llvm dev - Jun 2018 - Mach-O support in lld: what are the known issues?

Hello all,

I'm trying to better understand the state of Mach-O support in lld.

The lld docs state that "the linker supports ELF (Unix), PE/COFF (Windows),
Mach-O (macOS) and WebAssembly in descending order of completeness." [1]
True to that statement, I found an email on this list from Jan 2018 stating
that "MachO support in lld is not really ready for real world usage. It was
able to bootstrap itself a couple of years ago, but, it has not really been
maintained or further developed since." [2] And on LLVM Bugzilla, a comment
on one bug states "indeed the macOS version seems to be experimental, and
not to support LTO at all for the moment." [3]

I'm curious if anyone has more information on what else Mach-O support in
lld is missing. From the above links, I'm aware of a lack of support for
LTO. I also encountered the same Clang driver bugs as mentioned on that
Bugzilla report. Besides that, I can see a few memory leaks and incorrect
links have been reported on Bugzilla as well. [4]

Is there anything else that lld developers might be aware of? What work
needs to be done before ld64.lld is considered complete?

Thanks in advance for any information you can send my way! :)

- Brian Gesiak

[1] https://lld.llvm.org
[2] http://lists.llvm.org/pipermail/llvm-dev/2018-January/120216.html
[3] https://bugs.llvm.org/show_bug.cgi?id=32175
[4]
https://bugs.llvm.org/buglist.cgi?bug_status=UNCONFIRMED&bug_status=NEW&bug_status=ASSIGNED&bug_status=REOPENED&bug_status=RESOLVED&bug_status=VERIFIED&bug_status=CLOSED&component=MachO&list_id=139976&product=lld&query_format=advanced&resolution=---&resolution=LATER&resolution=REMIND
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Are you planning on working on LLD?

Over here in the Zig frontend, we've been relying on LLD Mach-O support but
at some point we'll have to either maintain LLD or write a Mach-O linker in
zig. So far we've been making it work with this hacky patch:
https://github.com/ziglang/zig/commit/1ba6e1641a4c5ea1d0d665fe500c9c66d69443a4

If nobody else cares about the LLD Mach-O code, then we'll be better off
doing the work in zig but if others are interested then it may be more
beneficial for the community to work together on the LLD codebase.

On Mon, Jun 4, 2018 at 7:06 PM, Brian Gesiak via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Hello all,
>
> I'm trying to better understand the state of Mach-O support in lld.
>
> The lld docs state that "the linker supports ELF (Unix), PE/COFF
> (Windows), Mach-O (macOS) and WebAssembly in descending order of
> completeness." [1] True to that statement, I found an email on this
list
> from Jan 2018 stating that "MachO support in lld is not really ready
for
> real world usage. It was able to bootstrap itself a couple of years ago,
> but, it has not really been maintained or further developed since."
[2] And
> on LLVM Bugzilla, a comment on one bug states "indeed the macOS
version
> seems to be experimental, and not to support LTO at all for the
moment."
> [3]
>
> I'm curious if anyone has more information on what else Mach-O support
in
> lld is missing. From the above links, I'm aware of a lack of support
for
> LTO. I also encountered the same Clang driver bugs as mentioned on that
> Bugzilla report. Besides that, I can see a few memory leaks and incorrect
> links have been reported on Bugzilla as well. [4]
>
> Is there anything else that lld developers might be aware of? What work
> needs to be done before ld64.lld is considered complete?
>
> Thanks in advance for any information you can send my way! :)
>
> - Brian Gesiak
>
> [1] https://lld.llvm.org
> [2] http://lists.llvm.org/pipermail/llvm-dev/2018-January/120216.html
> [3] https://bugs.llvm.org/show_bug.cgi?id=32175
> [4] https://bugs.llvm.org/buglist.cgi?bug_status>
UNCONFIRMED&bug_status=NEW&bug_status=ASSIGNED&bug_
> status=REOPENED&bug_status=RESOLVED&bug_status=VERIFIED&
> bug_status=CLOSED&component=MachO&list_id=139976&product>
lld&query_format=advanced&resolution=---&resolution>
LATER&resolution=REMIND
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
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Brian,

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.
There may be long-tail missing features as well. But I don't think
that's
the real issue. 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. As an example I made a last-minute cherrypick for some LLVM
release to fix a bug blocking Zig language without really understanding
what that patch is exactly doing. That's far from ideal...

