Specify dependency of my LLVM pass on the mem2reg pass

Question

I am writing a ModulePass and invoke it using opt -load. I would require that alloca has been promoted to registers when my pass runs, using the -mem2reg switch for opt.

There is a link which indicates that the PromoteMemoryToRegsiter pass is a transform pass and as such should not be required by my pass. That's a statement from 2010. Does that still hold?

One of the posts I found suggested something like

AU.addRequiredID(PromoteMemoryToRegister::MemoryToRegisterID);

but that contradicted the post I linked above.

So my question is, how to I express this dependency for my pass, if possible? How do I express, in general, such pass dependencies? And what's the difference between a transform pass and, well, another pass?

Answer 1

What's the difference between a transform pass and another pass?

A transform pass is a pass that may invalidate the results of other passes.

How to I express this dependency for my pass?

First of all, I recommend reading the pass-dependency section of the official "how to write a pass" guide. In any case, the correct way to add a dependency between transformation passes is to add one before the other in your pass manager (see the guide section on the pass manager), or, if you just invoke opt, then add all the passes you want in the order you want them to occur, e.g.:

opt -load mypass.so -mem2reg -mypass