[LLVMdev] Patch: PassManager should call add() instead of addLowerLevelRequiredPass()

[Devang Patel]

Hi Kevin,

On Mar 9, 2009, at 3:49 PM, Kevin Fan wrote:

> Hello,
>
> LLVM asserts in the following scenario.  Say there are three passes:
> A, B, C.  C requires A and B; B requires A; and B does not preserve A.
> A valid pass ordering would be A B A C.  However, opt gives the
> following assertion:
>
> $ opt -load libPassTest.so foo.bc -f -o /dev/null -c -debug- 
> pass=Structure
> opt: /u/loc/kevin/llvm/llvm/lib/VMCore/PassManager.cpp:1424: virtual
> void llvm::MPPassManager::addLowerLevelRequiredPass(llvm::Pass*,
> llvm::Pass*): Assertion `(P->getPotentialPassManagerType() <
> RequiredPass->getPotentialPassManagerType()) && "Unable to handle Pass
> that requires lower level Analysis pass"' failed.
>
> even though all passes are ModulePasses and do not require lower  
> level passes.
>
> The attached patch modifies PMDataManager::add() to check if the
> required pass is actually a lower level pass before calling
> addLowerLevelRequiredPass(); if not, it calls add() instead.  Not sure
> if this is the best fix, but it works for me.
>
> I also attached a test case with the three passes A, B, C described  
> above.
>
> Kevin
> <passmanager.diff><PassTest1.cpp>

This patch is not appropriate. At this point, the pass manager expects  
that all required passes at the same level (module pass, requiring  
module pass) are handled.

The pass manager uses simplistic approach and does not handle cases  
where required passes cancels each other. And that's what happens in  
your example. Pass B cancels, already schedule Pass A.

You've two alternatives:

1) Require pass B before requiring pass A. In other words,

replace

     virtual void getAnalysisUsage(AnalysisUsage &use) const {
       use.addRequired<A>();
       use.addRequired<B>();
     }

with

     virtual void getAnalysisUsage(AnalysisUsage &use) const {
       use.addRequired<B>();
       use.addRequired<A>();
     }



2) do

$ opt -load libPassTest.so foo.bc -f -o /dev/null -b -c -debug- 
pass=Structure

-
Devang