Paul Khuong mostly on Lisp

Starting to Hack on SBCL

SBCL was accepted as a mentoring organisation for Google’s Summer of Code 2013 (our list of project suggestions is here). This will be our first time, so that’s really great news. I’m also extremely surprised by the number of people who’ve expressed interest in working with us. I was going to reply to a bunch of emails individually, but I figure I should also centralise some of the stuff here.

EDIT: There’s a section with a bunch of general references.

EDIT 2: Added a note on genesis when playing with core formats.

Getting started

Setting up the basic tools

The first step is probably to install git, and clone our repo (the github mirror works well, and lets you fork to your own github account for easy publication). Then, building from source and installing SBCL (a local installation to $HOME works fine) is obviously useful experience, and will be useful to explore the source. Reading INSTALL should be enough to get started on Linux or OS X and x86[-64]. Other platforms may need more work, and might not be the best choice if you’re not interested in improving the port itself (although I’m told FreeBSD and Solaris work very well on x86[-64]). To build SBCL from source, you’ll need an SBCL binary (bootstrapping from other CLs should work, but support regularly bitrots away), and the usual C development tools (e.g. build-essential on debian). A fancy build (./make.sh --fancy) is probably the best choice for development.

You’ll also want to run the test suite often; better try it out now (cd tests; sh run-tests.sh) to make sure you can get it working. The test suite will barf if there’s non-ASCII characters in the environment. Oh my Zsh’s git customisation systematically trips me up, for example (I currently kludge it and start a bash from ~ and then run the tests).

Once SBCL HEAD is working and installed, it’s probably best to install emacs and SLIME. Quicklisp’s quicklisp-slime-helper can take care of installing SLIME. It is possible to work on SBCL without SLIME. However, SLIME has a lot of useful extensions, if only to explore the code base. If you’re not (and don’t wish to become) comfortable with emacs, it’s probably best to nevertheless use emacs and SLIME for the REPL, debugger, inspector, etc. and write code in your favourite editor. Later on, it’ll be useful to figure out how to make SLIME connect to a freshly-built SBCL.

Exploring the source

I often see newcomers try to read the source like a book, and, once they realise there’s a lot of code, try to figure out a good order to read the source. I don’t think that’s the best approach. SBCL is pretty huge, and I doubt anyone ever simultaneously holds the complete system in their head. RAM’s “The Python Compiler for CMU Common Lisp” is still useful as an overview, and SBCL’s internals manual is a good supplement. Once you get close to bootstrapping logic, Christophe Rhodes’s “SBCL: a Sanely-Bootstrappable Common Lisp” helps understand the exclamation marks. Past that, I believe it’s preferable to start out small, learn just enough to get the current task done, and accept that some things just work, without asking how (for now).

In that spirit, I’d say M-. (Alt period, Command period on some OS X emacsen) is the best way to explore most of SBCL’s source. SBCL’s build process preserves a lot of source location information, and M-. queries that information to jump to the definitions for any given symbol (M-, will pop back up to the previous location). For example, if you type “(truncate” at the REPL and hit M-. (with the point on or just after “truncate”), you’ll find the out of line definition for truncate in (mostly) regular Common Lisp, optimisation rules regarding truncate, and VOPs, assembly language templates, for truncate called with a few sets of argument and return types. The out of line definition isn’t that interesting. The transforms, however, are. (VOPs aren’t useful if one isn’t comfortable with the platform’s assembly language, and mostly self-explanatory otherwise.)

The one to “convert integer division to multiplication” is a very good example. One could M-. on deftransform, and go down a very long chain of definitions. Instead, I think it’s only essential to see that the form defines a new rule, like a compiler macro, such that compile-time values (lvars) that represent its two arguments are bound to x and y, and the rule only triggers if its first argument is known to be an unsigned word, and its second a constant unsigned word. If that’s satisfied, the transformation still only triggers if the speed optimisation quality is higher than both compilation speed and space (code size).

Then, the constant value for y is extracted and bound to y, and a conservative bound on the maximum value that x can take at runtime is computed. If truncate by y should be handled elsewhere, the transform gives up. Otherwise, it returns a form that will be wrapped in (lambda (x y) ...) and spliced in the call, instead of truncate.

To extend SBCL’s support for division by constants, it’s not necessary to understand more of SBCL’s compiler than the above. There’s no need to try and understand how deftransform works, only that it defines a rule to simplify calls to truncate. Similarly for lvar-value and lvar-type: the former extracts the value for constant lvars, and the latter the static type derived for that lvar (value at a program point). With time, knowledge will slowly accrete. However it’s possible, if not best, to start hacking without understanding the whole system. This approach will lead to a bit of cargo culting, but mentors and people on IRC will help make sure it doesn’t do any harm, and can explain more stuff if it’s interesting or à propos.

