llvm dev - Feb 2021 - How to get from "opt -O3" to "opt <a-single-pass>" with the new PM?

Hi,

I just wrote a PR about a crash that I only see with the new PM:
 https://bugs.llvm.org/show_bug.cgi?id=49185

Normally (with the old PM) when I get a crash with e.g. "opt -O3" I
use
-debug-pass=Arguments to get the list of passes run, and then I usually
try to reduce the amount of passes with the goal to reach a simpler opt
command line just running one pass to get a small and stable
reproducer.

With the new PM I have no idea how to do this. -debug-pass=Arguments
doesn't do anything, it doesn't give an error either, it's just
silent.

What is the intended workflow to reach a proper nice and small
reproducer that has a minumum of dependencies towards different passes
with the new PM?

Thanks,
Mikael

> On Feb 15, 2021, at 09:33, Mikael Holmén via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> Hi,
> 
> I just wrote a PR about a crash that I only see with the new PM:
> https://bugs.llvm.org/show_bug.cgi?id=49185
> 
> Normally (with the old PM) when I get a crash with e.g. "opt -O3"
I use
> -debug-pass=Arguments to get the list of passes run, and then I usually
> try to reduce the amount of passes with the goal to reach a simpler opt
> command line just running one pass to get a small and stable
> reproducer.
> 
> With the new PM I have no idea how to do this. -debug-pass=Arguments
> doesn't do anything, it doesn't give an error either, it's just
silent.
> 
> What is the intended workflow to reach a proper nice and small
> reproducer that has a minumum of dependencies towards different passes
> with the new PM?
Unfortunately there’s nothing similar to bugpoint’s crashing pass reduction
feature that works for the new pass manager at the moment. I am also not aware
of anyone working on this.

For the new PM, there’s -debug-pass-manager, which prints the passes that run,
but not in a format suitable to feed back to `opt`.

Arthur, Alina, do you know if there’s a way to get a string of passes that
corresponds to a all passes in a given pipeline, which can be passed directly to
`opt`? With that, it might also be easier to build a llvm-reduce-based pass
reduction tool as well.

Cheers,
Florian