Link with LLD

[Woyten]

This PR uses lld as the proposed linker for libtock. Using lld has a couple of advantages:

• We get rid of arm-none-eabi-ld as an external build tool. lld, in contrast, is shipped with Rust (rust: Import LLD for linking wasm objects rust-lang/rust#48125) and, therefore, does not require any additional installation steps.
• lld supports any target that LLVM supports.
• According to https://lld.llvm.org, lld is super fast.
• The resulting elf file contains a lot of sections that were missing when using arm-none-eabi-ld, such as the .bss section. My guess is that proper relocation will be possible with lld.

Note: You might need to remove $HOME/.xargo before building anything.

[Woyten]

I had to inline those to make lld happy.

[Woyten]

I don't know why but the pre-link-args section stopped having any effect on the linker, so I had to move that part to .cargo/config.

[alevy]

This is awesome, we need this too because it's a prerequisite to dealing with global variables (gnu ld doesn't know how to link LLVM's pic strategies that we need to use).

Will take a look soon, but high level if it works for the example apps I think we should merge and deal with bugs as they arise

[bradjc]

Seems like efl2tbf doesn't work with the new output files. When running blink I now get a panic: tock/tock#817

[Woyten]

We tested LLD on our NRF52DK on three different PCs running Ubuntu Linux.
What OS are you using?
Could you try running the code on an NRF52DK board?
Did you run git submodule update to get the newest xargo version?

[bradjc]

I on Mac, but I don't have a nRF52dk board. I updated the submodule to the correct commit (it has some change in a cargo.lock file but is otherwise the same).

$ cargo --version
cargo 0.26.0-nightly (1d6dfea44 2018-01-26)

I have no idea what linker script it is using, but it doesn't seem to be layout.ld in the repo.

What does the output of tockloader inspect-tab look like for you?

[Woyten]

If your repo is clean except for xargo/Cargo.lock (see japaric/xargo#201) the checkout should be fine.

The intention is that layout.ld of the libtock root is used. Before this PR, the script was referenced in the linker arguments section of thumbv7em-tock-eabi.json. But since LLD is used the linker arguments stopped working. Therefore, I moved them to .cargo/config which works for us.

My outputs:

$ cargo --version
cargo 0.26.0-nightly (1d6dfea44 2018-01-26)

$ rustc --version
rustc 1.26.0-nightly (c9334404f 2018-03-05)

$ tockloader inspect-tab target/blink.tab
Inspecting TABs...
TAB: blink
  build-date: 2018-03-18 16:44:11+00:00
  only-for-boards: 
  tab-version: 1
  supported architectures: cortex-m4
  TBF Header
    package_name          : blink
    version               :        2
    checksum              :            0x6e4c6c2d
    flags                 :        1          0x1
      enabled             : Yes
      sticky              : No
    header_size           :       44         0x2c
    init_fn_offset        :       37         0x25
    minimum_ram_size      :     4096       0x1000
    protected_size        :        0          0x0
    total_size            :     4096       0x1000

[bradjc]

I can't explain this but with a new checkout it works. #thanksrust

[bradjc]

I can't get this to work if I have the repo inside my /Users/bradjc/git folder. So that's cool.

[alevy]

@bradjc is it possible you have some sort of git ignore rule somewhere in your tree?

[bradjc]

Ok don't be dumb and set the RUSTFLAGS variable in your shell.

[Woyten]

@bradjc Can the pr/18 branch be deleted?