[llvm-dev] Mach-O support in lld: what are the known issues?

Jean-Daniel via llvm-dev llvm-dev at lists.llvm.org
Wed Jun 6 14:07:06 PDT 2018

> Le 6 juin 2018 à 22:37, Rui Ueyama <ruiu at google.com> a écrit :
> On Wed, Jun 6, 2018 at 11:41 AM Brian Gesiak <modocache at gmail.com <mailto:modocache at gmail.com>> wrote:
> Thanks for the response, Rui!
> On Tue, Jun 5, 2018 at 5:26 PM, Rui Ueyama <ruiu at google.com <mailto:ruiu at google.com>> wrote:
> Besides the features you pointed out, I think Xcode introduced a new way of listing dynamic linking symbols, and I believe lld doesn't support that.
> .tbd files, is that right? A colleague of mine pointed me to Apple's libtapi open source project [1], maybe I can learn more about these files from that library. In fact, there's been discussion about bringing libtapi to LLVM on the mailing list in the past, although I don't know if anything came of it. [2]
> I think the real issue is the lack of maintenance and ownership of the mach-O lld tree. There's no activities for the tree for years, though we've been making efforts to keep it compile and pass all the existing tests.
> Excellent, thanks for letting me know. This doesn't bother me, I'm happy to try contributing to it as best I can! I would also appreciate, as your time permits, whatever guidance you can provide. For example, benefitting from several years of hindsight, would you recommend keeping the ATOM-based lld approach? [3] Prior emails discussed moving Mach-O lld away from ATOM. [4] Has the success of ELF and COFF influenced your thinking on this in the years since, or is ATOM probably still the best fit for Mach-O?
> That's a good question. There was a big discussion as to the design of the new (now current ELF/COFF/wasm) and the ATOM-based lld a few years ago when I started working on the new one. At the time no one including me was really sure what design is desirable, and I was exploring the design space to something good. Today, we have three working, high-performance linkers for ELF, COFF and wasm based on the new design, which I think proves the design; it is easy to add new features, easy to understand, and it delivers what users want the most (i.e. speed). Given that, if I were you, I'd try to see if the new lld's design fits mach-O. You may need to tweak the design a little bit, but I'd imagine that the difference is not as significant as between ELF and wasm (which has a different concept of memory address space mainly for security). I'd also like to get input from Apple engineers as well.

I don’t remember but I think one of the main point was that EFL and Mach-O don’t have the same concept of section. IIRC, the concept of section in ELF was closer to the atom model than with Mach-O which uses few sections and put a lot of things in each one.

And a quick search gave me that:  http://llvm.1065342.n5.nabble.com/LLD-improvement-plan-td80788.html#a80871 <http://llvm.1065342.n5.nabble.com/LLD-improvement-plan-td80788.html#a80871>

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