Commits on Mar 4, 2018

1. rust: Import LLD for linking wasm objects …

   This commit imports the LLD project from LLVM to serve as the default linker for
   the `wasm32-unknown-unknown` target. The `binaryen` submoule is consequently
   removed along with "binaryen linker" support in rustc.
   
   Moving to LLD brings with it a number of benefits for wasm code:
   
   * LLD is itself an actual linker, so there's no need to compile all wasm code
     with LTO any more. As a result builds should be *much* speedier as LTO is no
     longer forcibly enabled for all builds of the wasm target.
   * LLD is quickly becoming an "official solution" for linking wasm code together.
     This, I believe at least, is intended to be the main supported linker for
     native code and wasm moving forward. Picking up support early on should help
     ensure that we can help LLD identify bugs and otherwise prove that it works
     great for all our use cases!
   * Improvements to the wasm toolchain are currently primarily focused around LLVM
     and LLD (from what I can tell at least), so it's in general much better to be
     on this bandwagon for bugfixes and new features.
   * Historical "hacks" like `wasm-gc` will soon no longer be necessary, LLD
     will [natively implement][gc] `--gc-sections` (better than `wasm-gc`!) which
     means a postprocessor is no longer needed to show off Rust's "small wasm
     binary size".
   
   LLD is added in a pretty standard way to rustc right now. A new rustbuild target
   was defined for building LLD, and this is executed when a compiler's sysroot is
   being assembled. LLD is compiled against the LLVM that we've got in tree, which
   means we're currently on the `release_60` branch, but this may get upgraded in
   the near future!
   
   LLD is placed into rustc's sysroot in a `bin` directory. This is similar to
   where `gcc.exe` can be found on Windows. This directory is automatically added
   to `PATH` whenever rustc executes the linker, allowing us to define a `WasmLd`
   linker which implements the interface that `wasm-ld`, LLD's frontend, expects.
   
   Like Emscripten the LLD target is currently only enabled for Tier 1 platforms,
   notably OSX/Windows/Linux, and will need to be installed manually for compiling
   to wasm on other platforms. LLD is by default turned off in rustbuild, and
   requires a `config.toml` option to be enabled to turn it on.
   
   Finally the unstable `#![wasm_import_memory]` attribute was also removed as LLD
   has a native option for controlling this.
   
   [gc]: https://reviews.llvm.org/D42511

   

   alexcrichton committed Mar 4, 2018
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2. rustc: Tweak default linker selection …

   This commit refactors how the path to the linker that we're going to invoke is
   selected. Previously all targets listed *both* a `LinkerFlavor` and a `linker`
   (path) option, but this meant that whenever you changed one you had to change
   the other. The purpose of this commit is to avoid coupling these where possible.
   
   Target specifications now only unconditionally define the *flavor* of the linker
   that they're using by default. If not otherwise specified each flavor now
   implies a particular default linker to run. As a result, this means that if
   you'd like to test out `ld` for example you should be able to do:
   
       rustc -Z linker-flavor=ld foo.rs
   
   whereas previously you had to do
   
       rustc -Z linker-flavor=ld -C linker=ld foo.rs
   
   This will hopefully make it a bit easier to tinker around with variants that
   should otherwise be well known to work, for example with LLD, `ld` on OSX, etc.

   

   alexcrichton committed Mar 4, 2018
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