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

Greg Clayton via llvm-dev
Thu, 23 Mar 2017 17:41:26 -0000

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>
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> 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
>