[LLVMdev] Can we establish layering for the LLD libraries? Current state is a bit of a mess...

[Chandler Carruth]

On Tue, Jan 20, 2015 at 5:35 PM, Nick Kledzik <kledzik at apple.com> wrote:

> On Jan 19, 2015, at 6:33 PM, Chandler Carruth <chandlerc at google.com>
> wrote:
>
> > I wanted to go through and map out the layering of LLD's libraries today
> and found that it's essentially impossible. I think some serious cleanup is
> needed here.
> >
> > Let's start with the purely link-level dependencies encoded in the CMake
> build:
> >
> > Curently the Core library depends on the ReaderWriter/Native library,
> which links against the ReaderWriter library, which links against the Core
> library. This clearly cannot work. The same cycle exists with Core -> YAML
> -> ReaderWrite -> Core.
> How are you determining these cycles?   How is Core dependent on YAML?
> (cause that seems wrong).
>

This is in the CMake build system today. You can see it with
CMakeLists.txt. Also you can look at Greg's patch for some of the issues
here.


>
> I just build all of lld in an Xcode projects (which compiles each .o file
> and links them all together).  I never see any of the layering...
>

The CMake build works on Mac OS X too. =] With Ninja, it is even quite
speedy (or so I understand).

I'm hopeful in the not-too-distant-future we'll actually start enforcing
even header file layering using Clang's Module's support (when it happens
to be the host compiler). When that happens this stuff should actually be
really explicit and enforced easily.

Until then, I also have access to some crazy (and sadly internal) tools
that check header file layering between libraries. Hopefully we just
modules soon though.


>
> > The situation seems a bit worse for includes. If you start from
> LinkingContext.h I think this becomes quite clear. This is ostensibly part
> of the Core library, but it has methods that produce types from the
> ReaderWriter library. Combined with the fact that ReaderWriter depends on
> Core (not the other way around) and ReaderWriter/ELF subclasses
> LinkingContext, I can't make heads or tails of what was intended here.
> >
> >
> > My vague guess is that Core should actually be two libraries. One that
> doesn't depend on anything (other than Config) and provides the very "core"
> types for LLD. And another, perhaps called "Linker" which is a much
> higher-level library and provides useful APIs for actually doing linking,
> depends on ReaderWriter and provides methods that manipulate it. I could
> even see needing to spilt the target libraries in a similar manner.
> >
> >
> > But I don't know LLD's design well, I'm just trying to stitch the build
> system back together in a reasonable way, so maybe I've missed things
> completely. So help me out. I'd like to understand a reasonable DAG in
> which to construct libraries for LLD. Having this should allow proper
> layering, layering checks, and eventually building with C++ modules. All of
> which seem essentially impossible today.
>
> One question I have is what should the granularity of the libraries be?
> At one point I asked about building an lld for OSX that only had mach-o
> support and got push-back that lld should always be a universal linker.  If
> that is the case, why have so many small libraries?
>
> Besides lld the command line tool, I can see lld libraries being used to
> construct a JIT linker.  And in that case, I can see wanting a very pared
> down linker - just one file format (not yaml) and no driver.


So, I think there is a good rationale for supporting both fine grained
libraries and LLD always being a universal / cross linker.

The libraries make it *possible* (as you indicate with the JIT stuff) to
build a narrowly targeted linker that has no excess functionality. We
should still choose to produce a full cross-linker binary called "lld"
which uses all the libraries.

Does that make sense?

While I think that generally layering also helps abstractly organize the
code, I don't think it is worth thinking about layering that serves no
purpose. So, if it isn't *possible* to use library A without using library
B as well, it doesn't make sense to separate them. I'm happy to separate
them if it is possible even if the users for this separation haven't yet
materialized.

-Chandler
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