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

[Mikael Holmén via llvm-dev]

On Wed, 2021-02-24 at 11:25 -0800, Arthur Eubanks wrote:
> There's no way to get a list of passes. Currently the name used to
> specify a pass, e.g. "instcombine" in -passes=instcombine is mostly a
> one way street to creating the pass, it's hard to go backwards from a
> pass to the original name due to the implementation. This could be
> fixed, but it'd be a fairly large change.
> 
> Personally I'd prefer an addition to https://reviews.llvm.org/D86657
> where we dump the IR on a crash to a file. Then with -debug-pass-manager, we can see which pass caused the crash. Then use llvm-reduce to reduce the IR, running only that one pass.
> 

Ok. So more or less the tool we have is -print-before-all -debug-pass-
manager and copy/paste the last input and rerun with the last
interesting pass. -print-on-crash makes it a little bit more convenient
when a pass crashes. I suppose this will work ok in many cases.


Another case is when a number of passes is needed to make a certain
pass expose something interesting.

Now and then I run into cases where
 opt -pass1 ... -passN-1 -passN
is needed to trigger something and splitting the opt command to
 opt -pass1 ... -passN-1 | opt passN
doesn't expose the problem. And the way I then find the needed passes
is to get the passlist with -debug-pass=Arguments and reduce the list
as far as I can. This is quite is rare though.

Thanks,
Mikael

> On Wed, Feb 24, 2021 at 1:28 AM Florian Hahn <florian_hahn at apple.com>
> wrote:
> > 
> > > 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