[llvm-dev] Testing utility for building and updating CFG

Jakub (Kuba) Kuderski via llvm-dev llvm-dev at lists.llvm.org
Wed Jun 28 18:47:42 PDT 2017

Tobias, David,

Thanks for sharing your opinions and concerns.

After some prototyping and an offline discussion with David, I think that
it would be better to take a few steps back of the initial proposal. At
this point it is not clear if and how such a tool would be useful for
testing things other than dominators. It would certainly be a burden to
commit and later maintain even a very small and simplistic DSL, especially
if it was to be exposed by other tools (e.g. opt).

I believe that a better way forward would be to create a small and simple
library-only solutions used only by dominator tests -- at least for now. I
created a prototype which you can find here: https://reviews.llvm.org/D34798

With this design it shouldn't be expensive to extend it later, when it
gains actual users (like the mentioned LoopInfo and NewGVN), or to even
create a think wrapper for handling a custom DSL.

Please let me know what you think.


On Wed, Jun 28, 2017 at 7:39 AM, David Blaikie <dblaikie at gmail.com> wrote:

> Off the cuff, I'd probably err on the side of improving/simplifying the
> API for building/querying the CFG if it's possible to go far enough along
> that route to address the issues (fuzzing could still be done there - by
> taking a byte array from libFuzzer and using the bytes to drive API calls
> to build a CFG, for example (is this byte less than <some constant> -> make
> an edge, etc)). But I realize there might be good reasons it's not
> practical - it'd be worth understanding/documenting those reasons before
> starting on a new tool/format/etc, I think.
> On Tue, Jun 27, 2017 at 3:32 PM Jakub (Kuba) Kuderski via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>> Hi folks,
>> I’m working on adding an API for incremental updates to DominatorTree
>> and I noticed that there isn’t a simple way to quickly build and update CFG
>> (just for testing) -- one has to either build CFG programmatically, or
>> write IR by hand and manually map pointers to basic blocks. The downside
>> is that it tends to be pretty verbose and not easy to update (e.g. adding a
>> new edge often involves changing the type of terminator instruction).
>> My idea is to create a simple format for building arbitrary CFG’s and new
>> utilities for updating it.
>> It can look something like this:
>> b ModuleName FunctionName // Build module ‘ModuleName’ with a single
>>                          // function ‘FunctionName’.
>> a entry if                // Add new basic blocks (entry and if) and
>>                          // connect them with an edge.
>> a if if.then              // Add new basic block (if.then) and create
>>                          // an edge from if to if.then
>> a if if.else
>> a if.then foo
>> a if.else foo
>> a bar                     // Add a new block bar, but don’t create
>>                          // any edges.
>> e                         // Finish constructing initial CFG
>> i foo bar                 // Insert and edge from foo to bar.
>> d if if.then              // Delete the edge from if to if.then.
>> i if bar                  // Insert an edge from if to bar.
>> Under the hood, the utility would build IR with just switch instructions.
>> Then you could assign it to a string and use in a unit test:
>> CFGBuilder B(MyString);
>> Function *F = B.getFunction();
>> while (!B.finished()) {
>>    Update U = B.applyNextUpdate(); // B changes the CFG.
>> // U stores type of update (insertion/deletion) and a pair of BB’s
>> // (From and To).
>>    doSomethingWithTheUpdate(U);
>> }
>> Other CFG passes (e.g. LoopInfo, NewGVN) could also use it for testing.
>> It would be also possible to hook it up to a fuzzer at some point in the
>> future.
>> What do you think about having such a utility in llvm?
>> ~Kuba
>> --
>> Jakub Kuderski
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

Jakub Kuderski
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