[llvm-dev] lld: ELF/COFF main() interface

[Rui Ueyama via llvm-dev]

Designing it a command makes things simpler because you can safely assume
that most functions always success, or otherwise the entire process
terminates. You can defer the operating system to clean up all resources
that was used by the process however it fails (with a few exceptions such
as temporary files.) I actually like to use the linker as an external
command since the operating system provides good isolation between my
process and the linker which could fail by an unknown bug or something.
Rewriting all of them as functions that return ErrorOr<Something> is
technically doable, but it needs strong justification, so I guess that
unlikely to change.

Why do you want to use that as a library? I don't think "because LLVM and
Clang allow that" is not a compelling argument since the linker and the
compiler are pretty different programs. I clearly see many reasons to use
LLVM and part of Clang as libraries, but they are not directly applicable
to LLD. The new linker is more like a command which is built on top of the
libraries that LLVM provides.

On Thu, Jan 7, 2016 at 8:28 AM, Arseny Kapoulkine <
arseny.kapoulkine at gmail.com> wrote:

> This is really unfortunate.
>
> I've read the discussion threads for the atom/chunk controversy and I feel
> like I understand the reasons for rewriting the linker, but this does not
> seem to have anything to do with whether the linker is usable as a library
> or not.
>
> As it stands, not only does lld have two completely different linkers (I'm
> treating COFF/ELF2 as one since they are really two different
> implementations of the same concept, AFAIU), but one is usable as a library
> (and even does not require round-tripping generated code through an object
> file! I was really happy to use that) and the other one isn't. Not sure
> what the future plans are for Mach-O linker (at this point it seems logical
> to rewrite that using the new designs but I'm not sure if it ever happens),
> so maybe at some point we'll just have one linker application instead of a
> library and an application.
>
> Anyway, since linker is the only missing piece for full compilation stack
> (source language to runnable executable), it's sad to see this specific
> part of LLVM not working as a library when everything else does.
>
> Are there specific concerns in terms of implementation that prevent new
> lld from being a library? I understand that using global variables and
> error() function is simpler, but the rest of LLVM does not do that and the
> codebase there is significantly larger. Am I missing any other issues
> except for the ones I mentioned in my original e-mail that will come up in
> a library-like usage scenario?
>
> Arseny
>
> On Thu, Jan 7, 2016 at 7:17 AM, Rui Ueyama <ruiu at google.com> wrote:
>
>> On Thu, Jan 7, 2016 at 7:03 AM, Arseny Kapoulkine via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>
>>> In the process of migrating from old lld ELF linker to new (previously
>>> ELF2) I noticed the interface lost several important features (ordered by
>>> importance for my use case):
>>>
>>> 1. Detecting errors in the first place. New linker seems to call exit(1)
>>> for any error.
>>>
>>> 2. Reporting messages to non-stderr outputs. Previously all link
>>> functions had a raw_ostream argument so it was possible to delay the error
>>> output, aggregate it for multiple linked files, output via a different
>>> format, etc.
>>>
>>> 3. Linking multiple outputs in parallel (useful for test drivers) in a
>>> single process. Not really an interface issue but there are at least two
>>> global pointers (Config & Driver) that refer to stack variables and are
>>> used in various places in the code.
>>>
>>> All of this seems to indicate a departure from the linker being useable
>>> as a library. To maintain the previous behavior you'd have to use a linker
>>> binary & popen.
>>>
>>> Is this a conscious design decision or a temporary limitation?
>>>
>>
>> That the new ELF and COFF linkers are designed as commands instead of
>> libraries is very much an intended design change.
>>
>>
>>> If it's a limitation, how would one go about fixing this? I'm not too
>>> familiar with the idiomatical error handling in LLVM. Normally in this
>>> situation I'd just throw from error() but lld is probably compiled without
>>> exceptions. Is ErrorOr the established practice? How does LLVM generally
>>> deal with error handling for parsers (like linker script parser) where it's
>>> a lot of mutually recursive functions where every single one can fail?
>>>
>>
>> Since it's as designed, please run the linker as an external command
>> using fork/exec (or wrappers of them) instead of trying to use that inside
>> the same process.
>>
>>
>>> Arseny
>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>
>>>
>>
>
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