[llvm-dev] New pass manager for optimization pipeline status and questions

Wed Jul 29 03:37:35 PDT 2020

Hi,

> Yes, I think big performance and code-size regressions need to be investigated. It will be hard to quantify what "big" is though, but we should definitely track such regressions.
> Due to the time-boxed approach we may need to discuss moving forward with the switch with some regressions deemed acceptable, but those considered blockers should be prioritized of course.

Agreed, SGTM.

> I think it's reasonable to add a firm switch before clang-13 and before the end of this year, with intermediary milestones (e.g. filing  blockers for user regressions in the next 1-2 months).
> I'm inclined to favor a tighter deadline, the motivation here being to ensure that working on potential blockers is prioritized with plenty of time to spare, so the switch remains time-boxed.

I am not entirely sure yet about the tighter deadline, but I am optimistic things might start falling into place soon. Let me explain.

I think you've noticed the code-size issue that I raised. Switching before that being fixed would be a real non-starter for us, but if there's consensus the community want to switch then we'll deal with that of course. Because (some) of these regression are so big, I am hoping we are missing something obvious and that with a few things fixed we solve most of these cases, which is why I am optimistic, and also because of next point:

After code-size, I'm now looking at performance, and things look a lot better there for us. I do see some up and down behaviour: some wins in some benchmarks overall, but also some regressions. I will need to do some more homework here, as I need to check if small downstream divergence play a role here, and need to eliminate that. In some benchmarks where we win overall, there are a few cases that we really don't want to regress, so I need to look into this. I think these are problems that we need to fix, but I will start raising issues for them just for visibility. Overall, there are less to none non-starters in this area I think.

But because these are time-consuming exercises, and it's the summer months, I am getting slightly nervous about the tight(er) deadline. :-)

Cheers,
Sjoerd.

________________________________
From: Alina Sbirlea <asbirlea at google.com>
Sent: 28 July 2020 18:23
To: Sjoerd Meijer <Sjoerd.Meijer at arm.com>
Cc: Philip Reames <listmail at philipreames.com>; Chandler Carruth <chandlerc at gmail.com>; Eric Christopher <echristo at gmail.com>; llvm-dev <llvm-dev at lists.llvm.org>
Subject: Re: [llvm-dev] New pass manager for optimization pipeline status and questions

On Fri, Jul 24, 2020 at 12:54 PM Sjoerd Meijer <Sjoerd.Meijer at arm.com<mailto:Sjoerd.Meijer at arm.com>> wrote:
Hi Alina,

I think this is an excellent direction, this is the direction we should take here.  Just a somewhat irrelevant disagreement on this though:

> Philip's point is spot on that we are deficient now in the testing of the LegacyPassManager,

I disagree because the LPM is still the default and I appreciated Hans' reply: "Defaults tend to be popular". But this is the direction I like:

> This means prioritizing those blockers over other LLVM work. The current umbrella bug is PR46649<https://bugs.llvm.org/show_bug.cgi?id=46649>.

Just checking: do you accept both performance and code-size regressions as blockers here?

Yes, I think big performance and code-size regressions need to be investigated. It will be hard to quantify what "big" is though, but we should definitely track such regressions.
Due to the time-boxed approach we may need to discuss moving forward with the switch with some regressions deemed acceptable, but those considered blockers should be prioritized of course.

> My point is that we want and should work with users to make the transition smooth, but we do very much need user (meaning companies using LLVM) involvement here in order to not delay the switch further.

That's clear, and agreed.
I would like to remark here that currently, when a commit regresses one benchmark that is important for someone, that is enough justification most of the time for a revert of that commit. That's why I surprised that it looked like we were not setting code-quality goals and requirements before switching. And what I would like to ask here is to provide reasonable enough time for people to look into switching to the NPM, to evaluate this, and then file bugs under PR46649. Just collecting data, evaluating problems, filings bugs can already time-consuming, and then I guess they need fixing too. This also needs to fit in people's plans right now.

