<html><head><meta http-equiv="Content-Type" content="text/html charset=us-ascii"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class="">As much as I'm not a fan of most XML things, this application of XPath is *inspired*.<div class=""><br class=""></div><div class="">This would be a great testing/query tool for tests.</div><div class=""><br class=""></div><div class="">It would also be a great way to prototype passes.</div><div class=""><br class=""></div><div class="">Looking forward to seeing something like this in llvm/tools/ !</div><div class=""><br class=""></div><div class="">Cheers</div><div class=""><br class=""><div><blockquote type="cite" class=""><div class="">On 1 Nov 2017, at 04:00, Sean Silva via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org" class="">llvm-dev@lists.llvm.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div dir="auto" class="">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: <a href="http://lists.llvm.org/pipermail/llvm-dev/2013-November/067720.html" target="_blank" class="">http://lists.llvm.org/<wbr class="">pipermail/llvm-dev/2013-<wbr class="">November/067720.html</a>).<div dir="auto" class=""><br class=""></div><div dir="auto" class="">Good idea using xpath to simplify the implementation and reuse existing languages/libraries as a starting point!</div></div><div class="gmail_extra"><br class=""><div class="gmail_quote">On Oct 29, 2017 6:47 AM, "Alessandro Di Federico via llvm-dev" <<a href="mailto:llvm-dev@lists.llvm.org" target="_blank" class="">llvm-dev@lists.llvm.org</a>> wrote:<br type="attribution" class=""><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">Hi, sometimes when dealing with LLVM IR getting to a desired point of<br class="">
the code is a bit cumbersome, in particular if you're instrumenting<br class="">
existing code. A lot of nested loops and if checks.<br class="">
<br class="">
Maybe all of this could be avoided by employing a query language. Since<br class="">
an LLVM module can be seen as a sort of tree with attributes, I think<br class="">
that reusing an existing query language for XML would be appropriate.<br class="">
<br class="">
In particular I choose XPath [1] since it's more expressive than, say,<br class="">
CSS selectors (e.g., you can move from the current element to the<br class="">
parent).<br class="">
<br class="">
Therefore, in a spare night, I took pugixml [2], a lightweight XML parser<br class="">
with XPath support, stripped away everything was XML-specific and<br class="">
adapted it so that it could query an arbitrary tree, as long as a class<br class="">
providing certain traits is provided.<br class="">
<br class="">
Attached you can find the class to query a LLVM module and example LLVM<br class="">
module (using LLVM 3.8, but newer versions should do to).<br class="">
<br class="">
The current implementation pretends that a module looks like the<br class="">
following XML tree (more or less):<br class="">
<br class="">
<main.ll><br class="">
<main><br class="">
<basicblock1><br class="">
<alloca /><br class="">
<alloca /><br class="">
...<br class="">
</basicblock1><br class="">
...<br class="">
</main><br class="">
</main.ll><br class="">
<br class="">
Additional information could be encoded in attributes.<br class="">
Please note that the queries are done on the LLVM IR directly, no XML<br class="">
tree is materialized.<br class="">
<br class="">
In the following you can find some examples:<br class="">
<br class="">
$ # Find all the basic blocks containing at least an alloca<br class="">
$ llvm-xpath '/main/*[count(alloca) > 0]' main.ll<br class="">
<br class="">
%1 = alloca i32, align 4<br class="">
%2 = alloca i32, align 4<br class="">
%i = alloca i32, align 4<br class="">
store i32 0, i32* %1, align 4<br class="">
store i32 %argc, i32* %2, align 4<br class="">
%3 = load i32, i32* %2, align 4<br class="">
store i32 %3, i32* %i, align 4<br class="">
br label %4<br class="">
<br class="">
$ # Find all store instructions<br class="">
$ llvm-xpath '/*/*/store'<br class="">
store i32 0, i32* %1, align 4<br class="">
store i32 %argc, i32* %2, align 4<br class="">
store i32 %3, i32* %i, align 4<br class="">
store i32 %6, i32* %i, align 4<br class="">
<br class="">
Obviously this doesn't have to be exclusively a command line tool, but<br class="">
we could have something like:<br class="">
<br class="">
for (auto *Store : TheModule.xpath<StoreInst>("/*<wbr class="">/*/store"))<br class="">
/* ... */<br class="">
<br class="">
I'm not releasing the full code yet since it's very much work in<br class="">
progress, but if anyone is interested in such a thing, just ping me.<br class="">
The applications could range from using it in existing code to just<br class="">
provide it for fast prototyping, e.g., in llvmcpy [3].<br class="">
<br class="">
Obviously there are some open questions, such as how to deal with<br class="">
operands, which could lead to an infinite tree, or how to organize<br class="">
attributes. But it should be doable.<br class="">
<br class="">
---<br class="">
Alessandro Di Federico<br class="">
PhD student at Politecnico di Milano<br class="">
<br class="">
[1] <a href="https://en.wikipedia.org/wiki/XPath" rel="noreferrer" target="_blank" class="">https://en.wikipedia.org/wiki/<wbr class="">XPath</a><br class="">
[2] <a href="https://pugixml.org/" rel="noreferrer" target="_blank" class="">https://pugixml.org/</a><br class="">
[3] <a href="https://github.com/revng/llvmcpy" rel="noreferrer" target="_blank" class="">https://github.com/revng/<wbr class="">llvmcpy</a><br class="">
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