[llvm-dev] Contributing LLD for Mach-O

Shoaib Meenai via llvm-dev
Thu, 05 Mar 2020 03:19:08 -0000

We’ve discussed this with Jim Grosbach at Apple (CC’d). My understanding is that
at this time, the officially supported linker for Apple’s platforms is ld64.
We’d love to collaborate with Apple (and any other interested parties, for that
matter) on LLD for Mach-O, and we’d be delighted if it were to become officially
supported at some point, but there’s a lot of work to be done first on reaching
feature parity with ld64 before that could even be considered :) Once we reach
feature parity, I can envision several good reasons both for sticking with ld64
and for switching to LLD. On our end, we aim to create a feature-complete Mach-O
linker. We also aim for the end product to be compelling enough that a switch
could be considered, and we’d be happy to work with Apple on that front if it
turns out to be.

What sort of compatibility test suite did you have in mind? We’re adding
lit-style unit tests as we add features (as is standard for LLVM), but we didn’t
have anything in mind beyond that right now.

We saw zld, and the speedups achieved by it make us hopeful of being able to
achieve similar results with LLD (since we’ll also have better parallelization
and better data structures in the form of the LLVM ones). (I’d also be curious
if zld’s improvements could be contributed back to ld64 so that everyone can
take advantage of them, but that’s completely tangential, of course.)

From: Dave Lee <davelee.com at gmail.com>
Date: Tuesday, March 3, 2020 at 7:37 AM
To: Shoaib Meenai <smeenai at fb.com>
Cc: "llvm-dev at lists.llvm.org" <llvm-dev at lists.llvm.org>,
Jez Ng <jezng at fb.com>
Subject: Re: [llvm-dev] Contributing LLD for Mach-O

Hi Shoaib,

Has there been any recent discussion with anyone on Apple's side? Is there a
way forward that results in a single unified open source linker? If not at the
start, how would it work if they later want to take maintainership?

Have you thought about a compatibility test suite? If so, I'm curious what
the approach will be.

Also for what it's worth, there's a recent fork of ld64's recent
source drop, which is optimized for incremental compilation. It also uses
non-STL data structures, parallelism, and a disk cache. See
https://github.com/michaeleisel/zld

On Fri, Feb 28, 2020 at 1:32 PM Shoaib Meenai via llvm-dev <llvm-dev at
lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
Hi all,

We’re planning to contribute a new implementation of LLD for Mach-O, using the
same design as the COFF and ELF ports. This design has proven to work very well
for those ports, and we’re keen to explore it for Mach-O as well. Our work is
based on an initial prototype created by Peter Collingbourne and Rui Ueyama.

Our initial commit is up for review at
https://reviews.llvm.org/D75382<https://urldefense.proofpoint.com/v2/url?u=https-3A__reviews.llvm.org_D75382&d=DwMFaQ&c=5VD0RTtNlTh3ycd41b3MUw&r=o3kDXzdBUE3ljQXKeTWOMw&m=Du_g8hPBNR5iofm4w6TR-gFx_aFK-4RNxpEmlx-FMNc&s=z1H0UW2F3uSaikSViTE-eRlQmyNhhrkHNq0aj2VxbfE&e=>.
We’ve intentionally stripped down this initial commit as much as possible to
ease reviewing; we’ve kept it to the absolute minimum needed to produce and test
a working macOS x86-64 executable for that prints “Hello World” via a syscall.
We have several short-term follow-ups planned to add important functionality,
such as linking against archives, universal binaries, dylibs, and tbd files,
performing subsection splitting (atomization), and producing dylibs. The
follow-ups should give a good sense of the overall design while still keeping
each piece easily reviewable and testable individually. Our end goal is to
create a full-featured Mach-O linker, and we’ll be working toward that goal over
the next several months (and years, in all likelihood). We’d appreciate feedback
and reviews.
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