[llvm-dev] pass invalidation

[Yuxi Chen via llvm-dev]

Hi,

Thanks for your reply.
But I still don't know how a transform pass updates a new analysis pass after it modifies the IR. Can you explain it clearly? I am not familiar with pass management and invocation.

Best,
Yuxi
________________________________
From: John Criswell [jtcriswel at gmail.com]
Sent: Sunday, June 19, 2016 10:05 AM
To: Mehdi Amini; Yuxi Chen
Cc: llvm-dev at lists.llvm.org; llvmdev-bounces at cs.uiuc.edu; llvmdev at cs.uiuc.edu
Subject: Re: [llvm-dev] pass invalidation

On 6/19/16 4:28 AM, Mehdi Amini via llvm-dev wrote:

On Jun 18, 2016, at 10:44 PM, Yuxi Chen via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:

Hi All,

When I use llvm, I encounter a problem like "unable to schedule pass A required by C"
I investigated deeper. It's like:
I have three passes, say A, B, C(all are on function level)
A would modify IR code. (change instruction order)

For pass B,
I would use the result of pass A, I use addRequired<B>(), and &getAnalysis<B>(), it works.

void getAnalysisUsage(AU){
AU.addRequired<A>();
}


For pass C, it will use the results of pass A and B.
I use the way as used for pass B, but it failed, even for LoopInfo analysis pass(which is the built-in analysis pass).
void getAnalysisUsage(AU){
AU.addRequired<A>();
AU.addRequired<B>();
}


It seems because A would modify IR code, so for pass C, I need first load pass A then pass B, otherwise it will be invalidated.
However, when I change the using order, I still got error "unable to schedule pass A required by C".

Does anyone encounter the same problem before and have a solution?
Any help is appreciated.

Depending on other transformations isn’t recommended, and isn’t supported by the soon-new-passmanager I believe.
The expectation is that the passes are added in order to the pass manager by the client.

Depending on transformation passes isn't supported by the legacy PassManager, either.  Occasionally some passes can get away with it, but it often results in unschedule-able pass pipelines as above.

If your transform pass does something to the code, other passes should either infer what it did by examining the IR.  the IR contains the definitive information about the program (because it is the program).

Alternatively, you could create an analysis pass upon which both your transform and analysis passes depend.  The transform pass would update this new analysis pass with information on what it transformed; your later analysis passes could then query this information.  This approach is fragile, but it could work.

Regards,

John Criswell


In you case, I expect that it would “work” by removing the dependency from C to A. If C requires B and B requires A, by scheduling C you’ll get A, B, C in sequence.

—
Mehdi




Best,
Yuxi
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--
John Criswell
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Department of Computer Science, University of Rochester
http://www.cs.rochester.edu/u/criswell
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