On Mon, Jun 4, 2018 at 4:07 PM Brian Gesiak via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Hello all,
>
> I'm trying to better understand the state of Mach-O support in lld.
>
> The lld docs state that "the linker supports ELF (Unix), PE/COFF
> (Windows), Mach-O (macOS) and WebAssembly in descending order of
> completeness." [1] True to that statement, I found an email on this
list
> from Jan 2018 stating that "MachO support in lld is not really ready
for
> real world usage. It was able to bootstrap itself a couple of years ago,
> but, it has not really been maintained or further developed since."
[2] And
> on LLVM Bugzilla, a comment on one bug states "indeed the macOS
version
> seems to be experimental, and not to support LTO at all for the
moment."
> [3]
>
> I'm curious if anyone has more information on what else Mach-O support
in
> lld is missing. From the above links, I'm aware of a lack of support
for
> LTO. I also encountered the same Clang driver bugs as mentioned on that
> Bugzilla report. Besides that, I can see a few memory leaks and incorrect
> links have been reported on Bugzilla as well. [4]
>
> Is there anything else that lld developers might be aware of? What work
> needs to be done before ld64.lld is considered complete?
>
> Thanks in advance for any information you can send my way! :)
>
> - Brian Gesiak
>
> [1] https://lld.llvm.org
> [2] http://lists.llvm.org/pipermail/llvm-dev/2018-January/120216.html
> [3] https://bugs.llvm.org/show_bug.cgi?id=32175
> [4]
>
https://bugs.llvm.org/buglist.cgi?bug_status=UNCONFIRMED&bug_status=NEW&bug_status=ASSIGNED&bug_status=REOPENED&bug_status=RESOLVED&bug_status=VERIFIED&bug_status=CLOSED&component=MachO&list_id=139976&product=lld&query_format=advanced&resolution=---&resolution=LATER&resolution=REMIND
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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I'd be interested in the existence of a high-quality, open-source, portable
linker for apple platforms, but not enough to help make that happen.

If I _was_ gonna work on something related to that, I'd probably be
inclined to instead add any required features to allow an ELF linker to
target a notional darwin-elf target, and to have clang emit darwin-elf
object files, and then write a binary converter to convert the emitted ELF
binary to a Mach-O binary, so that it can actually run on a platform that
exists rather than the platform I'd prefer to exist. But that's probably
just me being crazy, and I'm not going to work on it. :)



On Tue, Jun 5, 2018 at 11:19 AM Andrew Kelley via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Are you planning on working on LLD?
>
> Over here in the Zig frontend, we've been relying on LLD Mach-O support
> but at some point we'll have to either maintain LLD or write a Mach-O
> linker in zig. So far we've been making it work with this hacky patch:
>
https://github.com/ziglang/zig/commit/1ba6e1641a4c5ea1d0d665fe500c9c66d69443a4
>
> If nobody else cares about the LLD Mach-O code, then we'll be better
off
> doing the work in zig but if others are interested then it may be more
> beneficial for the community to work together on the LLD codebase.
>
> On Mon, Jun 4, 2018 at 7:06 PM, Brian Gesiak via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Hello all,
>>
>> I'm trying to better understand the state of Mach-O support in lld.
>>
>> The lld docs state that "the linker supports ELF (Unix), PE/COFF
>> (Windows), Mach-O (macOS) and WebAssembly in descending order of
>> completeness." [1] True to that statement, I found an email on
this list
>> from Jan 2018 stating that "MachO support in lld is not really
ready for
>> real world usage. It was able to bootstrap itself a couple of years
ago,
>> but, it has not really been maintained or further developed
since." [2] And
>> on LLVM Bugzilla, a comment on one bug states "indeed the macOS
version
>> seems to be experimental, and not to support LTO at all for the
moment."
>> [3]
>>
>> I'm curious if anyone has more information on what else Mach-O
support in
>> lld is missing. From the above links, I'm aware of a lack of
support for
>> LTO. I also encountered the same Clang driver bugs as mentioned on that
>> Bugzilla report. Besides that, I can see a few memory leaks and
incorrect
>> links have been reported on Bugzilla as well. [4]
>>
>> Is there anything else that lld developers might be aware of? What work
>> needs to be done before ld64.lld is considered complete?
>>
>> Thanks in advance for any information you can send my way! :)
>>
>> - Brian Gesiak
>>
>> [1] https://lld.llvm.org
>> [2] http://lists.llvm.org/pipermail/llvm-dev/2018-January/120216.html
>> [3] https://bugs.llvm.org/show_bug.cgi?id=32175
>> [4]
>>
https://bugs.llvm.org/buglist.cgi?bug_status=UNCONFIRMED&bug_status=NEW&bug_status=ASSIGNED&bug_status=REOPENED&bug_status=RESOLVED&bug_status=VERIFIED&bug_status=CLOSED&component=MachO&list_id=139976&product=lld&query_format=advanced&resolution=---&resolution=LATER&resolution=REMIND
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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Thanks for the response, Rui!

On Tue, Jun 5, 2018 at 5:26 PM, Rui Ueyama <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?

On Wed, Jun 6, 2018 at 8:10 PM, Jean-Daniel via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
>
> The list of feature exclusive to the Apple platform and Mac-O in the
> linker is astonishingly long. I started listing the missing features in lld
> at some points, but stop midway.
>
Jean-Daniel, do you still happen to have this list somewhere? I'd love to
read it, even if you did stop midway! Besides .tbd files, for example
there's the matter of supporting "fat binaries," which contain
code for
multiple instruction sets.

Also, thanks to everyone on the thread who expressed interest! I'm still
exploring and don't want to commit to anything just yet, but suffice it to
say I'm interested in lld Mach-O as well.

- Brian Gesiak

[1] https://opensource.apple.com/source/tapi/tapi-1.30/
[2] https://lists.llvm.org/pipermail/llvm-dev/2017-September/117264.html
[3] https://lld.llvm.org/AtomLLD.html
[4] http://lists.llvm.org/pipermail/llvm-dev/2015-May/085115.html
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