CODEWITHSUNDEEP
c++clangllvmllvm-irlibtooling
user2333829
user2333829

Reputation: 1401

How can I make the LLVM IR of a function available to my program?

I'm working on a library which I'd like certain introspection features to be available. Let's say I'm compiling with clang, so I have access to libtooling or whatever.

What I'd like specifically is for someone to be able to view the LLVM IR of an already-compiled function as part of the program. I know that, when compiling, I can use -emit-llvm to get the IR. But that saves it to a file. What I'd like is for the LLVM IR to be embedded in and retrievable from the program itself -- e.g. my_function_object.llvm_ir()

Is such a thing possible? Thanks!

Upvotes: 1

Views: 565

Answers (1)

Theodoros Chatzigiannakis
Theodoros Chatzigiannakis

Reputation: 29233

You're basically trying to have reflection to your program. Reflection requires the existence of metadata in your binary. This doesn't exist out of the box in LLVM, as far as I know.

To achieve an effect like this, you could create a global key-value dictionary in your program, exposed via an exported function - something like IRInstruction* retrieve_llvm_ir_stream(char* name).

This dictionary would map some kind of identifier (for example, the exported name) of a given function to an in-memory array that represents the IR stream of that function (each instruction represented as a custom IRInstruction struct, for example). The types and functions of the representation format (like the custom IRInstruction struct) will have to be included in your source.

At the step of the IR generation, this dictionary will be empty. Immediately after the IR generation step, you'll need to add a custom build step: open the IR file and populate the dictionary with the data - for each exported function of your program, inject its name as a key to the dictionary and its IR stream as a value. The IR stream would be generated from the definitions of your functions, as read by your custom build tool (which would leverage the LLVM API to read the generated IR and convert it to your format).

Then, proceed to the assembler and linker as before.

Upvotes: 2

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