non-object "magical
    // files" *after* all of the object files in the archive. The reason for
    // this is as follows:
    //
    // * When performing LTO, this archive will be modified to remove
    //   objects from above. The reason for this is described below.
    //
    // * When the system linker looks at an archive, it will attempt to
    //   determine the architecture of the archive in order to see whether its
    //   linkable.
    //
    //   The algorithm for this detection is: iterate over the files in the
    //   archive. Skip magical SYMDEF names. Interpret the first file as an
    //   object file. Read architecture from the object file.
    //
    // * As one can probably see, if "metadata" and "foo.bc" were placed
    //   before all of the objects, then the architecture of this archive would
    //   not be correctly inferred once 'foo.o' is removed.
    //
    // Basically, all this means is that this code should not move above the
    // code above.
    match trans {
        Some(trans) => {
            // Instead of putting the metadata in an object file section, rlibs
            // contain the metadata in a separate file. We use a temp directory
            // here so concurrent builds in the same directory don't try to use
            // the same filename for metadata (stomping over one another)
            let metadata = tmpdir.join(sess.cstore.metadata_filename());
            match fs::File::create(&metadata).and_then(|mut f| {
                f.write_all(&trans.metadata)
            }) {
                Ok(..) => {}
                Err(e) => {
                    sess.fatal(&format!("failed to write {}: {}",
                                        metadata.display(), e));
                }
            }
            ab.add_file(&metadata);

            // For LTO purposes, the bytecode of this library is also inserted
            // into the archive.  If codegen_units > 1, we insert each of the
            // bitcode files.
            for obj in objects {
                // Note that we make sure that the bytecode filename in the
                // archive is never exactly 16 bytes long by adding a 16 byte
                // extension to it. This is to work around a bug in LLDB that
                // would cause it to crash if the name of a file in an archive
                // was exactly 16 bytes.
                let bc_filename = obj.with_extension("bc");
                let bc_deflated_filename = tmpdir.join({
                    obj.with_extension("bytecode.deflate").file_name().unwrap()
                });

                let mut bc_data = Vec::new();
                match fs::File::open(&bc_filename).and_then(|mut f| {
                    f.read_to_end(&mut bc_data)
                }) {
                    Ok(..) => {}
                    Err(e) => sess.fatal(&format!("failed to read bytecode: {}",
                                                 e))
                }

                let bc_data_deflated = flate::deflate_bytes(&bc_data[..]);

                let mut bc_file_deflated = match fs::File::create(&bc_deflated_filename) {
                    Ok(file) => file,
                    Err(e) => {
                        sess.fatal(&format!("failed to create compressed \
                                             bytecode file: {}", e))
                    }
                };

                match write_rlib_bytecode_object_v1(&mut bc_file_deflated,
                                                    &bc_data_deflated) {
                    Ok(()) => {}
                    Err(e) => {
                        sess.fatal(&format!("failed to write compressed \
                                             bytecode: {}", e));
                    }
                };

                ab.add_file(&bc_deflated_filename);

                // See the bottom of back::write::run_passes for an explanation
                // of when we do and don't keep .0.bc files around.
                let user_wants_numbered_bitcode =
                        sess.opts.output_types.contains_key(&OutputType::Bitcode) &&
                        sess.opts.cg.codegen_units > 1;
                if !sess.opts.cg.save_temps && !user_wants_numbered_bitcode {
                    remove(sess, &bc_filename);
                }
            }

            // After adding all files to the archive, we need to update the
            // symbol table of the archive. This currently dies on OSX (see
            // #11162), and isn't necessary there anyway
            if !sess.target.target.options.is_like_osx || ab.using_llvm() {
                ab.update_symbols();
            }
        }

