[llvm-dev] Legacy PM deprecation for optimization pipeline timeline

[Arthur Eubanks via llvm-dev]

On Mon, Apr 26, 2021 at 1:48 PM Mats Larsen <me at supergrecko.com> wrote:

> Hello everyone
>
> I wouldn't mind looking into hooking the new PM infrastructure into the
> LLVM C API. However, I would like to know a little more about the task.
> I've had a look at the new PM infrastructure, and from taking a quick
> glance at the available APIs and how it's used in the tests I assume we'd
> look to do something similar to the existing legacy PM hooks.
>
> Things are a bit different as far as I've understood because we have
> Module, CGSCC, Function, and Loop passes, each of which may be adapted into
> "higher" kinds, eg FunctionPM -> ModulePM, all of which differs a bit from
> the legacy PM. From what I've gathered, we'll need to set up the PMs
> themselves as you mentioned and we'll have to hook a set (all?) of the new
> PM passes into this. Would it be possible to get a more comprehensive list
> of what needs to be done?
>
> I'm pretty new around here so I'll probably require some guidance further
> down the line.
>>
>> <https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev>
>>
> Awesome, I'm happy to do reviews and answer any further questions.

I'm imagining a new PassBuilder.h in llvm/include/llvm-c where
PassManagerBuilder.h was.
Initially I was thinking we'd need wrappers around the various types of
pass managers as you've mentioned, as well as the adaptors
(e.g. createModuleToFunctionPassAdaptor()). And each of the LLVMAdd*Pass()
would need a corresponding new PM alternative that adds the pass to the
proper pass manager type.

But now that I think about it some more, we might be able to bypass all of
that and instead use the new PM's pass pipeline text parsing (the same
thing that `opt -passes=foo` uses). This seems exactly the sort of thing it
would be useful for. We'll bypass the need to create wrappers for the
different types of pass managers since we just get back a ModulePassManager
from the PassBuilder.
The code in NewPMDriver.cpp should be a good starting place to figure out
what's necessary to create a PassBuilder, then how to get a
ModulePassManager from it and run it on some IR.
I imagine first you'll need some way to setup the options to pass to the
PassBuilder, so the C wrapper would probably own various things like the
various
*AnalysisManagers, PassInstrumentationCallbacks, StandardInstrumentations,
and PipelineTuningOptions. Then the user can ask for some pipeline via a
string, whether that's an individual pass like "instcombine" or a full
pipeline like "default<O2>", and you create a PassBuilder from the
options, then tell it to create the ModulePassManager via
PassBuilder::parsePassPipeline().
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