[llvm-dev] A query language for LLVM IR (XPath)

[Chris Lattner via llvm-dev]

It would be really interesting to see this as an extension of FileCheck.  Having the ability to write these sorts of predicates in CHECK: lines would be pretty cool, and could make existing regex checks a lot more principled.

-Chris

> On Oct 31, 2017, at 3:21 PM, Dean Michael Berris via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> As much as I'm not a fan of most XML things, this application of XPath is *inspired*.
> 
> This would be a great testing/query tool for tests.
> 
> It would also be a great way to prototype passes.
> 
> Looking forward to seeing something like this in llvm/tools/ !
> 
> Cheers
> 
>> On 1 Nov 2017, at 04:00, Sean Silva via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>> 
>> This is so cool! I once had a similar idea but the way I was thinking about it ended up more complex than I had time to implement (I sketched it here: http://lists.llvm.org/pipermail/llvm-dev/2013-November/067720.html <http://lists.llvm.org/pipermail/llvm-dev/2013-November/067720.html>).
>> 
>> Good idea using xpath to simplify the implementation and reuse existing languages/libraries as a starting point!
>> 
>> On Oct 29, 2017 6:47 AM, "Alessandro Di Federico via llvm-dev" <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>> Hi, sometimes when dealing with LLVM IR getting to a desired point of
>> the code is a bit cumbersome, in particular if you're instrumenting
>> existing code. A lot of nested loops and if checks.
>> 
>> Maybe all of this could be avoided by employing a query language. Since
>> an LLVM module can be seen as a sort of tree with attributes, I think
>> that reusing an existing query language for XML would be appropriate.
>> 
>> In particular I choose XPath [1] since it's more expressive than, say,
>> CSS selectors (e.g., you can move from the current element to the
>> parent).
>> 
>> Therefore, in a spare night, I took pugixml [2], a lightweight XML parser
>> with XPath support, stripped away everything was XML-specific and
>> adapted it so that it could query an arbitrary tree, as long as a class
>> providing certain traits is provided.
>> 
>> Attached you can find the class to query a LLVM module and example LLVM
>> module (using LLVM 3.8, but newer versions should do to).
>> 
>> The current implementation pretends that a module looks like the
>> following XML tree (more or less):
>> 
>>     <main.ll>
>>       <main>
>>         <basicblock1>
>>           <alloca />
>>           <alloca />
>>           ...
>>         </basicblock1>
>>         ...
>>       </main>
>>     </main.ll>
>> 
>> Additional information could be encoded in attributes.
>> Please note that the queries are done on the LLVM IR directly, no XML
>> tree is materialized.
>> 
>> In the following you can find some examples:
>> 
>>     $ # Find all the basic blocks containing at least an alloca
>>     $ llvm-xpath '/main/*[count(alloca) > 0]' main.ll
>> 
>>       %1 = alloca i32, align 4
>>       %2 = alloca i32, align 4
>>       %i = alloca i32, align 4
>>       store i32 0, i32* %1, align 4
>>       store i32 %argc, i32* %2, align 4
>>       %3 = load i32, i32* %2, align 4
>>       store i32 %3, i32* %i, align 4
>>       br label %4
>> 
>>     $ # Find all store instructions
>>     $ llvm-xpath '/*/*/store'
>>       store i32 0, i32* %1, align 4
>>       store i32 %argc, i32* %2, align 4
>>       store i32 %3, i32* %i, align 4
>>       store i32 %6, i32* %i, align 4
>> 
>> Obviously this doesn't have to be exclusively a command line tool, but
>> we could have something like:
>> 
>>     for (auto *Store : TheModule.xpath<StoreInst>("/*/*/store"))
>>       /* ... */
>> 
>> I'm not releasing the full code yet since it's very much work in
>> progress, but if anyone is interested in such a thing, just ping me.
>> The applications could range from using it in existing code to just
>> provide it for fast prototyping, e.g., in llvmcpy [3].
>> 
>> Obviously there are some open questions, such as how to deal with
>> operands, which could lead to an infinite tree, or how to organize
>> attributes. But it should be doable.
>> 
>> ---
>> Alessandro Di Federico
>> PhD student at Politecnico di Milano
>> 
>> [1] https://en.wikipedia.org/wiki/XPath <https://en.wikipedia.org/wiki/XPath>
>> [2] https://pugixml.org/ <https://pugixml.org/>
>> [3] https://github.com/revng/llvmcpy <https://github.com/revng/llvmcpy>
>> 
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> 
> -- Dean
> 
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