But it sounds reasonable to me that this is time-boxed. Given that switching is quite some work I think, switching before the clang-12 release would be unreasonable, and if clang-13 is in half a year from now, that already sounds perhaps somewhat reasonable, but might be tight.

I think it's reasonable to add a firm switch before clang-13 and before the end of this year, with intermediary milestones (e.g. filing blockers for user regressions in the next 1-2 months).
I'm inclined to favor a tighter deadline, the motivation here being to ensure that working on potential blockers is prioritized with plenty of time to spare, so the switch remains time-boxed.

Best,
Alina

Thanks,
Sjoerd.

________________________________
From: Alina Sbirlea <asbirlea at google.com<mailto:asbirlea at google.com>>
Sent: 24 July 2020 19:51
To: Sjoerd Meijer <Sjoerd.Meijer at arm.com<mailto:Sjoerd.Meijer at arm.com>>
Cc: Philip Reames <listmail at philipreames.com<mailto:listmail at philipreames.com>>; Chandler Carruth <chandlerc at gmail.com<mailto:chandlerc at gmail.com>>; Eric Christopher <echristo at gmail.com<mailto:echristo at gmail.com>>; llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>
Subject: Re: [llvm-dev] New pass manager for optimization pipeline status and questions

Hi all,

The current plan is to prioritize enabling the NPM as soon as possible, and that includes addressing any blockers that are known or arise. This means prioritizing those blockers over other LLVM work. The current umbrella bug is PR46649<https://bugs.llvm.org/show_bug.cgi?id=46649>.

Philip's point is spot on that we are deficient now in the testing of the LegacyPassManager, because so many have already made the switch (FWIW Google switched more than 2 years ago).

It's not constructive for the LLVM community to just flip the switch and break current LPM users. The purpose of these communications to llvm-dev and the bug tracking is to be informative as to the planned direction and make as quick of a progress as possible.
Please keep in mind that the work on the NPM has been going on for many years and many customers have switched years ago, and delaying this for even an additional year is not acceptable for the code health and stability of LLVM.

My point is that we want and should work with users to make the transition smooth, but we do very much need user (meaning companies using LLVM) involvement here in order to not delay the switch further.

Best,
Alina

On Thu, Jul 23, 2020 at 2:59 AM Sjoerd Meijer <Sjoerd.Meijer at arm.com<mailto:Sjoerd.Meijer at arm.com>> wrote:
I am not in favour of just flipping the switch and then deal with all the fall-out, because we see major regressions that would be unacceptable for our users. Thus, not only would this be very disruptive, also our releases are based on a certain trunk versions, so we would need to revert back to the legacy PM downstream and thus diverge from upstream which wouldn't be ideal for us. About the regressions, see the message/thread that I kicked off earlier (http://lists.llvm.org/pipermail/llvm-dev/2020-July/143646.html) which was quickly followed up by this thread.

I would like to see here if we are interesting in defining a few criteria that must be met before we switch:

  1.  Correctness, which obviously always must come first: looks like this is covered by bots that are running with the NPM, and by downstream users. >From the latest messages, I am getting we are there, or nearly there.
  2.  Performance (i.e. optimising for speed),
  3.  Code-size.

With 1) correctness box covered and ticked, is now the time to look at codegen quality: 2) performance and 3) code-size? Would it be reasonable that we create a plan or timeline to address this, and thus allow time that these issues can be addressed?

We are now ready to start tuning the NPM for code-size. Perhaps we are late to the NPM party (but that was a priority and bandwidth issue), but perhaps with correctness fixed this is actually the right time. I only ran numbers for code-size, and haven't even looked at performance numbers yet, which we would also need to do and takes time.

Cheers,
Sjoerd.

