Calling pass from Clang

[palmerc]

When you call opt directly you use --passes and the list of desired passes, but in some older code I've seen you could call from clang with -mllvm followed by some additional options added directly to PassManagerBuilder in the form of cl::opt items. This was also how you could then pass along additional options to the pass. This code was using the legacy pass manager. What is the current way to specify the passes and options when using clang?

[banach-space]

Hello @palmerc ,

Thank you for stopping by! :) Unfortunately, the short answer is that I don't know.

Running plugins from clang It's a very important use-case and I am curious myself. I've labeled this as an enhancement and will try to produce some examples and/or docs (or at least get a better understanding of the available options). I can't commit to any timelines and volunteers are always very welcome :)

Passing command line options to plugins with clang All of llvm-tutor relies on opt to load and run the plugins. clang is a completely different driver, so the underlying logic around command-line options is also very different. I haven't really looked into it yet.

In opt, AFAIK, it is not possible to pass command line options to plugins when using the new PM. This was brought up in the past:

• https://lists.llvm.org/pipermail/llvm-dev/2019-September/134973.html (slightly different question, but the answer applies here as well)
• https://www.reddit.com/r/LLVM/comments/eh6m1t/new_pass_manager_new_interesting_behavior_with/

This might have changed since I looked into it originally (LLVM ~9).

-Andrzej

[palmerc]

Cool! I’m looking into this and I will report my findings.

On 17 Jul 2021, at 17:34, Andrzej Warzyński @.***> wrote:

 Hello @palmerc ,

Thank you for stopping by! :) Unfortunately, the short answer is that I don't know.

Running plugins from clang It's a very important use-case and I am curious myself. I've labeled this as an enhancement and will try to produce some examples and/or docs (or at least get a better understanding of the available options). I can't commit to any timelines and volunteers are always very welcome :)

Passing command line options to plugins with clang All of llvm-tutor relies on opt to load and run the plugins. clang is a completely different driver, so the underlying logic around command-line options is also very different. I haven't really looked into it yet.

In opt, AFAIK, it is not possible to pass command line options to plugins when using the new PM. This was brought up in the past:

https://lists.llvm.org/pipermail/llvm-dev/2019-September/134973.html (slightly different question, but the answer applies here as well) https://www.reddit.com/r/LLVM/comments/eh6m1t/new_pass_manager_new_interesting_behavior_with/ This might have changed since I looked into it originally (LLVM ~9).

-Andrzej

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[palmerc]

Initial look. You can get options back to LLVM by calling clang with -mllvm followed by the option. You would then need to add your pass into PassManagerBuilder by specifying an option cl::opt and then in void PassManagerBuilder::populateModulePassManager(legacy::PassManagerBase &MPM) you would call MPM.add() there is a function pass analogue named populateFunctionPassManager

[banach-space]

Thanks for the update! Do you know where are options for LLVM defined? (e.g. -load). And who is responsible for calling populateModulePassManager? Is that supposed to happen in a plugin?

[banach-space]

FYI: https://lists.llvm.org/pipermail/cfe-dev/2020-November/067166.html

[KuanKuanQAQ]

Sorry to bother you both. Recently, I've been trying to use LLVM to compile the Linux kernel and integrate my LLVM pass into some kernel modules to enhance security. As far as I know, it's not feasible to use opt when compiling the kernel, so I specifically need a method to invoke the LLVM pass via clang. I noticed that this issue has been around for a few years and was wondering if there have been any updates.

[KuanKuanQAQ]

In fact, I have already made some attempts. For example, using HelloWorld.cpp and the registerVectorizerStartEPCallback function, it is possible to invoke this LLVM pass with clang.

llvm::PassPluginLibraryInfo getHelloWorldPluginInfo() {
  return {LLVM_PLUGIN_API_VERSION, "HelloWorld", LLVM_VERSION_STRING,
          [](PassBuilder &PB) {
            PB.registerVectorizerStartEPCallback(
                [](FunctionPassManager &PM, OptimizationLevel Level) {
                  PM.addPass(HelloWorld());
                });
          }};
}

We can compile like this: clang -fpass-plugin=libHelloWorld.so input_for_hello.c

[banach-space]

Sounds like you have a solution?

[KuanKuanQAQ]

I’m not sure if this is an official method, as it only allows the use of register*EPCallback and not registerAnalysisRegistrationCallback or registerPipelineParsingCallback. This means that only transform passes can be registered, and not analysis passes. I believe there should be an official method to pass arguments to opt, but when I tested it, neither -mllvm nor -Xclang worked.

[banach-space]

This is more of a question for https://discourse.llvm.org/ ;-) Please post there.