Finding where the compiler lives

Working on the compiler itself is a bit more work. I think the best approach is to go in src/compiler/main.lisp and look for compile-component. ir1-phases loops on a component and performs high-level optimisations until fixpoint (or we get tired of waiting), while %compile-component handles the conversion to IR2 and then to machine code. The compilation pipeline hasn’t really changed since the Python paper was written, and the subphases each have their own function (and file). M-. on stuff that sounds interesting is probably the best approach at the IR2 level.

Runtime stuff

The C and assembly runtime lives in src/runtime/. There’s a lot of stuff that’s symlinked or generated during the build, so it’s probably best to look at it after a successful build. Sadly, we don’t track source locations there, but {c,e,whatever}tags works; so does grep.

GC stuff is in the obvious suspects (gc-common, gencgc, gc, etc.), but may end up affecting core loading/saving (core, coreparse, save). Depending on what in the core loading code is affected, code in genesis (the initial bootstrap that reads fasl files from the cross compiler and builds the initial core file) might also have to be modified (mostly in src/compiler/generic/genesis.lisp). That’s… more work. Like the project suggestions list says, when we change things in the runtime, it sometimes ends up affecting a lot of other components.

GDB tends to be less than useful, because of the many tricks SBCL plays on itself. It’s usually hard to beat pen, paper, and printf. At least, rebuilding the C runtime is quick: if the feature :sb-after-xc-core is enabled (which already happens for --fancy builds), slam.sh should be able to rebuild only the C runtime, and then continue the bootstrap with the rest of SBCL from the previous build. That mostly leaves PCL to build, so the whole thing should takes less than a minute on a decent machine.

Some references

I was replying to an email when I realised that some general compiler references would be useful, in addition to project- and SBCL- specific tips.

Christian Queinnec’s Lisp in Small Pieces gives a good overview of issues regarding compiling Lisp-like languages. Andrew Appel’s Modern Compiler Implementation in ML is more, well, modern (I hear the versions in C and Java have the same text, but the code isn’t as nice… and ML is a very nice language for writing compilers). I also remember liking Appel’s Compiling with Continuations, but I don’t know if it’s particularly useful for CL or the projects we suggest.

For more complicated stuff, I believe Stephen Muchnick’s Advanced Compiler Design and Implementation would have been really nice to have, instead of slogging through code and dozens of papers. Allen & Kennedy’s Optimizing Compilers for Modern Architectures: A Dependence-based Approach is another really good read, but I’m not sure how useful it would be when working on SBCL: we still have a lot of work to do before reaching for the really sophisticated stuff (and what sophistication there is is fairly non-standard).

I believe the Rabbit and Orbit compilers have influenced the design of CMUCL and SBCL. The Lambda papers provide some historical perspective, and the RABBIT and ORBIT theses are linked here.

What little magic remains in SBCL and CMUCL is the type derivation (constraint propagation) pass, and how it’s used to exploit a repository of source-to-source transformations (deftransforms). The rest is bog-standard tech from the 70s or 80s. When trying to understand SBCL’s type derivation pass at a very high level, I remember finding Henry Baker’s The Nimble Type Inferencer for Common Lisp-84 very useful, even though it describes a scheme that doesn’t quite work for Common Lisp (it’s very hard to propagate information backward while respecting the final standard). Kaplan and Ullman’s A Scheme for the Automatic Inference of Variable Types was also helpful.

Getting help

Over the years, I’ve seen a couple of people come in with great ambition, and give up after some time, seemingly without having made any progress. I believe a large part of the problem is that they tried to understand all of SBCL instead of just learning the bare minimum to get hacking, and that their goal was too big. I already wrote that SBCL is probably best approached bit by bit, with some guidance from people who’ve been there before, and I hope the projects we suggest can all lead to visible progress quickly, after a couple days or two weeks of work at most.

Still, before investing my time, I like to see the other person also give some of theirs to SBCL. This is why, as I wrote on the mailing list last week, I’m much more inclined to help someone who’s already built SBCL on their own and has submitted a patch that’s been committed or is being improved on the mailing list. I absolutely do not care what the patch is; it can be new code, a bugfix for a highly unlikely corner case, better documentation, or spelling and grammar corrections in comments. The bugs tagged as easy in our bugtracker may provide some inspiration. However trivial a patch might seem, it’s still a sign that someone is willing to put the work in to concretely make SBCL better, and I like that… it’s also a minimal test to make sure the person is able to work with our toolchain. (This isn’t SBCL policy for GSoC. It’s simply how I feel about these things.)

Again, I’m amazed by the number of people who wish to hack on SBCL this summer (as part of Google’s Summer of Code or otherwise). Because of that, I think it’s important to note that this is our first year, and so we’ll likely not have more than two or three spots. However, I always like seeing more contributors, and I hope anyone who’d like to contribute will always be guided, GSoC or not.

Finally, I’ll note that Google’s Summer of Code program was only a good excuse to write up our list of projects: they’re simply suggestions to incite programmers to see what they can do that is useful for SBCL and, most importantly, is interesting for them. Anyone should feel welcome to work on any of these projects, even if they’re not eligible or chosen for GSoC. They’re also only suggestions; if someone has their own idea, we can likely help them out just the same.

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