[WIP] internal lld linker #36120

this commit embeds lld, the LLVM linker, into rustc. If the `-Z use-lld` flag is passed to rustc, then rustc will use lld (via a library call), instead of an external linker like cc, to link a Rust executable. This has been tested by building the following minimal program: ``` rust #![feature(lang_items)] #![feature(no_core)] #![feature(start)] #![no_core] #[link(name = "c")] extern {} #[start] fn start(_: isize, _: *const *const u8) -> isize { 0 } #[lang = "copy"] trait Copy {} #[lang = "sized"] trait Sized {} ``` Note that lld doesn't implicitly pass libraries (-lc) or startup object (crt1.o) like cc does, so one has to manually pass these to lld via `-C link-args`: ``` $ rustc -Z print-link-args \ -Z use-lld \ -C link-args='-L /usr/lib/x86_64-linux-gnu /usr/lib/x86_64-linux-gnu/Scrt1.o /usr/lib/x86_64-linux-gnu/crti.o' zero.rs ["lld", "--as-needed", "-z", "noexecstack", "-L", "/root/rust/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/x86_64-unknown-linux-gnu/lib", "zero.0.o", "-o", "zero", "--gc-sections", "--pie", "-L", "/root/rust/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/x86_64-unknown-linux-gnu/lib", "--Bstatic", "--Bdynamic", "-l", "c", "-L", "/usr/lib/x86_64-linux-gnu", "/usr/lib/x86_64-linux-gnu/Scrt1.o", "/usr/lib/x86_64-linux-gnu/crti.o", "-l", "compiler-rt"] ``` Which links correctly! But executing the produced binary yields a segfault :-(. There appears to be a problem when lld mixes Rust code and C code but I haven't investigated further. So was this an exercise in futility? No, not so fast. I did one more test: a Rust program with zero C dependencies, i.e. a Rust "kernel". Basically, I grabbed the "Hello, world" program from Intermezzos chapter 3 [1] and turned it into a Cargo project [2] then proceeded to link it using lld: ``` $ cargo rustc --target x86_64 --release -- -Z print-link-args -Z use-lld ["lld", "-Tlayout.ld", "-L", "/root/rust/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/x86_64/lib", "/root/eighty-six/target/x86_64/release/kernel.0.o", "-o", "/root/eighty-six/target/x86_64/release/kernel", "--gc-sections", "-L", "/root/eighty-six/target/x86_64/release/deps", "-L", "/root/eighty-six/target/release/build/kernel-24c773dcbde95e48/out", "-L", "/root/rust/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/x86_64/lib", "--Bstatic", "--whole-archive", "-l", "boot", "--no-whole-archive", "--Bdynamic", "/root/eighty-six/target/x86_64/release/deps/libcore.rlib"] ``` This linked correctly and after turning the executable into a ISO image, it also worked perfectly under QEMU! ### Extra notes I've only tested ELF executables, not dylibs, MachO or COFF. lld supports ELFs, MachO and COFF formats. So in theory, with an internal lld we could drop the dependency on a external linker pretty much on all the platforms we support. lld is a multi-arch linker so it should be possible to link foreign (e.g. arm or mips) binaries as well but I haven't tested this yet. I actually intended to test this by linking Rust programs for the Cortex-M, as I have experience making C free programs for that target, but lld doesn't support Thumb (yet). Relevant news: "lld now implements almost all of the functionality necessary to function as a system linker for FreeBSD/amd64" -- FreeBSD folk @ LLVM mailing list [3]. The e-mail also mentions that other support for other archs is not quite ready for prime time. [1]: http://intermezzos.github.io/book/booting-up.html [2]: https://github.com/japaric/eighty-six/tree/lld2 [3]: http://lists.llvm.org/pipermail/llvm-dev/2016-August/103998.html

just to test this on the try bots

should fix build on windows-gnu

should hopefully fix linking the rustc_lld dylib on osx

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[WIP] internal lld linker #36120

[WIP] internal lld linker #36120

Commits on Sep 15, 2016