[llvm-dev] [GSoC 2017] Clang-based diff tool project

[Johannes Altmanninger via llvm-dev]

Hello,

I am currently studying Computer Science at TU Eindhoven. I am doing a
course that involves programming assignments on parts of LLVM such as
lowering, scheduling and optimization. For this year's Google Summer of
Code I plan to submit a proposal to implement a clang-based diff tool
[1].

I think it really pays off to have decent developer tools available, as
they can save tons of time. Clang tooling has obviously been very
successful.  I think it would be a good idea to develop a diff tool that
considers the structure of the code, as opposed to just the lines. Plain
old diff only thinks in terms of "additions" and "deletions", although
it would be more natural to also consider "updates" and "moves".

So a structural diff would work solely on the AST, hence formatting
changes are ignored. It would allow to highlight the exact location of a
change, and not a whole line. Furthermore, it would allow to compare
pieces of code with the same structure (think subclasses).

Besides some papers with clever AST-matching algorithms, a quick web
search yielded [2], which is a proof-of-concept implementation of a
structural comparison algorithm.  I think it demonstrates rather nicely
what could be done: movement of chunks of code can be easily traced.

Anyway, one could make all kinds of nice visualizations using a AST diff
tool, however, I think the initial focus should probably be on creating
one with a similar output to traditional diff, with the difference that
updates and moves are displayed in a easily readable way, which already
could improve developer productivity and happiness.

As of now I have one question: The output of the tool is meant just for
humans to read (and not for actual patching), right?

To sum up, this could be a very interesting project for me to work on,
and the result will hopefully be useful to a wide range of developers. I
would appreciate any feedback. Also, suggestions on how the diff output
should be presented are welcome. Thank you!

Johannes


[1] http://llvm.org/OpenProjects.html#clang-diff-tool
[2] https://yinwang0.wordpress.com/2012/01/03/ydiff/