[llvm-dev] OptBisect implementation for new pass manager

Kaylor, Andrew via llvm-dev
Wed, 26 Sep 2018 23:18:59 -0000

We absolutely need to be able to generate executable programs using opt-bisect,
so some mechanism for not skipping required passes is needed. It might be nice
to have a mode where no passes are skipped and the IR/MIR is dumped when the
bisect limit is reached, but I don't see that as a requirement.

Regarding opt-in versus opt-out, I think we want to make this as easy and
transparent to pass developers as possible. It would be nice to have the
mechanism be opt-out so that passes that were added with no awareness of
opt-bisect would be automatically included. However, there is a small wrinkle to
this. I can't defend this as a reasonable design choice, but the
SelectionDAGISel pass has a sort of hybrid behavior. It can't actually be
skipped, but it OptBisect says it should be skipped it drops the optimization
level to OptNone. That's a machine function pass, so it doesn't matter
so much right now. It's just something to think about.

One of the reasons that we combined the optnone handling and the opt-bisect
handling is that we specifically wanted these two behaviors to be linked. The
exact rule we would like to use for opt bisect is that no pass which runs at O0
is skipped by opt-bisect. There's a test that verifies this. Conversely, if
a pass is able to respect the optnone attribute then it should also be skippable
by opt-bisect. Of course, I would be open to considering a use case where this
reasoning isn't desirable.


With regard to there being one OptBisect object per compilation pipeline, I have
some concerns. Specifically, the behavior of opt-bisect depends on the sequence
of passes run before the limit is reached being consistent and repeatable. My
inclination would be to not allow parallel compilation when opt-bisect is
enabled. I can imagine cases where you might specifically want to debug
something that only happens in a parallel build, but it's more difficult to
imagine something that only happens in a parallel build and doesn't depend
on interactions between threads. In such a case, would we be able to guarantee
that the sequence of passes and any interaction between pipelines was
repeatable. Basically, here I feel like I'm exploring a hypothetical idea
where other people have specific use cases. If so, please explain the use case
to me.

-Andy

-----Original Message-----
From: Fedor Sergeev [mailto:fedor.sergeev at azul.com]
Sent: Wednesday, September 26, 2018 9:54 AM
To: llvm-dev <llvm-dev at lists.llvm.org>; Zhizhou Yang <zhizhouy at
google.com>; David Greene <dag at cray.com>; David Blaikie <dblaikie
at gmail.com>; Kaylor, Andrew <andrew.kaylor at intel.com>; Chandler
Carruth <chandlerc at gmail.com>
Subject: OptBisect implementation for new pass manager

Greetings!

As the generic Pass Instrumentation framework for new pass manager is finally
*in*, I'm glad to start the discussion on implementation of -opt-bisect
through that framework.

As it has already been discovered while porting other features (namely,
-time-passes)
blindly copying the currently existing legacy implementation is most likely not
a perfect way forward. Now is a chance to take a fresh look at the overall
approach and perhaps do better, without the restrictions that legacy pass
manager framework imposed on the implementation.

Kind of a summary of what we have now:
   - There is a single OptBisect object, requested through LLVMContext
     (managed as ManagedStatic).

   - OptBisect is defined in lib/IR, but does use analyses,
     which is a known layering issue

   - Pass hierarchy provides skipModule etc helper functions

   - Individual passes opt-in to OptBisect activities by manually calling skip*
helper functions
     whenever appropriate

With current state of new-pm PassInstrumentation potential OptBisect
implementation will have the following properties/issues:
   - OptBisect object that exists per compilation pipeline, managed similar to
PassBuilder/PassManagers
     (which makes it more suitable for use in parallel compilations)

   - no more layering issues imposed by implementation since instrumentations by
design
     can live anywhere - lib/Analysis, lib/Passes etc

   - since Codegen is still legacy-only we will have to make a joint
implementation that
     provides a sequential passes numbering through both new-PM IR and legacy
Codegen pipelines

   - as of right now there is no mechanism for opt-in/opt-out, so it needs to be
designed/implemented
     Here I would like to ask:
         - what would be preferable - opt-in or opt-out?

         - with legacy implementation passes opt-in both for bisect and
attribute-optnone support at once.
           Do we need to follow that in new-pm implementation?

Also, I would like to ask whether people see current user interface for
opt-bisect limiting?
Do we need better controls for more sophisticated bisection?
Basically I'm looking for any ideas on improving opt-bisect user experience
that might affect design approaches we take on the initial implementation.

regards,
   Fedor.