        None => {}
    }

    ab
}

fn write_rlib_bytecode_object_v1(writer: &mut Write,
                                 bc_data_deflated: &[u8]) -> io::Result<()> {
    let bc_data_deflated_size: u64 = bc_data_deflated.len() as u64;

    try!(writer.write_all(RLIB_BYTECODE_OBJECT_MAGIC));
    try!(writer.write_all(&[1, 0, 0, 0]));
    try!(writer.write_all(&[
        (bc_data_deflated_size >>  0) as u8,
        (bc_data_deflated_size >>  8) as u8,
        (bc_data_deflated_size >> 16) as u8,
        (bc_data_deflated_size >> 24) as u8,
        (bc_data_deflated_size >> 32) as u8,
        (bc_data_deflated_size >> 40) as u8,
        (bc_data_deflated_size >> 48) as u8,
        (bc_data_deflated_size >> 56) as u8,
    ]));
    try!(writer.write_all(&bc_data_deflated));

    let number_of_bytes_written_so_far =
        RLIB_BYTECODE_OBJECT_MAGIC.len() +                // magic id
        mem::size_of_val(&RLIB_BYTECODE_OBJECT_VERSION) + // version
        mem::size_of_val(&bc_data_deflated_size) +        // data size field
        bc_data_deflated_size as usize;                    // actual data

    // If the number of bytes written to the object so far is odd, add a
    // padding byte to make it even. This works around a crash bug in LLDB
    // (see issue #15950)
    if number_of_bytes_written_so_far % 2 == 1 {
        try!(writer.write_all(&[0]));
    }

    return Ok(());
}

// Create a static archive
//
// This is essentially the same thing as an rlib, but it also involves adding
// all of the upstream crates' objects into the archive. This will slurp in
// all of the native libraries of upstream dependencies as well.
//
// Additionally, there's no way for us to link dynamic libraries, so we warn
// about all dynamic library dependencies that they're not linked in.
//
// There's no need to include metadata in a static archive, so ensure to not
// link in the metadata object file (and also don't prepare the archive with a
// metadata file).
fn link_staticlib(sess: &Session, objects: &[PathBuf], out_filename: &Path,
                  tempdir: &Path) {
    let mut ab = link_rlib(sess, None, objects, out_filename, tempdir);
    if sess.target.target.options.is_like_osx && !ab.using_llvm() {
        ab.build();
    }
    if !sess.target.target.options.no_compiler_rt {
        ab.add_native_library("compiler-rt");
    }

    let mut all_native_libs = vec![];

    each_linked_rlib(sess, &mut |cnum, path| {
        let name = sess.cstore.crate_name(cnum);
        ab.add_rlib(path, &name, sess.lto()).unwrap();

        let native_libs = sess.cstore.native_libraries(cnum);
        all_native_libs.extend(native_libs);
    });

    ab.update_symbols();
    ab.build();

    if !all_native_libs.is_empty() {
        sess.note_without_error("link against the following native artifacts when linking against \
                                 this static library");
        sess.note_without_error("the order and any duplication can be significant on some \
                                 platforms, and so may need to be preserved");
    }

    for &(kind, ref lib) in &all_native_libs {
        let name = match kind {
            NativeLibraryKind::NativeStatic => "static library",
            NativeLibraryKind::NativeUnknown => "library",
            NativeLibraryKind::NativeFramework => "framework",
        };
        sess.note_without_error(&format!("{}: {}", name, *lib));
    }
}

// Create a dynamic library or executable
//
// This will invoke the system linker/cc to create the resulting file. This
// links to all upstream files as well.
fn link_natively(sess: &Session, dylib: bool,
                 objects: &[PathBuf], out_filename: &Path,
                 trans: &CrateTranslation,
                 outputs: &OutputFilenames,
                 tmpdir: &Path) {
    info!("preparing dylib? ({}) from {:?} to {:?}", dylib, objects,
          out_filename);

    // The invocations of cc share some flags across platforms
    let (pname, mut cmd, extra) = get_linker(sess);
    cmd.env("PATH", command_path(sess, extra));

    let root = sess.target_filesearch(PathKind::Native).get_lib_path();
    cmd.args(&sess.target.target.options.pre_link_args);

    let pre_link_objects = if dylib {
        &sess.target.target.options.pre_link_objects_dll
    } else {
        &sess.target.target.options.pre_link_objects_exe
    };
    for obj in pre_link_objects {
        cmd.arg(root.join(obj));
    }

