[lldb-dev] Circular library dependencies

Zachary Turner zturner at google.com
Tue Mar 17 13:39:09 PDT 2015

I agree that it should be organized by functionality, but I think it's
possible to organize it by functionality in a way that better linking falls
out naturally as a consequence.  Also I'm not really talking about a major
restructuring of every project, but just hitting a few key points.  A good
example is how source/lldb has 2 random cpp files dropped in, lldb-log.cpp
and lldb.cpp.  The way the CMake build works is that we treat all source
files in a particular directory (and sometimes in sub-directories) as
belonging to the same target, and each target is compiled to a .a file (or
.lib file on Windows).  So source/lldb.cpp and source/lldb-log.cpp get put
into something I've called lldbBase.a / .lib.  Now, since *everything* in
the codebase depends on logging, we have to link everything against
lldbBase, which also means lldb.o.  And lldb.o depends on pretty much
everything in the entire project.

The way this came up in context is that our unit test runner builds
multiple executables, one for each component being tested.  This makes the
link time scale really horribly, because it's having to look at every .lib
in all of LLDB just to link against one class.

In any case, the organization also doesn't make much sense.  You can't use
either source/lldb-log.cpp or source/Core/Log.cpp without the other, so
seems to me they should both be in source/Core.  and lldb.cpp is kind of
its own thing that is only necessary for global LLDB initialization, so it
could be off by itself in like source/Initialize or something.

So that's one example of something I wanted to fix.  So to sum up, I agree
we shouldn't do anything that's purely an optimization and has no benefit
to code organization, but I think there's some places where the
organization could improve and the rest would happen naturally.

On Tue, Mar 17, 2015 at 1:17 PM Greg Clayton <gclayton at apple.com> wrote:

> > On Mar 17, 2015, at 11:25 AM, Zachary Turner <zturner at google.com> wrote:
> >
> > Trying to look at ways to reduce the link time of LLDB, I wrote a small
> python script last night whose purpose is to get a rough idea of what
> LLDB's dependency graph looks like.  A lot of this is CMake specific, but
> the idea isn't much different either way.
> >
> > It does this by walking the source and incldue folder.  For each folder
> entered, it determines what .a this compiles to by looking for the
> corresponding CMakeLists.txt and doing a rudimentary parse.
> >
> > Then, for each .cpp and .h file found, scan the file for #include
> statements, and map the include file to the corresponding location under
> the source tree, using this to determine which .a file the #include is a
> primary member of.  For each of these found, add that .a file to the list
> of dependencies for the containing .a.
> >
> > Basically what I found is that every folder more or less depends on
> every other folder, which is pretty unfortunate.
> >
> > I have some ideas about how to make some improvements, but I want to see
> what people think first.  Aside from the obvious benefits of just making
> the code be better layered and more separable, it also would reduce link
> times quite a bit I think.  And there are lots of cases such as lldb-server
> or unit tests where we want to link in as little as possible, as opposed to
> the monolithic LLDB executable which wants to link in pretty much
> everything.
> >
> > Thoughts?
> On MacOSX the linking process is quick, I am not sure how fast linking is
> on other platforms.
> I really want the code organized by functionality not optimized for
> linking. Everything in LLDB links against just about everything else and
> there should be no limits imposed on what can link with what.
> Linking static libraries is only for tools that want to use the dangerous
> internals of LLDB. Really most tools should link against our public API. I
> know that lldb-server is one of the tools that needs the internals and it
> should stay that way.
> I would rather not see any changes/reorganization going on to optimize
> linking unless it makes a lot of sense organizationally and I think we have
> decent organization in our source tree.
> Greg
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