[llvm-dev] New pass manager, documentation?

[Jon Chesterfield via llvm-dev]

Hi,

I'm trying to work out how to use the new pass manager. For example, what is the rough equivalent to addPreISel() in the old one for running target specific passes shortly before instruction selection, or how do I run a module pass shortly before a function pass.

I have a module pass which works if run shortly before instruction selection, but not if it is run via EP_ModuleOptimizerEarly. My current plan is to make the pass significantly more complicated, so that it can handle being run repeatedly from suboptimal points in the pipeline, but I can keep the simple/stupid version instead if I can work out how the pass manager works.

Thanks,

Jon

[David Blaikie via llvm-dev]

+Arthur

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[via llvm-dev]

> For example, what is the rough equivalent to addPreISel() in the old one for running target specific passes shortly before instruction selection

Hello,

Code generation passes including target-specific IR passes (usually these specified in derived TargetPassConfig class) are not using the new pass manager. I'm working on switching the code generation to use the new pass manager but it is not quite there yet. I guess we should document this fact in the new pass manager documentation because I think this question comes up once in a while.

- Yuanfang

________________________________________
From: llvm-dev <llvm-dev...@lists.llvm.org> on behalf of Jon Chesterfield via llvm-dev <llvm...@lists.llvm.org>
Sent: Wednesday, February 24, 2021 9:31 AM
To: llvm-dev
Subject: [llvm-dev] New pass manager, documentation?

Hi,

Thanks,

Jon

[Arthur Eubanks via llvm-dev]

Yes we should definitely document this somewhere. Does it make sense to add onto https://llvm.org/docs/WritingAnLLVMBackend.html? Or create a new page since these hooks aren't only used by backends, but also used by Clang to add sanitizers and other Clang-specific passes?

[Arthur Eubanks via llvm-dev]

Oh and to answer the original question, +1 to what Yuanfang said, but you mentioned considering EP_ModuleOptimizerEarly, so what about keeping it in the middle-end at EP_OptimizerLast (the NPM equivalent being registerOptimizerLastEPCallback()) rather than moving it to the backend? I'm not sure if you consider your pass to be a middle-end or a backend pass.

[Jon Chesterfield via llvm-dev]

Thank you all. That is interesting in several ways - I had missed the subtlety that the middle end is changing / has changed to the new pass manager before the back end.

In the immediate sense, this solves my problem because I therefore don't need to run the pass through the new manager after all. I can leave the plumbing in place so that it can be run through opt with the new pass manager, but don't need it to actually be run. Once the back ends can/do use the new manger there will hopefully be a corresponding hook I can put the invocation. That can be tomorrow's task however.

Thank you for the suggestion of EP_OptimizerLast. It doesn't help this particular pass but likely will others.

Re: updating docs, I found the link to the new pass manager, https://llvm.org/docs/WritingAnLLVMNewPMPass.html, so would have benefited from a warning there that this is not yet in use across back ends.

Thanks!

Jon

[Arthur Eubanks via llvm-dev]

https://reviews.llvm.org/D97515 is adding some documentation around using the new PM, reviews for clarity/anything missing are appreciated :)