LLVM how to execute code in a module before any other code

Question

I want to instrument some code that gets executed before any other code in my module.

I thought about calling the code in the start of the main function. But there is not always a main function or it is not always named "main". Or it is a library and it doesn't even have a main function.

Are there some other, smarter ways?

http://llvm.org/docs/LangRef.html#the-llvm-global-ctors-global-variable — Ismail Badawi, Oct 09 '16 at 18:45
@IsmailBadawi that seems to be a good answer. Do you have any idea how I can append to this variable from an compiler pass? — Fee, Oct 09 '16 at 19:52

score 3 · Accepted Answer · edited May 23 '17 at 12:16

3

You can put the code you want to run early into a function and add that function to llvm.global_ctors. This is the equivalent of using __attribute__((constructor)) in C or C++.

To do this from a pass, you can use the llvm::appendToGlobalCtors function, which is declared in llvm/Transforms/Utils/ModuleUtils.h.

edited May 23 '17 at 12:16

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answered Oct 09 '16 at 19:58

Ismail Badawi

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LLVM how to execute code in a module before any other code

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