The Shape of Code

A few weeks ago I received an inquiry about running a course/workshop on compiler writing. This does not does not happen very often and it reminded me that many years ago the ACCU asked if I would run a mentored group on compiler writing, I was busy writing a book at the time. The inquiry got me thinking it would be fun to run a compiler writing mentored group over a 6-9 month period and I emailed the general ACCU reflector asking if anybody was interested in joining such a group (any reader wanting to join the group has to be a member of the ACCU).

Over the weekend I had a brainwave for a project, automatic compiler test generation coupled with a program source code minimizer (I need a better name for this bit). Automatic test generation sounds great in theory but in practice whittling down the source code of those programs that result in a fault being exhibited, to create a usable sized test case that is practical for debugging purposes can be a major effort. What is needed is a tool to automatically do the whittling, i.e., a test case minimizer.

A simple algorithm for whittling down the source of a large test program is to continually throw away that half/third/quarter of the code that is not needed for the fault to manifest itself. A compiler project that took as input source code, removed half/third/quarter of the code and generated output that could be compiled and executed is realistic. The input/reduce/output process could be repeated until the generated source was considered to have reached some minima. Ok, this will soak up some cpu time, but computers are cheap and people are expensive.

Where does the test source code come from? Easy, it is generated from the same yacc grammar that the compiler, written by the mentored group member, uses to parse its input. Fortunately such a generation tool is available and ready to use.

The beauty is using the same grammar to generate tests and parse input. This means there is no need to worry about which language subset to use initially and support for additional language syntax can be added incrementally.

Experience shows that automatically generated test programs quickly uncover faults in production compilers, even when working with language subsets. Compiler implementors are loath to spend time cutting down a large program to find the statement/expression where the fault lies, this project will produce a tool that does the job for them.

So to recap, the mentored group is going to write one or more automatic source code generators that will be used to stress test compilers written by other people (e.g., gcc and Microsoft). Group members will also write their own compiler that reads in this automatically generated source code, throws some of it away and writes out syntactically/semantically correct source code. Various scripts will be be written to glue this all together.

Group members can pick the language they want to work with. The initial subset could just include supports for integer types, if-statements and binary operators.

If you had trouble making any sense all this, don’t join the group.