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

[Chandler Carruth via llvm-dev]

On Thu, Jan 21, 2016 at 10:49 AM Rafael Espíndola <
rafael.espindola at gmail.com> wrote:

> > There are probably others, but this is the gist of it. Now, you could
> still
> > design everything with the simplest imaginable API, that is incredibly
> > narrow and specialized for a *single* user. But there are still
> fundamentals
> > of the style of code that are absolutely necessary to build a library.
> And
> > the only way to make sure we get this right, is to have the single user
> of
> > the code use it as a library and keep all the business logic inside the
> > library.
> >
> > This pattern is fundamental to literally every part of LLVM, including
> > Clang, LLDB, and thus far LLD. I think it is a core principle of the
> project
> > as a whole. I think that unless LLD continues to follow this principle,
> it
> > doesn't really fit in the LLVM project at all.
>
> The single user so far is the one the people actually coding the
> project care for. I seems odd to say that it doesn't fit in the LLVM
> project when it has attracted a lot of contributors and hit some
> important milestones.
>

I don't think that every open source effort relating to compilers belongs
in the LLVM project. I think that they would have to fit with the
overarching goals and design of the LLVM project as a whole. This includes,
among other things, being modular and reusable.


>
> > So, I encourage LLD to keep its interfaces highly specialized for the
> users
> > it actually has -- and indeed today that may be exactly one user, the
> > command line linker.
>
> We have a highly specialized api consisting of one function:
> elf2::link(ArrayRef<const char *> Args). That fits 100% of the uses we
> have. If there is ever another use we can evaluate the cost of
> supporting it, but first we need to actually write the linker.
>

Note that I'm perfectly happy with this *interface* today, provided it is
genuinely built as a library and can be used in that context. See below.


>
> Note that this is history replaying itself in a bigger scale. We used
> to have a fancy library to handle archives and llvm-ar was written on
> top of it. It was the worst ar implementation by far. It had horrible
> error handling, incompatible options and produced ar files with
> indexes that no linker could use.
>
> I nuked the library and wrote llvm-ar as the trivial program that it
> is. To the best of my knowledge it was then the fastest ar in
> existence, actually useful (linkers can use it's .a files) and far
> easier to maintain.
>

The fact that it was in a library, IMO, is completely orthogonal from the
fact that the design of that library ended up not working.

Bad library code is indeed bad. That doesn't mean that it is terrible hard
to write good library code, as you say:


> When the effort to support windows came up, there was a need to create
> archives from within lld since link.exe can run lib.exe. The
> maintainable code was easy to refactor into one library function
> llvm::writeArchive. If another use ever show up, we evaluate it. If
> not, we keep the very narrow interface.
>

Yes, +1 to narrow interface, but I think it should always be *in a
library*. That impacts much more than just the interface.


>
> > Finally, I will directly state that we (Google) have a specific interest
> in
> > both linking LLD libraries into the Clang executable rather than having
> > separate binaries, and in invoking LLD to link many different executables
> > from a single process. So there is at least one concrete user here today.
> > Now, the API we would need for both of these is *exactly* the API that
> the
> > existing command line linker would need. But the functionality would
> have to
> > be reasonable to access via a library call.
>
> Given that clang can fork, I assume that this new clang+lld can fork.
>

No, it cannot in all cases. We have genuine use cases where forking isn't
realistically an option. As an example, imagine that you want to use
something like ClangMR (which I presented ages ago) but actually *link*
code massively at scale to do post-link analysis of the binaries? There are
environments where we need to be able to run the linker on multiple
different threads in a single address space and collect the linked object
in an in-memory buffer.

Also, one of the other possible motivations of using LLD directly from
Clang would be to avoid process overhead on operating systems where that is
a much more significant part of the compile time cost. We could today
actually take the fork out of the Clang driver because the Clang frontend
*is* designed in this way. But we would also need LLD to work in this way.
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