FunctionPass requiring SCCs

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1

I have a FunctionPass F that needs a list of all the SCCs for use in its doFinalization() method. Let's say I write a CallGraphSCCPass C that creates an array of all SCCs. Let C be required by F, and let F call getAnalysis<C>() from its doFinalization() method. Am I guaranteed that C's runOnSCC() method will have executed on all SCCs before F's doFinalization() method? In other words, when F calls getAnalysis<C>() from its doFinalization() method, am I guaranteed that C will have been able to build the array of SCCs? If not, how else might I structure things?

Regards,
Ryan

2

Hi Ryan,

I have a FunctionPass F that needs a list of all the SCCs for use in its
doFinalization() method. Let's say I write a CallGraphSCCPass C that
creates an array of all SCCs. Let C be required by F, and let F call
getAnalysis<C>() from its doFinalization() method. Am I guaranteed that
C's runOnSCC() method will have executed on all SCCs before F's
doFinalization() method? In other words, when F calls getAnalysis<C>()
from its doFinalization() method, am I guaranteed that C will have been
able to build the array of SCCs? If not, how else might I structure things?

That should work. I've written such code, something like:
bool doFinalization(Module &M) {
  CallGraph &CG = getAnalysis<CallGraph>();
  for (scc_iterator<CallGraph*> SCCI = scc_begin(&CG),
    SCCE = scc_end(&CG); SCCI != SCCE; ++SCCI) {
    unsigned size = (*SCCI).size();
    for (unsigned i = 0; i < size; ++i) {
      Function *F = (*SCCI)[i]->getFunction();
      ......
    }
  }
  ....
}

Check out scc_* iterators. Also note that the call graph
is not aware of the indirect calls, so you will need to write your
own CG implementation if you need to handle function pointers
soundly.

Domagoj Babic

3

Chris, is this true? If so, it seems like a bad property for the CallGraphSCCPass framework.

--Vikram
http://www.cs.uiuc.edu/~vadve
http://llvm.cs.uiuc.edu/

4

This isn't true. The *default* implementation of the callgraph interface handles function pointers soundly, just non-aggressively: it assumes they can point to any function. Of course you can provide your own implementation of the CallGraph interface, or use something more aggressive (e.g. DSA).

If you're seeing cases this isn't happening, please file a bug.

-Chris

5

Chris,

Thx for correcting me. I don't remember how I came to that conclusion.

Domagoj