    {
        let mut linker = if sess.target.target.options.is_like_msvc {
            Box::new(MsvcLinker { cmd: &mut cmd, sess: &sess }) as Box<Linker>
        } else {
            Box::new(GnuLinker { cmd: &mut cmd, sess: &sess }) as Box<Linker>
        };
        link_args(&mut *linker, sess, dylib, tmpdir,
                  objects, out_filename, trans, outputs);
        if !sess.target.target.options.no_compiler_rt {
            linker.link_staticlib("compiler-rt");
        }
    }
    cmd.args(&sess.target.target.options.late_link_args);
    for obj in &sess.target.target.options.post_link_objects {
        cmd.arg(root.join(obj));
    }
    cmd.args(&sess.target.target.options.post_link_args);

    if sess.opts.debugging_opts.print_link_args {
        println!("{:?}", &cmd);
    }

    // May have not found libraries in the right formats.
    sess.abort_if_errors();

    // Invoke the system linker
    info!("{:?}", &cmd);
    let prog = time(sess.time_passes(), "running linker", || cmd.output());
    match prog {
        Ok(prog) => {
            fn escape_string(s: &[u8]) -> String {
                str::from_utf8(s).map(|s| s.to_owned())
                    .unwrap_or_else(|_| {
                        let mut x = "Non-UTF-8 output: ".to_string();
                        x.extend(s.iter()
                                 .flat_map(|&b| ascii::escape_default(b))
                                 .map(|b| char::from_u32(b as u32).unwrap()));
                        x
                    })
            }
            if !prog.status.success() {
                let mut output = prog.stderr.clone();
                output.extend_from_slice(&prog.stdout);
                sess.struct_err(&format!("linking with `{}` failed: {}",
                                         pname,
                                         prog.status))
                    .note(&format!("{:?}", &cmd))
                    .note(&*escape_string(&output[..]))
                    .emit();
                sess.abort_if_errors();
            }
            info!("linker stderr:\n{}", escape_string(&prog.stderr[..]));
            info!("linker stdout:\n{}", escape_string(&prog.stdout[..]));
        },
        Err(e) => {
            sess.fatal(&format!("could not exec the linker `{}`: {}", pname, e));
        }
    }


    // On OSX, debuggers need this utility to get run to do some munging of
    // the symbols
    if sess.target.target.options.is_like_osx && sess.opts.debuginfo != NoDebugInfo {
        match Command::new("dsymutil").arg(out_filename).output() {
            Ok(..) => {}
            Err(e) => sess.fatal(&format!("failed to run dsymutil: {}", e)),
        }
    }
}

fn link_args(cmd: &mut Linker,
             sess: &Session,
             dylib: bool,
             tmpdir: &Path,
             objects: &[PathBuf],
             out_filename: &Path,
             trans: &CrateTranslation,
             outputs: &OutputFilenames) {

    // The default library location, we need this to find the runtime.
    // The location of crates will be determined as needed.
    let lib_path = sess.target_filesearch(PathKind::All).get_lib_path();

    // target descriptor
    let t = &sess.target.target;

    cmd.include_path(&fix_windows_verbatim_for_gcc(&lib_path));
    for obj in objects {
        cmd.add_object(obj);
    }
    cmd.output_filename(out_filename);

    // If we're building a dynamic library then some platforms need to make sure
    // that all symbols are exported correctly from the dynamic library.
    if dylib {
        cmd.export_symbols(sess, trans, tmpdir);
    }

    // When linking a dynamic library, we put the metadata into a section of the
    // executable. This metadata is in a separate object file from the main
    // object file, so we link that in here.
    if dylib {
        cmd.add_object(&outputs.with_extension("metadata.o"));
    }

    // Try to strip as much out of the generated object by removing unused
    // sections if possible. See more comments in linker.rs
    cmd.gc_sections(dylib);

    let used_link_args = sess.cstore.used_link_args();

    if !dylib && t.options.position_independent_executables {
        let empty_vec = Vec::new();
        let empty_str = String::new();
        let args = sess.opts.cg.link_args.as_ref().unwrap_or(&empty_vec);
        let mut args = args.iter().chain(used_link_args.iter());
        let relocation_model = sess.opts.cg.relocation_model.as_ref()
                                   .unwrap_or(&empty_str);
        if (t.options.relocation_model == "pic" || *relocation_model == "pic")
            && !args.any(|x| *x == "-static") {
            cmd.position_independent_executable();
        }
    }

