[llvm-dev] A query language for LLVM IR (XPath)
Petr Penzin via llvm-dev
llvm-dev at lists.llvm.org
Tue Oct 31 22:42:01 PDT 2017
At some point I tried doing something like this for debug information
verification (via XML, the effort did not go very far). I would be very
interested in using and contributing to this code.
-Petr
On 10/31/2017 22:16, Chris Lattner via llvm-dev wrote:
> 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 <mailto: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/
>>> [3] https://github.com/revng/llvmcpy
>>> <https://github.com/revng/llvmcpy>
>>>
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>>>
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>>
>> -- Dean
>>
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