How can I map OCaml bytecode to its original source code location?

Question

Is there some nice feature of the format or library for going from some part of the bytecode to the line of code it originally came from? This would obviously be useful for debugging and error messages.

In particular, I'm looking at how hard it would be to add support for source maps to js_of_ocaml.

Answer 1

When compiled with debug information enabled (option -g), the bytecode carries so-called "event" structures marking for example the function entry and return point, which provide source location and typing information.

As a proof of concept of how to inspect this information, I have created a small branch of the ocamlpp tool (a small utility by Benoît Vaugon to inspect bytecode files) that prints this debug information alongside the bytecode instructions.

I have no idea whether js_of_ocaml takes the necessary steps to preserve this location information throughout the compilation process. You should probably contact the maintainer, Jérôme Vouillon, to ask for more information.

Answer 2

js_of_ocaml -debuginfo uses debug_event in a bytecode to write the line of code in comment.