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

[Dean Michael Berris via llvm-dev]

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> 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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