[llvm-dev] LLD status update and performance chart

[Sean Silva via llvm-dev]

On Tue, Dec 13, 2016 at 8:10 PM, Andrew Kelley <superjoe30 at gmail.com> wrote:

>
> On Tue, Dec 13, 2016 at 10:59 PM, Sean Silva via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>>
>> Yes. Rui has bent over backwards every time a real user has come to us
>> and said "we need X". The historical precedent here is that LLD is open to
>> many kinds of changes, but not on theoretical grounds.
>>
>> Admittedly this leads to a somewhat conservative design for the linker
>> w.r.t. enabling new use cases. However, having been involved directly or
>> indirectly with LLD development over the last 4 or 5 years, the number of
>> new use cases that have been proposed fall into a very small number of
>> categories:
>>
>> - I want to have "main() in a library" for a static linker. LLD/ELF
>> currently provides this functionality (with some unfortunate caveats w.r.t.
>> re-entrancy and other stuff, but the core functionality is there, and LLD
>> has catered to reasonable requests to remove or mitigate the caveats).
>>
>
> My previous understanding is that LLD is a drop in replacement for every
> system linker - COFF, ELF, MACH-O, for every architecture that LLVM
> supports.
>
> 1. Is what I said above in the scope of the LLD project?
>

That is certainly in scope.


>
> 2. How close is the LLD project to reaching that state?
>

To varying degrees.

COFF: LLD can self-host and also build chromium and is generally considered
a stable windows linker. For windows-specific reasons, debug info (PDB) is
particularly challenging and still being worked on. Without something like
the big open-source package managers on ELF platforms it's harder to
perform really thorough testing of the long tail of issues though.
ELF: LLD is a respectable linker basically on par with gold for most use
cases. And LLD is even beyond gold for some things; for example, gold can't
link a working FreeBSD kernel and LLD can (on the other hand, gold can link
a working Linux kernel, and I don't think LLD can yet; if somebody is
interested in working on this, I can share how Michael and I debugged the
issues with the mislinked FreeBSD kernel, which involved bootloader
debugging and other stuff). There's a tail of features being worked on, but
you can use it now.
MachO: There is relatively little activity upstream, but last I talked with
Lang, it is progressing nicely. He can probably give more precise status
info. I don't remember if it can self-host yet.



>
> 3. Is there anything preventing "main() in a library" from supporting all
> of these platforms and architectures?
>
>
All 3 already provide this. If you look in main() in lld/tools/lld/lld.cpp
you can see the corresponding calls to {elf,coff,mach_o}::link. There are
some caveats for ELF and especially COFF. Basically, the ELF and COFF ones
are not re-entrant currently, and the COFF one currently uses fatal error
handling for common errors (ELF did too, but a user requested that it not
do so and it was fixed).

-- Sean Silva
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