[llvm-dev] optnone/skipping passes in the new pass manager

[Robinson, Paul via llvm-dev]

Maybe you could change the default PM in opt and see what fails?
--paulr

From: Arthur Eubanks <aeubanks at google.com>
Sent: Monday, June 8, 2020 5:52 PM
To: Robinson, Paul <paul.robinson at sony.com>
Cc: Chandler Carruth <chandlerc at gmail.com>; llvm-dev at lists.llvm.org
Subject: Re: [llvm-dev] optnone/skipping passes in the new pass manager



On Mon, Jun 8, 2020 at 7:11 AM Robinson, Paul <paul.robinson at sony.com<mailto:paul.robinson at sony.com>> wrote:
When the optnone design was being discussed, Chandler specifically rejected having the pass manager involved in the decision (which was the original proposed design).  Assuming he still feels the same way now, if the existing `skipFunction` calls aren’t being executed under the new pass manager, then each pass that has that call will need to be modified accordingly (added to the NPM path or moved to some common point).  It would be best if the `skipFunction` calls were handled consistently in all passes so that it would become part of the normal pass boilerplate.
Makes sense, thanks for the background. I'll see if there's a clean way to make this functionality easy to use in the NPM.

I suspect any `skipFunction` or opt-bisection tests have been written to force the old pass manager, which is why defaulting to the new pass manager doesn’t fail anywhere.
I took a closer look at some optnone tests and they use opt instead of clang. opt still defaults to the legacy pass manager regardless of clang's default.
--paulr

From: llvm-dev <llvm-dev-bounces at lists.llvm.org<mailto:llvm-dev-bounces at lists.llvm.org>> On Behalf Of Arthur Eubanks via llvm-dev
Sent: Sunday, June 7, 2020 7:59 PM
To: llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>
Subject: [llvm-dev] optnone/skipping passes in the new pass manager

Looking through some of the remaining test failures under the new pass manager, I've narrowed down one of the failures in GWPAsan(!) to the fact that the new pass manager doesn't properly skip passes like the old pass manager. For example, when a function is marked optnone, or when using https://llvm.org/docs/OptBisect.html<https://urldefense.com/v3/__https:/llvm.org/docs/OptBisect.html__;!!JmoZiZGBv3RvKRSx!t3zrtZFFWm0ifdWgL9SiWSNETodW3pMSJ8m8YWqK139cicFp_U1juO0D90-VinpUWg$>.

Lots of passes (e.g. SROA) will do the following under the legacy pass manager:

  bool runOnFunction(Function &F) override {
    if (skipFunction(F))
      return false;
    // do pass
  }

What's the right way to proceed with this? There are 50-100 calls to skipFunction() in legacy passes. This doesn't even account for other types of IR units, like skipModule(Module&).

I suppose it's possible to manually go in and add in the same check in the new passes, but that seems tedious (and how do you test that at scale? clearly there aren't many tests for it right now since check-llvm passes under the new pass manager). An alternative of skipping passes at the pass manager level would require marking each pass as necessary/optional (I think...).
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