Requiring a pass to run before/after a pass? (Adding PHIs and updating uses)

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#1

Is there a simple way to require a pass, e.g., Reg2Mem/Mem2Reg, to run before/after my transformation pass? Or do I do something like:

struct myOpt {
myOpt() {
mBefore = createDemoteRegisterToMemoryPass();
mAfter = createPromoteMemoryToRegisterPass();
}

getAnalysisUsage(AU) {
AU.addRequired(my stuff);
mBefore.getAnalysisUsage(AU);
mAfter.getAnalysisUsage(AU);
}

runOnFunction(aF) {
changed = mBefore(F);
do my stuff;
changed |= mAfter(F);
}
}

I’m trying to transform a CFG where A flows into B and C, and I’m making a copy of A, A_copy, which will also flow into B and C. Right now I’m making a merge point mergeB where both A and A_copy flow into it, and it then flows into B. Same for C.

A
/
B C

A A_copy

/ \ |
mergeB mergeC

B C

For any values %v deffed in A and A_copy, mergeB has the phis %v.phi.B, and I replace the original uses of %v in B with %v.phi.B. However, if both B and C use %v, I need to make sure B gets updated to use %v.phi.B and C gets %v.phi.C (and not %v.phi.B).

I figured I could maybe make the code cleaner by pushing values into alloca slots then doing my transformation and converting back to registers with Reg2Mem then Mem2Reg.

Ed

#2

Is there a simple way to require a pass, e.g., Reg2Mem/Mem2Reg, to run before/after my transformation pass? Or do I do something like:

One simplest way is to handle this is to add these passes around your pass in the pass manager.

pm.add(Reg2Mem)
pm.add(MyPass)
pm.add(Mem2Reg)

#3

How about in the context of opt assuming myOpt is built as part of
llvm and not a separate plugin. It probably wouldn't be ideal to
modify opt.cpp to do something like..

void addPass(PassManager &PM, Pass *P) {
  if (isa<myOpt>(P) {
    addPass(PM, beforePass);
    PM.add(P);
    addPass(PM, afterPass);
  }

And even then, that only fixes opt.

Perhaps Passes can get to the appropriate PassManager by going through
Pass's virtual assignPassManager method?

Ed