[llvm-dev] pass invalidation

John Criswell via llvm-dev llvm-dev at lists.llvm.org
Tue Jun 21 09:01:15 PDT 2016

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 

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.


John Criswell

> 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
> 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

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