    // Pass optimization flags down to the linker.
    cmd.optimize();

    // Pass debuginfo flags down to the linker.
    cmd.debuginfo();

    // We want to prevent the compiler from accidentally leaking in any system
    // libraries, so we explicitly ask gcc to not link to any libraries by
    // default. Note that this does not happen for windows because windows pulls
    // in some large number of libraries and I couldn't quite figure out which
    // subset we wanted.
    if t.options.no_default_libraries {
        cmd.no_default_libraries();
    }

    // Take careful note of the ordering of the arguments we pass to the linker
    // here. Linkers will assume that things on the left depend on things to the
    // right. Things on the right cannot depend on things on the left. This is
    // all formally implemented in terms of resolving symbols (libs on the right
    // resolve unknown symbols of libs on the left, but not vice versa).
    //
    // For this reason, we have organized the arguments we pass to the linker as
    // such:
    //
    //  1. The local object that LLVM just generated
    //  2. Local native libraries
    //  3. Upstream rust libraries
    //  4. Upstream native libraries
    //
    // The rationale behind this ordering is that those items lower down in the
    // list can't depend on items higher up in the list. For example nothing can
    // depend on what we just generated (e.g. that'd be a circular dependency).
    // Upstream rust libraries are not allowed to depend on our local native
    // libraries as that would violate the structure of the DAG, in that
    // scenario they are required to link to them as well in a shared fashion.
    //
    // Note that upstream rust libraries may contain native dependencies as
    // well, but they also can't depend on what we just started to add to the
    // link line. And finally upstream native libraries can't depend on anything
    // in this DAG so far because they're only dylibs and dylibs can only depend
    // on other dylibs (e.g. other native deps).
    add_local_native_libraries(cmd, sess);
    add_upstream_rust_crates(cmd, sess, dylib, tmpdir);
    add_upstream_native_libraries(cmd, sess);

    // # Telling the linker what we're doing

    if dylib {
        cmd.build_dylib(out_filename);
    }

    // FIXME (#2397): At some point we want to rpath our guesses as to
    // where extern libraries might live, based on the
    // addl_lib_search_paths
    if sess.opts.cg.rpath {
        let sysroot = sess.sysroot();
        let target_triple = &sess.opts.target_triple;
        let mut get_install_prefix_lib_path = || {
            let install_prefix = option_env!("CFG_PREFIX").expect("CFG_PREFIX");
            let tlib = filesearch::relative_target_lib_path(sysroot, target_triple);
            let mut path = PathBuf::from(install_prefix);
            path.push(&tlib);

            path
        };
        let mut rpath_config = RPathConfig {
            used_crates: sess.cstore.used_crates(LinkagePreference::RequireDynamic),
            out_filename: out_filename.to_path_buf(),
            has_rpath: sess.target.target.options.has_rpath,
            is_like_osx: sess.target.target.options.is_like_osx,
            linker_is_gnu: sess.target.target.options.linker_is_gnu,
            get_install_prefix_lib_path: &mut get_install_prefix_lib_path,
        };
        cmd.args(&rpath::get_rpath_flags(&mut rpath_config));
    }

    // Finally add all the linker arguments provided on the command line along
    // with any #[link_args] attributes found inside the crate
    if let Some(ref args) = sess.opts.cg.link_args {
        cmd.args(args);
    }
    cmd.args(&used_link_args);
}

// # Native library linking
//
// User-supplied library search paths (-L on the command line). These are
// the same paths used to find Rust crates, so some of them may have been
// added already by the previous crate linking code. This only allows them
// to be found at compile time so it is still entirely up to outside
// forces to make sure that library can be found at runtime.
//
// Also note that the native libraries linked here are only the ones located
// in the current crate. Upstream crates with native library dependencies
// may have their native library pulled in above.
fn add_local_native_libraries(cmd: &mut Linker, sess: &Session) {
    sess.target_filesearch(PathKind::All).for_each_lib_search_path(|path, k| {
        match k {
            PathKind::Framework => { cmd.framework_path(path); }
            _ => { cmd.include_path(&fix_windows_verbatim_for_gcc(path)); }
        }
    });

    let libs = sess.cstore.used_libraries();

    let staticlibs = libs.iter().filter_map(|&(ref l, kind)| {
        if kind == NativeLibraryKind::NativeStatic {Some(l)} else {None}
    });
    let others = libs.iter().filter(|&&(_, kind)| {
        kind != NativeLibraryKind::NativeStatic
    });

