[cfe-dev] [llvm-dev] [RFC] Open sourcing and contributing TAPI back to the LLVM community

Jake Ehrlich via cfe-dev cfe-dev at lists.llvm.org
Tue Apr 10 14:33:01 PDT 2018


Benifits of TBD:
1) It's human readable and diffs on TBDs correspond to changes in the ABI.
Diffs can be automatically added to review processes to ensure that changes
to the ABI are reviewed. The TBDs also document your precise ABI.
2) The size is smaller which means they can be shipped in an SDK instead of
binaries to reduce the size of an SDK
3) Stubs are producible from TBDs (or should be) which means stubs for
linking can be produced even if we don't directly support them in LLD. This
lets you ship the smaller TBD files in place of larger binaries and still
link things without direct linker support (assuming you already ship a
toolchain with your SDK or expect your users to have this tool)

Since stubs are producible from TBDs I don't really see a downside. I think
we need both, I was going to propose a yaml based representation for ELF
for the above reasons anyhow.

On Tue, Apr 10, 2018 at 1:14 PM Andrew Kelley via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> On Mon, Apr 9, 2018 at 10:11 PM, John Ericson via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> > Regardless of any of that, given that TBD files _are_ an integral part
>> of the apple platform, supporting them is certainly a necessity in order to
>> have a working apple linker. So, if making LLD work for Apple/MachO is the
>> justification for adding TBD support to LLVM, that seems self-evidently a
>> reasonable thing to do. On the other hand, it looks like the LLD mach-o
>> code is unmaintained and nobody seems to be much interested in it. And
>> having code for reading TBD files in LLVM seems not terribly interesting,
>> unless it is as part of a project to make the LLD MachO linker actually
>> functional and supported.
>>
>> Yes. I hope this can be reason enough. Hobbyists could push for LLD
>> support for Mach-O besides Apple, and if LLD is to displace other linkers
>> this is a necessary component as you say. Better to upstream now before the
>> code diverges than more work later? Conversely if nothing happens, I doubt
>> libtapi would be a greater drag on the codebase than the MachO LLD code, so
>> whatever cost/benefit analysis exists for keeping that around could also
>> apply to this.
>
>
>
> Speaking for the Zig project here, our goal is to support
> cross-compilation for any target, on any target, without requiring
> installation of any target-specific SDK. So, for example, these use cases:
>  * on linux, compile & link a binary targeting macos
>  * on windows, compile & link a binary targeting macos
>
> This works today, although it depends on a patch to LLD to fix the MACH-O
> linker that is not high enough quality to upstream.
>
> So we have a vested interest in improving the MACH-O linker, and in fact a
> Zig community member has fixed at least one bug in MACH-O LLD:
> reviews.llvm.org/D35387
>
> I don't fully understand how TBD or TAPI works, but I hope that it results
> in improvements to the MACH-O linker.
>
>
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