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

[Johannes Altmanninger via llvm-dev]

So I have been playing around with this some more.

> Yes. But we developed software as libraries usually. Practically I
> expect the main part of the work to write some piece of API that
> generate an “in-memory” representation of the diff.

Yes, of course, I think the representation will be just a set of
changes, where a change is a insert/update/removal of a node.

> In the future I hope it’d enable other graphical diff client to
> plug-in, or git-merge resolution tools as well.
Yes, merge tools would be a nice application for this, as Hal also
pointed out. Of course it is easy to make the output machine readable or
just use the API in the first place.

Unfortunately it is not possible to (de-)serialize the AST from/to XML
(or some other structured data format). If it was, one could use
existing structural diff tools to make prototypes or help test the final
version.

I have another question as I have never written a clang tool before: One
could use LibClang or LibTooling, the former seems easier and probably
good enough (?)

Furthermore, should I work on a small patch / fix an easy bug to show
that I am comfortable with the development process? I am a bit lost with
the bug tracker, so if you can suggest things to work I will try that.
Other than that I can work on a prototype for the tool but that takes
some time.  I found one small typo in clang, I guess I could submit
that..

Johannes


diff --git a/include/clang/AST/RecursiveASTVisitor.h b/include/clang/AST/RecursiveASTVisitor.h
index 1b5850a05b..c31e4b0bb4 100644
--- a/include/clang/AST/RecursiveASTVisitor.h
+++ b/include/clang/AST/RecursiveASTVisitor.h
@@ -83,7 +83,7 @@ namespace clang {
       return false;                                                            \
   } while (false)

-/// \brief A class that does preordor or postorder
+/// \brief A class that does preorder or postorder
 /// depth-first traversal on the entire Clang AST and visits each node.
 ///
 /// This class performs three distinct tasks:



Mehdi Amini <mehdi.amini at apple.com> writes:

> (+CC: Greg Clayton who gave me this idea in the first place)
>
>> On Mar 20, 2017, at 3:20 PM, Johannes Altmanninger via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>> 
>> 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].
>
> Great! I look forward to see this :)
>
>> 
>> 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?
>
> Yes. But we developed software as libraries usually. Practically I expect the main part of the work to write some piece of API that generate an “in-memory” representation of the diff.
>
> A tool that is generating a textual-human readable output is likely the first client of this API and is likely critical to be able to functionally test it in the early development. In the future I hope it’d enable other graphical diff client to plug-in, or git-merge resolution tools as well.
>
> Best,
>
> — 
> Mehdi