    // 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.
    cmd.hint_static();

    let search_path = archive_search_paths(sess);
    for l in staticlibs {
        // 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.
        cmd.link_whole_staticlib(l, &search_path);
    }

    cmd.hint_dynamic();

    for &(ref l, kind) in others {
        match kind {
            NativeLibraryKind::NativeUnknown => cmd.link_dylib(l),
            NativeLibraryKind::NativeFramework => cmd.link_framework(l),
            NativeLibraryKind::NativeStatic => unreachable!(),
        }
    }
}

// # Rust Crate linking
//
// Rust crates are not considered at all when creating an rlib output. All
// dependencies will be linked when producing the final output (instead of
// the intermediate rlib version)
fn add_upstream_rust_crates(cmd: &mut Linker, sess: &Session,
                            dylib: bool, tmpdir: &Path) {
    // All of the heavy lifting has previously been accomplished by the
    // dependency_format module of the compiler. This is just crawling the
    // output of that module, adding crates as necessary.
    //
    // Linking to a rlib involves just passing it to the linker (the linker
    // will slurp up the object files inside), and linking to a dynamic library
    // involves just passing the right -l flag.

    let formats = sess.dependency_formats.borrow();
    let data = if dylib {
        formats.get(&config::CrateTypeDylib).unwrap()
    } else {
        formats.get(&config::CrateTypeExecutable).unwrap()
    };

    // Invoke get_used_crates to ensure that we get a topological sorting of
    // crates.
    let deps = sess.cstore.used_crates(LinkagePreference::RequireDynamic);

    for &(cnum, _) in &deps {
        // We may not pass all crates through to the linker. Some crates may
        // appear statically in an existing dylib, meaning we'll pick up all the
        // symbols from the dylib.
        let src = sess.cstore.used_crate_source(cnum);
        match data[cnum as usize - 1] {
            Linkage::NotLinked |
            Linkage::IncludedFromDylib => {}
            Linkage::Static => {
                add_static_crate(cmd, sess, tmpdir, dylib, &src.rlib.unwrap().0)
            }
            Linkage::Dynamic => {
                add_dynamic_crate(cmd, sess, &src.dylib.unwrap().0)
            }
        }
    }

    // Converts a library file-stem into a cc -l argument
    fn unlib<'a>(config: &config::Config, stem: &'a str) -> &'a str {
        if stem.starts_with("lib") && !config.target.options.is_like_windows {
            &stem[3..]
        } else {
            stem
        }
    }

    // Adds the static "rlib" versions of all crates to the command line.
    // There's a bit of magic which happens here specifically related to LTO and
    // dynamic libraries. Specifically:
    //
    // * For LTO, we remove upstream object files.
    // * For dylibs we remove metadata and bytecode from upstream rlibs
    //
    // When performing LTO, all of the bytecode from the upstream libraries has
    // already been included in our object file output. As a result we need to
    // remove the object files in the upstream libraries so the linker doesn't
    // try to include them twice (or whine about duplicate symbols). We must
    // continue to include the rest of the rlib, however, as it may contain
    // static native libraries which must be linked in.
    //
    // When making a dynamic library, linkers by default don't include any
    // object files in an archive if they're not necessary to resolve the link.
    // We basically want to convert the archive (rlib) to a dylib, though, so we
    // *do* want everything included in the output, regardless of whether the
    // linker thinks it's needed or not. As a result we must use the
    // --whole-archive option (or the platform equivalent). When using this
    // option the linker will fail if there are non-objects in the archive (such
    // as our own metadata and/or bytecode). All in all, for rlibs to be
    // entirely included in dylibs, we need to remove all non-object files.
    //
    // Note, however, that if we're not doing LTO or we're not producing a dylib
    // (aka we're making an executable), we can just pass the rlib blindly to
    // the linker (fast) because it's fine if it's not actually included as
    // we're at the end of the dependency chain.
    fn add_static_crate(cmd: &mut Linker, sess: &Session, tmpdir: &Path,
                        dylib: bool, cratepath: &Path) {
        if !sess.lto() && !dylib {
            cmd.link_rlib(&fix_windows_verbatim_for_gcc(cratepath));
            return
        }

