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

[Mats Larsen via llvm-dev]

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.

Best regards
Mats

On Thu, Apr 22, 2021 at 11:17 PM Arthur Eubanks via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Splitting this off from the other thread.
>
> We should have a deprecation timeline that revolves around the major
> releases. I'd say we announce the legacy PM for the optimization pipeline
> to be deprecated the major release after the new PM has been the default.
> Looks like the new PM switch made it into LLVM 12.
>
> The major blocker right now is that the C API doesn't have hooks into the
> new PM infrastructure. llvm/include/llvm-c/Transforms/PassManagerBuilder.h
> should be fairly straightforward to port to the new PM, but the API to add
> individual passes (e.g. llvm/include/llvm-c/Transforms/Scalar.h) needs to
> distinguish between the different types of passes, e.g. module vs function
> pass. The new PM has explicit pass nesting, so we'll need to make sure that
> we add a function pass to a function pass manager. Or we could simplify
> things and force each pass added via the C API to run in isolation (e.g.
> two adjacent function passes would run completely separately rather than
> being interleaved function-by-function), which doesn't match how pipelines
> are constructed everywhere else, but it's already an adhoc API.
>
> At some point after the deprecation announcement we should start cleaning
> up tests for passes in the optimization pipeline to use `opt -passes=foo`
> rather than `opt -foo`.
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