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

Edward Lee eslee3 at uiuc.edu
Thu Oct 16 08:29:09 PDT 2008

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);

  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.

 / \
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.

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