        let dst = tmpdir.join(cratepath.file_name().unwrap());
        let name = cratepath.file_name().unwrap().to_str().unwrap();
        let name = &name[3..name.len() - 5]; // chop off lib/.rlib

        time(sess.time_passes(), &format!("altering {}.rlib", name), || {
            let cfg = archive_config(sess, &dst, Some(cratepath));
            let mut archive = ArchiveBuilder::new(cfg);
            archive.remove_file(sess.cstore.metadata_filename());
            archive.update_symbols();

            let mut any_objects = false;
            for f in archive.src_files() {
                if f.ends_with("bytecode.deflate") {
                    archive.remove_file(&f);
                    continue
                }
                let canonical = f.replace("-", "_");
                let canonical_name = name.replace("-", "_");
                if sess.lto() && canonical.starts_with(&canonical_name) &&
                   canonical.ends_with(".o") {
                    let num = &f[name.len()..f.len() - 2];
                    if num.len() > 0 && num[1..].parse::<u32>().is_ok() {
                        archive.remove_file(&f);
                        continue
                    }
                }
                any_objects = true;
            }

            if any_objects {
                archive.build();
                cmd.link_whole_rlib(&fix_windows_verbatim_for_gcc(&dst));
            }
        });
    }

    // Same thing as above, but for dynamic crates instead of static crates.
    fn add_dynamic_crate(cmd: &mut Linker, sess: &Session, cratepath: &Path) {
        // If we're performing LTO, then it should have been previously required
        // that all upstream rust dependencies were available in an rlib format.
        assert!(!sess.lto());

        // Just need to tell the linker about where the library lives and
        // what its name is
        let parent = cratepath.parent();
        if let Some(dir) = parent {
            cmd.include_path(&fix_windows_verbatim_for_gcc(dir));
        }
        let filestem = cratepath.file_stem().unwrap().to_str().unwrap();
        cmd.link_rust_dylib(&unlib(&sess.target, filestem),
                            parent.unwrap_or(Path::new("")));
    }
}

// Link in all of our upstream crates' native dependencies. Remember that
// all of these upstream native dependencies are all non-static
// dependencies. We've got two cases then:
//
// 1. The upstream crate is an rlib. In this case we *must* link in the
// native dependency because the rlib is just an archive.
//
// 2. The upstream crate is a dylib. In order to use the dylib, we have to
// have the dependency present on the system somewhere. Thus, we don't
// gain a whole lot from not linking in the dynamic dependency to this
// crate as well.
//
// The use case for this is a little subtle. In theory the native
// dependencies of a crate are purely an implementation detail of the crate
// itself, but the problem arises with generic and inlined functions. If a
// generic function calls a native function, then the generic function must
// be instantiated in the target crate, meaning that the native symbol must
// also be resolved in the target crate.
fn add_upstream_native_libraries(cmd: &mut Linker, sess: &Session) {
    // Be sure to use a topological sorting of crates because there may be
    // interdependencies between native libraries. When passing -nodefaultlibs,
    // for example, almost all native libraries depend on libc, so we have to
    // make sure that's all the way at the right (liblibc is near the base of
    // the dependency chain).
    //
    // This passes RequireStatic, but the actual requirement doesn't matter,
    // we're just getting an ordering of crate numbers, we're not worried about
    // the paths.
    let crates = sess.cstore.used_crates(LinkagePreference::RequireStatic);
    for (cnum, _) in crates {
        let libs = sess.cstore.native_libraries(cnum);
        for &(kind, ref lib) in &libs {
            match kind {
                NativeLibraryKind::NativeUnknown => cmd.link_dylib(lib),
                NativeLibraryKind::NativeFramework => cmd.link_framework(lib),
                NativeLibraryKind::NativeStatic => {
                    sess.bug("statics shouldn't be propagated");
                }
            }
        }
    }
}