llvm dev - Jan 2015 - [LLVMdev] Can we establish layering for the LLD libraries? Current state is a bit of a mess...

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.


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.

-Chandler
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Hi Chandler,
> Let's start with the purely link-level dependencies encoded in the
CMake build
I've been playing around a bit with this.  Looks like we can solve
most of the issues by dissolving lldReaderWriter.
Reader.cpp/Writer.cpp go into lldCore and the rest go into lldDriver.
I don't know if that makes conceptual sense, but it's what breaks the
cyclic dependencies.

-Greg

On Mon, 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.
>
>
> 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.
>
> -Chandler
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>

On Tue, Jan 20, 2015 at 4:02 PM, Greg Fitzgerald <garious at gmail.com>
wrote:
> Hi Chandler,
>
> > Let's start with the purely link-level dependencies encoded in the
CMake
> build
>
> I've been playing around a bit with this.  Looks like we can solve
> most of the issues by dissolving lldReaderWriter.
> Reader.cpp/Writer.cpp go into lldCore and the rest go into lldDriver.
> I don't know if that makes conceptual sense, but it's what breaks
the
> cyclic dependencies.
>
Does that break just the *link time* dependencies?

If you form a DAG of cross-library header inclusion and a DAG of the link
time dependencies, they should be compatible, and that's the layering we
should use.

I don't know much about LLD -- maybe it would help for you to describe the
DAG you're envisioning?
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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).


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 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.

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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On Tue, Jan 20, 2015 at 05:35:53PM -0800, Nick Kledzik
wrote:
> How are you determining these cycles?   How is Core dependent on YAML?
> (cause that seems wrong).
Try building LLVM + LLD with shared libraries on Linux. That will
reliably fail due to dependency cycles between the components.

Joerg