[llvm-dev] pass invalidation

[Yuxi Chen via llvm-dev]

Hi Prof. John Criswell,

Thanks for your patience and detailed information.

Best,
Yuxi
________________________________
From: John Criswell [jtcriswel at gmail.com]
Sent: Wednesday, June 22, 2016 11:47 AM
To: Yuxi Chen
Cc: llvm-dev at lists.llvm.org
Subject: Re: [llvm-dev] pass invalidation

On 6/22/16 1:02 AM, Yuxi Chen wrote:
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.

Just to check, have you read the "How to Write an LLVM Pass" document on the LLVM web page?

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?

A pass is a C++ object.  For a FunctionPass, the pass manager will create the pass object once and then call its runOnFunction() method for every function in the program.  Additionally, if a ModulePass requires a FunctionPass, then the pass manager will call the runOnFunction() method on any function that the ModulePass requests.

If you are using opt, use the -debug-pass=Structure argument to make opt print out the structure of passes.  That will probably make it more clear as to how passes are scheduled and run.

Finally, if you're confused about when your passes are run, it might be better to write your passes as a ModulePass first.  You can almost never go wrong writing a ModulePass, and they are simpler to understand than FunctionPasses.


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.

Your transform passes can safely use any existing LLVM passes that do not modify the IR (such as AliasAnalysis and LoopInfo).  The only real restriction is that you want to avoid using getAnalysis<>() to get pointers/references to passes that modify the IR.

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.

What, specifically, do your analysis passes need to know?  Do they need to know which IR is the modified IR and which was left unmodified, or does it need to know something else?  Can it infer everything it needs to know just by looking at the IR?

If your analysis passes can determine everything they need to know by looking at the Module or Function passed into their runOnModule()/runOnFunction() methods, then you have no problem (and, in fact, you don't need your transform passes to communicate any additional information to your analysis passes).

However, if you need your transform pass to communicate information to your analysis passes, then you need to do something more sophisticated.  Copying LLVM IR would be a bad idea (too much memory consumption); you would probably record pointers to the relevant IR objects instead.

Perhaps an example will be helpful.

Let's say that you write a pass (call it Pass A) that creates a clone of every function in a program.  You have an analysis pass (call it Pass B) that takes each function and finds the clone that Pass A created.  Pass A could implement a data structure that maps original functions to the clones it created and then provide a method to Pass B that would query this information.  However, that would require Pass B to use addRequired<>() and getAnalysis<>() to get a pointer to Pass A.  That could create a scheduling conflict that the PassManager cannot handle (e.g., Pass A invalidates another pass that Pass B requires).

Instead, you create a Pass C that contains an empty map of functions to their clones.  The runOnModule() method of Pass C does nothing.  Pass C provides a method to record a new function->clone in its internal map, and it provides another method that takes a function and returns a pointer to its clone.  Pass A and Pass B both require Pass C as a dependency in their getAnalysisUsage<>() methods.  Pass A tells Pass C about every clone it creates; Pass B queries Pass C any time it wants to lookup the clone of a function.  Additionally, Pass A states that it preserves Pass C in its getAnalysisUsage<>() method.

In this example, Pass C is simply a pass through which Pass A and B communicate without creating a scheduling conflict for the pass manager.  It is needed because Pass B needs information which is readily available in Pass A that cannot be easily inferred from the LLVM Module that Pass B analyzes.

There are, of course, alternatives to this approach.  Pass A could put metadata on the clones it creates that indicate that they are clones of other functions; Pass B then looks for this metadata.  As long as other transforms don't remove the metadata, this works.

Regards,

John Criswell


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