[llvm-dev] pass invalidation

[Mehdi Amini via llvm-dev]

> On Jun 19, 2016, at 12:59 AM, serge guelton via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> On Sun, Jun 19, 2016 at 05:44:01AM +0000, Yuxi Chen via llvm-dev 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.
> 
> Hi,
> 
> It looks like A is not an analyse, as it modifies IR code, while
> analysis don't modify IR. You cannot specify this kind of dependencies
> in the getAnalysisUsage method.
> 
> What you can do though is create a pass ABC that spawns a new
> Passmanager that calls A, B and C in that order, something like:
> 
>    llvm::legacy::FunctionPassManager FPM(Module);
>    FPM.add(createA());
>    FPM.add(createB());
>    FPM.add(createC());
> 
>    bool modified = false;
>    modified |= FPM.doInitialization();
>    for (Function &F : Module)
>      modified |= FPM.run(F);
>    modified |= FPM.doFinalization();


Indeed, this would work quite elegantly.
However be aware that it prevents from sharing analyses (easily…) with the outer pass-manager, so it can cost some compile time.

I think it should be possible to set it up in a nicer way with the new pass manager though.

— 
Mehdi

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