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

Mehdi Amini via llvm-dev
Thu, 30 Mar 2017 15:03:25 -0000

> On Mar 30, 2017, at 6:56 AM, Vassil Vassilev <v.g.vassilev at
gmail.com> wrote:
>
> Hi,
>
>   This seems a very exciting project.

Do I take that you’re volunteering to mentor it? ;-)


>
>   As part of GSoC16 Raphael developed a code clone detection tool
(https://docs.google.com/presentation/d/1mJ6dA6XmAQ8s8Zqm_j518yoW-_QZ72e69fPG1u_nbj8/edit#slide=id.g35f391192_00
<https://docs.google.com/presentation/d/1mJ6dA6XmAQ8s8Zqm_j518yoW-_QZ72e69fPG1u_nbj8/edit#slide=id.g35f391192_00>).
We are working on turning the infrastructure into a reusable set of components
(https://reviews.llvm.org/D23418 <https://reviews.llvm.org/D23418>).
>
>   Raphael hacked together a few lines of code, addressing Greg's
proposal based on D23418.
>
>
> r1	r2
> int main(int argc, const char **argv) {
>   switch (argc) {
>   }
>   if (argc > 2) {
>     return 1;
>   }
>   while (false);
>   int funkyVariable = 1;
>   funkyVariable++;
> }
> int main(int argc, const char **argv) {
>   if (argc > 2) {
>     return 1;
>   }
>   switch (argc) {
>   }
>   while (false);
>   int funkyVariable = 1;
>   funkyVariable++;
> }
>   ./clangDiff
>   Change: SwitchStmt moved from line 2 to line 5
>   Change: IfStmt moved from line 4 to line 2
>

Neat! :)


>
>   Here is how it looks
https://gist.github.com/Teemperor/b252bae4b2544f57d6bb9580a0e890e4
<https://gist.github.com/Teemperor/b252bae4b2544f57d6bb9580a0e890e4>
>
>   Let us know if we can help you further with this!

I’d be happy if you could take the lead. Johannes asked earlier how to start in
clang and show his ability, any bug to fix or small improvement to implement you
can suggest?
He also asked about libclang vs libtooling, not sure if anyone already answered.

—
Mehdi



>
> -- Vassil and Raphael
>
> On 23/03/17 18:41, Greg Clayton via llvm-dev wrote:
>> My original idea was to write a semantic diff tool that just does some
simple things up front:
>>
>> create an MD5 from all top level blocks of the code. Start by just
finding matching blocks of code ('{' and '}', '(' and
')') and remember the source locations for these and their MD5 values.
Run a normal diff on the code and see what blocks the diffs fall into. Then try
to figure out where things moved by possibly delving deeper into each block that
matched something from the diff. Also if any blocks moved to a completely
different location, try and figure that out by matching the MD5 of any blocks.
>>
>> For example if you had:
>>
>> int main(int argc, const char **argv) {
>>   if (argc > 2) {
>>   }
>>   switch (argc) {
>>   }
>> }
>>
>> You would first make MD5s for the '(' and ')' in the
"main" line and for the '{' at the end of the main line, and
ending at the end of the code. Now the code looks like:
>>
>> int main(int argc, const char **argv) {
>>   switch (argc) {
>>   }
>>   if (argc > 2) {
>>   }
>> }
>>
>> The diff would show that the "if" is gone and a new
"if" is found after the switch in the new version of the file. We
would notice that the diff appears inside the block from the first:
>>
>> {
>>   if (argc > 2) {
>>   }
>>   switch (argc) {
>>   }
>> }
>>
>> And in the block from the second:
>>
>> {
>>   switch (argc) {
>>   }
>>   if (argc > 2) {
>>   }
>> }
>>
>> So we would then compute the MD5 for the blocks inside each of these
blocks and try to match things up. The MD5 would of course remove spaces that
aren't in strings and only compute the MD5 from the characters that make
sense. This simple type of approach could almost work on any language without
the need to be able to correctly compile each file with all the right options.
>>
>> Greg
>>
>>> On Mar 20, 2017, at 4:47 PM, Mehdi Amini <mehdi.amini at
apple.com> <mailto:mehdi.amini at apple.com> wrote:
>>>
>>> (+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> <mailto: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
>>>
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