[llvm-dev] LCSSA verification for the top-level loops

[Igor Laevsky via llvm-dev]

Hi Michael,


Unfortunately there are loop print passes (LoopPassPrinter and PrintLoopPassWrapper) which don’t require LCSSA. And I think it’s important to support ability to print loops without mandatory LCSSA verification.
In this case we can leave "if(getAnalaysisIfAvaliable<LCSSAVerifier>())" as you suggested originally. However, what do we lose if we require all loop passes to require and preserve LCSSA? Why can't we just require LCSSA in the loop printers as well? We discussed that with Chandler some time ago, and there even was an idea to make it an IR property (i.e. IR would be considered invalid if LCSSA is broken) - it's still just an idea though.

I think it's useful to assume that loop pass printer never changes the IR and always prints it exactly as it is. Unless, of course, we introduce general invariant that loops are always in the LCSSA form. But current problem alone seems like a weak motivation for doing such thing.
I’ll proceed with “getAnalaysisIfAvaliable” approach then and will send a review in the near future.

- Igor


Probably loop printers can be converted to function passes, but in that case we will execute them only once per LPPassManager, which is also not an option. Now I’m not sure how to better bypass this issue. Do you have any ideas?

Michael


- Michael



Thanks,
Michael


— Igor

On 14 Oct 2016, at 01:57, Mikhail Zolotukhin <mzolotukhin at apple.com<mailto:mzolotukhin at apple.com>> wrote:

Hi Igor,

I like the second option more, and I think we can do something like this, or even more general. I'd suggest creating a new loop pass, called e.g. LoopVerification, and manually (at least for now) add it as a last pass to every instance of LPM. The pass will directly call isRecursivelyInLCSSA for the current loop, and later probably can also check other properties like LoopSimplify This way, the verification will be executed when all loop passes have finished working with a loop and before going to another loop. We can also schedule it in between some passes if we'd like to, and we can schedule it after all loop passes, placing it into a separate LPM instance. What do you think?

For now I don't have a good idea on how to utilize PassManager to schedule such verifiers automatically though:-(

- Michael

PS: Did you mean to send it to the list? Chandler might have good ideas here.


On Oct 13, 2016, at 8:30 AM, Igor Laevsky <igor at azulsystems.com<mailto:igor at azulsystems.com>> wrote:

Hi Michael,

In the https://reviews.llvm.org/D25364 we were discussing possibility of verifying LCSSA only for the top-level loops. I started implementing this but process appears to be a bit tricky. Problem is that I can’t find a way to check that LPPassManager contains LCSSA pass without introducing circular dependency between TransformUtils and Analysis libraries. I have couple of solutions in mind but wanted to ask your opinion on those, maybe we will be able to arrive at better decision.

1. I can introduce list of loops which LCSSAWrapperPass::verifyAnalysis needs to verify. It will be stored in the LoopInfo and populated in the LPPassManager main loop. After verification LCSSAWrapperPass will remove loop from this list.


2. There could be separate analysis “LCSSAVerification” on which LCSSA pass will depend. That way I will be able to recognise it inside LPPassManager and manually call verification of the current loop. Not sure how well our pass managers can handle such dependencies.

None of those seem particularly appealing to me. What do you think? Is there any better way?

Thanks,
Igor.

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