How to write a custom LTO pass?

I’d like to write an inter-modular analysis pass. My understanding is that this should take the form of an LTO pass. However, I have been unable to find any resources on the mechanics of writing and running a custom LTO pass. Does anyone have any pointers?

Thanks,
Steve

All you need to do is to write a ModulePass and modify the libLTO plugin to run it. Writing a ModulePass is described in the “How to Write an LLVM Pass” document in the LLVM documentation. To modify libLTO, you’ll need to read the documents on how to install it, and you’ll need to modify the plugin (in llvm/tools/LTO, I think). Alternatively, you can modify the LLVM library that defines which passes get run when you use -O2 and -flto optimization, but the result is the same: you’ll need to install a new libLTO plugin. Regards, John Criswell

It works in debug mode. I left the std::cout statements in and compiled in
sim mode. ir2v aborted.

A similar abort happened with more than 8 memcpy’s to cpu_ptrs. I don’t know if
there are other limits we should be diagnosing or documenting.

-jon

I'd like to write an inter-modular analysis pass. My understanding is that
this should take the form of an LTO pass. However, I have been unable to
find any resources on the mechanics of writing and running a custom LTO
pass. Does anyone have any pointers?

All you need to do is to write a ModulePass and modify the libLTO plugin
to run it. Writing a ModulePass is described in the "How to Write an LLVM
Pass" document in the LLVM documentation.

Great, that makes sense.

To modify libLTO, you'll need to read the documents on how to install it,
and you'll need to modify the plugin (in llvm/tools/LTO, I think).

Alternatively, you can modify the LLVM library that defines which passes

get run when you use -O2 and -flto optimization, but the result is the
same: you'll need to install a new libLTO plugin.

Just to check my understanding: I will have to change LLVM source code
(probably to some .cpp file in llvm/tools/LTO or in llvm/lib/LTO) and
recompile in order to run my custom LTO pass? I'm double checking because
for custom intra-modular analysis passes, it is not a requirement to change
LLVM source code. Instead, one just adds the name of the .so to the opt
invocation. (
http://llvm.org/docs/WritingAnLLVMPass.html#running-a-pass-with-opt).

Thanks,
Steve

The opt command (which is really a utility program and not part of the compiler toolchain proper) loads LLVM passes; I believe Clang (which is part of the compiler toolchain proper) can as well. However, to the best of my knowledge, there is no way to tell the gold linker to load another .so file with an LLVM pass and run it during link-time optimization. If there is a way, I’d like to know what it is. :slight_smile: Regards, John Criswell