[llvm-dev] Adding a new target to 'llvm-lld'

[ORiordan, Martin via llvm-dev]

Thanks Peter,

Fortunately I am only interested in ELF so COFF won't be important to me.  I really appreciate your guidance, and offer of assistance with questions I may have.  I don't think that I will be able to start work on this for another couple of weeks, and this means that I will be using the v6.0.0 release (I don't track head).

All the best,

	MartinO

-----Original Message-----
From: Peter Smith [mailto:peter.smith at linaro.org] 
Sent: Thursday, January 25, 2018 5:55 PM
To: ORiordan, Martin <martin.oriordan at intel.com>
Cc: llvm-dev <llvm-dev at lists.llvm.org>
Subject: Re: [llvm-dev] Adding a new target to 'llvm-lld'

I'm not aware of a written guide either. In 2016 I did a talk on that subject at the LLVM Cauldron
(http://llvm.org/devmtg/2016-09/#schedule) although quite a bit has changed since then so I'm hesitant to recommend it apart from general principles.

I can't speak for the COFF side as I've only worked on the ELF side of LLD; my suggestions on where to start:
- Properties of the target that are common to all ELF linkers, such as how relocation directives work, are defined in sub-classes of TargetInfo. At present each sub-class is implemented in Arch/<Target>.cpp/ such as Arch/ARM.cpp.
- The driver will instantiate a Target based on the ELF machine type.

If your target can get something basic working without implementing custom or advanced features then it is possible to get something up and running quickly and without many lines of code. For example for Arm, I was able to pick an Arm state only (no interworking) dynamically linked (no TLS or GNU_IFUNC support) hello world up and running by just implementing the PLT sequences and the relocations that I needed.

What happens next depends on how close your target is to an existing one, both in how it implements general features like TLS, and whether it needs custom linker generated content such as Thunks. Further things to consider are whether the default ELF layout suits your target.

The parts that needed most modifications for Arm were:
- Relocations.cpp; TLS, GNU_IFUNC often need customisation.
- Writer.cpp ; The overall controller of the layout and address allocation.
- SyntheticSections.cpp ; Where linker generated sections are created.
- Thunks.cpp ; Creation of linker generated trampolines between functions.
- InputSection.cpp ; Support relocation calculations specific to Arm.
I needed to add support for a couple of Arm specific section types to InputFiles.cpp, but otherwise the generic Elf loading parts of LLD worked without modification.

Searching for Config->EMachine will give you a good idea where something custom has had to be put in for one of the targets. Stepping through run() in Writer.cpp will give you a good idea of what order LLD does various passes.

Hope this helps and I'll be happy to try and answer any questions.

Peter


On 25 January 2018 at 15:38, ORiordan, Martin via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> Hi LLVM-Devs,
>
> I am considering switching to using 'llvm-lld' instead of Gnu 'ld' in a future revision of our out-of-tree target, and I am wondering is there a getting started guide for how to go about extending 'llvm-lld' to support an additional target.
>
> Thanks,
>
>         MartinO
>
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