Re: [Haskell-cafe] More Language.C work for Google's Summer of Code

That's very good to hear! When it comes to preprocessing and exact printing, I think that there are various stages of completeness that we could support. 1) Add support for parsing comments to the Language.C parser. Keep using an external pre-processor but tell it to leave comments in the source code. The cpphs pre-processor can do this. The trickiest bit here would have to do with where to record the comments in the AST. What AST node is a given comment associate with? We could probably come up with some general rules, and perhaps certain comments, in weird locations, would still be ignored. 2) Support correct column numbers for source locations. This falls short of complete macro support, but covers one of the key problems that macros introduce. The mcpp preprocessor [1] has a special diagnostic mode where it adds special comments describing the origin of code that resulted from macro expansion. If the parser retained comments, we could use this information to help with exact pretty- printing. 3) Modify the pretty-printer to take position information into account when pretty-printing (at least optionally). As long as macro definitions themselves (as well as #ifdef, etc.) are not in the AST, the output will still not be exactly the same as the input, but it'll come closer. 4) Add full support for parsing and expanding macros internally, so that both macro definitions and expansions appear in the Language.C AST. This is probably a huge project, partly because macros do not have to obey the tree structure of the C language in any way. This is perhaps beyond the scope of a summer project, but the other steps could help prepare for it in the future, and still fully address some of the problems caused by the preprocessor along the way. Do you think you'd be interested in some subset or variation of 1, 2, and 3? Are there other ideas you have? Things I've missed? Things you'd do differently? Thanks, Aaron [1] http://mcpp.sourceforge.net/ On Mar 30, 2010, at 1:46 PM, Edward Amsden wrote:

I'd be very much interested in working on this library for GSoC. I'm currently working on an idea for another project, but I'm not certain how widely beneficial it would be. The preprocessor and pretty-printing projects sound especially intriguing.

On Tue, Mar 30, 2010 at 1:30 PM, Aaron Tomb wrote:

...

Hello,

I'm wondering whether there's anyone on the list with an interest in doing additional work on the Language.C library for the Summer of Code. There are a few enhancements that I'd be very interested seeing, and I'd love be a mentor for such a project if there's a student interested in working on them.

The first is to integrate preprocessing into the library. Currently, the library calls out to GCC to preprocess source files before parsing them. This has some unfortunate consequences, however, because comments and macro information are lost. A number of program analyses could benefit from metadata encoded in comments, because C doesn't have any sort of formal annotation mechanism, but in the current state we have to resort to ugly hacks (at best) to get at the contents of comments. Also, effective diagnostic messages need to be closely tied to original source code. In the presence of pre-processed macros, column number information is unreliable, so it can be difficult to describe to a user exactly what portion of a program a particular analysis refers to. An integrated preprocessor could retain comments and remember information about macros, eliminating both of these problems.

The second possible project is to create a nicer interface for traversals over Language.C ASTs. Currently, the symbol table is built to include only information about global declarations and those other declarations currently in scope. Therefore, when performing multiple traversals over an AST, each traversal must re-analyze all global declarations and the entire AST of the function of interest. A better solution might be to build a traversal that creates a single symbol table describing all declarations in a translation unit (including function- and block-scoped variables), for easy reference during further traversals. It may also be valuable to have this traversal produce a slightly-simplified AST in the process. I'm not thinking of anything as radical as the simplifications performed by something like CIL, however. It might simply be enough to transform variable references into a form suitable for easy lookup in a complete symbol table like I've just described. Other simple transformations such as making all implicit casts explicit, or normalizing compound initializers, could also be good.

A third possibility, which would probably depend on the integrated preprocessor, would be to create an exact pretty-printer. That is, a pretty-printing function such that pretty . parse is the identity. Currently, parse . pretty should be the identity, but it's not true the other way around. An exact pretty-printer would be very useful in creating rich presentations of C source code --- think LXR on steroids.

If you're interested in any combination of these, or anything similar, let me know. The deadline is approaching quickly, but I'd be happy to work together with a student to flesh any of these out into a full proposal.

Thanks, Aaron

-- Aaron Tomb Galois, Inc. (http://www.galois.com) atomb@galois.com Phone: (503) 808-7206 Fax: (503) 350-0833

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