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About CEDET, Completion, and compilers
Eric M. Ludlam
About CEDET, Completion, and compilers
Wed, 12 Mar 2014 23:04:40 -0400
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The long thread about "Emacs Contributions" that ended up being about
Clang, CEDET, completion etc was interesting, and reveled problems
people run into with CEDET, and possibly some misconceptions about what
CEDET is with respect to the "completion" feature. I thought I would
try and help explain a couple things, and I'll wrap up with my thoughts
on some TODO items that will help make CEDET better for anyone who wants
to help out.
For those who don't know me, I wrote the first versions of CEDET and
maintain it over in the SourceForge repository. http://cedet.sf.net has
lots of nifty explanations and pictures for anyone wanting to know more.
To start, CEDET is a group of very different tools, each of which covers
a particular topic, and which work together to create a suite of
features many associate with a modern IDE. That includes project
management, tagging, smart completion, and code-generation tooling.
Each of these elements is built on a framework where the raw data
needed, such as project definitions, tag lists, and templates is
represented in an Emacs friendly data structure. One of CEDET's most
important features is that data representation and the way it keeps it
up to date. It allows a wide range of back-ends or tools to be
implemented in Emacs with minimal fuss.
When someone talks about using Clang or other external compiler for
"smart completion", there is a layer in CEDET where that tool can be
integrated in as a new smart-completion back-end, allowing any
user-facing tool built using CEDET APIs to work with this new tool. I
don't really see using an external compiler as "competing" with CEDET.
It is just a tool that fits into one corner of the big picture.
External tools are likely to fail in many cases (ie - like when it isn't
installed on a machine), and CEDET will simply revert back to the core
Anyway, while using an external completion engine is nice for someone
who just wants completion, it leaves out support for tagging, and the
wide range of features you get with it. Fortunately, it is also
possible to integrate in a compiler (if desired) to produce tags as
well. In CEDET there is the tagging parser for C/C++ written in Emacs
Lisp, plus an Exuberent CTags parser that can be used in its stead.
Exuberent ctags is off by default simply because it isn't good enough,
but it can be used for basic tagging for files not in buffers. If a
compiler such as gcc were to be extended to help support CEDET,
preferably (from my point of view) by creating CEDET compatible tags,
that would also allow CEDET to parse files not in buffers, but it may
not be robust enough to handle incremental parsing as you type the way
the Emacs parser does. Fortunately, they do not need to be mutually
exclusive. Both can be used at the same time and work together because
the underlying data structure is well defined.
This is all just basic modularization stuff applied to a particular
problem. Lets me discuss where CEDET tends to fall down. My hope is
that this could inspire someone interested in participating who just
needs a starting point.
Detecting & Managing projects:
You can't do completion without knowing the bounds and shape of your
sources. One of the most common questions on the mailing list tends to
be about setting up a "project" to get the tooling to work. There are
so many ways to compile code, and the Make systems are generally opaque
making this a challenging support issue for EDE. While I think the
basic premise in EDE (the project management tool) is good, there is a
diversity of support that is missing. A good project here is making the
task of creating a project for Emacs to describe your code a bit easier.
Quality of the C++ parser:
It was frequently noted that C++ is hard to write parsers for.
Fortunately, the current parser that is written in Emacs Lisp covers the
basics, and because it is by nature very forgiving, filling in the gaps
has proven within the reach of a wide range of people. Anyone who
discovers a missing c++ feature should feel confident about being able
to update the parser.
Quality of C++ completions:
The core completion engine here is also in pretty good shape, right up
until you start playing around with templates and some other exciting
c++ features. David Engster has gotten quite a bit working here, but
I'll defer to him to explain this in more detail for anyone who wants to
Performance, and Really Large projects:
There is a core problem in the way CEDET manages tags, which is that it
keeps them all in memory. For large code bases, after working in them
for a while, CEDET will make Emacs' memory use go up so much that a
garbage collect spends more time swapping that collecting.
Fortunately, the entire system has been abstracted through some Emacs
data structures that represent databases of tag tables. I have done
some work to create databases using external tools such as GNU Global
which fit into the system transparently providing tag data. One
possible solution would be to develop a nice on-disk representation that
is easy to search to move the bulk of the data out of the Emacs process.
Refactoring is non-existent:
I did start down a road with "semantic-symref" tools that can do
primitive refactoring, but some of the key features, such as resolving
polymorphism issues hasn't been implemented. The data is there, the
logic just hasn't been implemented. At the moment it is mostly a fancy
Language X isn't supported:
The system that supports C++ has been generalized, and supports many
languages. C++, Java, and Emacs Lisp are perhaps the best supported so
far. There are a lot of opportunities to add support for new languages,
or enhance an existing language support.
I hope this data is helpful.
- About CEDET, Completion, and compilers,
Eric M. Ludlam <=
- Re: About CEDET, Completion, and compilers, Andreas Röhler, 2014/03/13
- Re: About CEDET, Completion, and compilers, João Távora, 2014/03/13
- Re: About CEDET, Completion, and compilers, Eric M. Ludlam, 2014/03/13
- EIEIO, Daniel Colascione, 2014/03/13
- Re: EIEIO, Eric M. Ludlam, 2014/03/13
- Re: EIEIO, David Engster, 2014/03/14
- Re: EIEIO, Eric Abrahamsen, 2014/03/14
- Re: EIEIO, Eric Schulte, 2014/03/14
- Re: EIEIO, Thien-Thi Nguyen, 2014/03/15