[llvm-dev] pass invalidation

Tue Jun 21 23:02:52 PDT 2016

Hi Prof. John Criswell,

Really appreciate your detailed reply.
Yes, I am using llvm to analyse C code for my research, I am quite new for llvm and clang.
I still have several questions.
1. To my understanding, if we add pass in getAnalysisUsage method(like LoopInfo), every time(for function pass), when we invoke runOnFunction(), llvm would automatically load result of LoopInfo, right? But when runOnFunction is invoked? It's in constructor?

2. right now, my passes includes several transform passes and analysis passes. For transform passes, they also use some built-in analysis passes, like AliasAnalysis, LoopInfo. My transform passes are to move some instructions around based on some analysis passes. Then other analysis passes would use those modified IR code.  Your suggestion is to dump information needed by my analysis passes into a new RK pass. I am not clear about it. Do you mean dump the modified IR code? Then pass those modified IR into my analysis pass? If so, if my transform pass analyses IR based on basicblock, after analysing every basicblock, I need dump something? Seem I misunderstood.

3. Is there elegant way to handle it? I tried like A = new transformPass(), but if I use in this way, I can't use analysis pass needed in transformPass, because llvm doesn't invoke getAnalysisUsage(). I guess your suggestion is the best way to do that(invocation of transform pass and analysis pass). But I still don't know how to deal with this problem.  Usually, I keep runOnFunction to do nothing expect some initialization. A concrete example are followed:

AnalysisA{
   doAnalysis();
   runOnFunction(){}
    getAnalysisUsage(){
      AU.addRequired(LoopInfo);
   }
}

TransformB{
  doCodeMove(){}
  runOnFunction(){}
  getAnalysisUsage(){
    AU.addrequired(AnalysisA);
  }
}

AnalysisC{
   doAnalysis();
   runOnFunction(){}
    getAnalysisUsage(){
      AU.addRequired(LoopInfo);
      AU.addRequired(AnalysisA);
      AU.addRequired(TransformB);
   }
}

It helps a lot if you can give more suggestion. I am struggle with this problem for a really long time.

Best,
Yuxi

________________________________
From: John Criswell [jtcriswel at gmail.com]
Sent: Tuesday, June 21, 2016 11:01 AM
To: 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 6/20/16 3:46 PM, Yuxi Chen wrote:
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.

Passes can have methods that allow their internal state to be updated by other passes (the same way that their state can be queried by other passes).  For example, the alias analysis passes have methods for querying alias information as well as methods that allow other passes to update the aliasing information when they make changes to the IR (so that friendly optimization passes don't invalidate alias analysis information when they make simple changes).

Right now, your transform pass has a method which your other passes are using to query information (I am guessing that your transform pass is recording information on what it has done).  I am suggesting that you create a new pass (call it "RK" for "Record Keeper") that implements this method (call it getInfo()).  Additionally, the RK pass also implements a method called setInfo() which the transform pass uses to record any information that later passes will need.  In their getAnalysisUsage<>() method, your passes preserve the results of the RK pass.

In this way, your transform pass modifies the IR and dumps any information needed by your analysis passes into the RK pass.  The RK pass does not modify the IR, so it doesn't create an impossible-to-schedule pass pipeline like your transform pass does.

If this isn't clear, please let me know.  I see that you're from UChicago, so I'm guessing that you need this for a research project.

Regards,

John Criswell

Best,
Yuxi
________________________________
From: John Criswell [jtcriswel at gmail.com<mailto:jtcriswel at gmail.com>]
Sent: Sunday, June 19, 2016 10:05 AM
To: Mehdi Amini; Yuxi Chen
Cc: llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>; llvmdev-bounces at cs.uiuc.edu<mailto:llvmdev-bounces at cs.uiuc.edu>; llvmdev at cs.uiuc.edu<mailto: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
Assistant Professor
Department of Computer Science, University of Rochester
http://www.cs.rochester.edu/u/criswell

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