________________________________
From: llvm-dev <llvm-dev-bounces at lists.llvm.org<mailto:llvm-dev-bounces at lists.llvm.org>> on behalf of Eric Christopher via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>
Sent: 23 July 2020 02:05
To: Philip Reames <listmail at philipreames.com<mailto:listmail at philipreames.com>>; Alina Sbirlea <asbirlea at google.com<mailto:asbirlea at google.com>>; Chandler Carruth <chandlerc at gmail.com<mailto:chandlerc at gmail.com>>
Cc: llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>
Subject: Re: [llvm-dev] New pass manager for optimization pipeline status and questions

FWIW I'm in favor of this direction while making sure that we keep focus on removing the vestiges of the old pass manager for the code health impact to the project.

-eric

On Wed, Jul 22, 2020 at 3:15 PM Philip Reames via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:

(I'm probably going to derail your thread, sorry about that.)

I think at this point, we should just bite the bullet and make the switch to NPM by default for Clang's optimization pipeline.  Today.

Why?  Because many of our downstream consumers have already switched.  Google has.  We (Azul) have.  I think I've heard the same for a couple other major contributors.  Why does this matter?  Testing.  At the current moment, the vast majority of testing the project gets is exercising NPM, not LPM.

NPM is functionally complete for Clang optimization.  There might be a few missing cases around the sanitizers, but last I heard those were on the edge of being fixed.

I think we should make the switch, and deal with any fall out as regressions.  If we made the change immediately after a release branch, we'd have several months to address any major issues before the next release.

Philip

On 7/22/20 2:39 PM, Arthur Eubanks via llvm-dev wrote:
Hi all,

I wanted to give a quick update on the status of NPM for the IR optimization pipeline and ask some questions.

In the past I believe there were thoughts that NPM was basically ready because all of check-llvm and check-clang passed when -DENABLE_EXPERIMENTAL_NEW_PASS_MANAGER=ON was specified. But that CMake flag did not apply to opt and any tests running something like `opt -foo-pass -bar-pass` (which is the vast majority of check-llvm tests) were still using the legacy PM. The intended way to use NPM was to use the -passes flag, e.g. `opt -passes='foo,bar'`.

I've added a -enable-new-pm flag to opt to force running NPM passes even when `opt -foo-pass` is used. This is because I didn't want to go through every single test and figure out which ones should be using both -foo-pass and -passes=foo. Switching on -enable-new-pm currently leads to ~1800 check-llvm failures. I've documented the failed tests count per directory in https://bugs.llvm.org/show_bug.cgi?id=46651 (some have been fixed since that was posted).

This has led to real bugs in NPM being discovered and fixed (e.g. some optnone issues).

But a large portion of the remaining failures are because codegen-only passes haven't been ported to NPM yet. That's fine for the optimization pipeline NPM transition since it doesn't affect the optimization pipeline, but it does present an issue with the approach of the -enable-new-pm flag (which would by default become true alongside the NPM transition). Lots of tests are testing codegen-specific passes via opt (e.g. `opt -amdgpu-lower-intrinsics`) and they can't use NPM (yet).

I was thinking either we have a way of identifying codegen-only passes and revert back to the legacy PM in opt whenever we see one, or we go back to considering the originally intended approach of adding an equivalent `-passes=` RUN to all tests that should be also running under NPM.

I'm not sure of a nice and clean solution to identify codegen-only passes. We could go and update every instance of INITIALIZE_PASS to take another parameter indicating if it's codegen-only. Or we could just have a central list somewhere where we check if the pass is in some hardcoded list or has some prefix (e.g. "x86-").

The approach of adding equivalent `-passes=` RUN lines to all relevant tests seems daunting, but not exactly sure how daunting. Maybe it's possible to script something and see what fails? We'd still need some way to identify codegen-only passes to make sure we don't miss anything, and we'd need to distinguish between analyses and normal passes. Also, it would slow down test execution since we'd run a lot more tests twice, but maybe that's not such a big deal? Maybe it's good to have most tests running against the legacy PM even when NPM is on by default?

Thoughts?

This is split off from http://lists.llvm.org/pipermail/llvm-dev/2020-July/143395.html.

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