Help with a pass

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1

Hi all,
I was able to create a pass following [1]. Now goal is amend the pass and try to dump the call graph. I think I have properly amended the source extending my pass from CallGraphSCCPass but unfortunately every time I run it, it crashes. I tried with llvm-6 and llvm-7. I notice that if I change EP_EarlyAsPossible to anything else it does not crash but I don’t see the expected string printed by the runOnSCC.

Could you help me to solve this problem? How should I create the pass?

Thanks for your time

[1] https://www.cs.cornell.edu/~asampson/blog/llvm.html

2

There’s two things going on here:

Firstly, extension points interact in a slightly odd way with optimization levels:

  • At -O0 the only extension points used are EP_EarlyAsPossible and EP_EnabledOnOptLevel0

  • At -O1 and higher all extension points are used except EP_EnabledOnOptLevel0

so when you’re not seeing the crash it’s because you’re using an optimization level (probably -O0 because that’s

the default) where the extension point you’re using isn’t used.

Secondly, CGSCC passes can only be added to a GCSCC pass manager. If you use a debug build of clang then the

crash becomes an assertion failure which (somewhat) tells you this:

CallGraphSCCPass.cpp:575: virtual void llvm::CallGraphSCCPass::assignPassManager(llvm::PMStack&, llvm::PassManagerType): Assertion `!PMS.empty() && “Unable to handle Call Graph Pass”’ failed.

From some experimentation it looks like it’s only EP_EarlyAsPossible where we get this error, because (looking

at PassManagerBuilder::populateFunctionPassManager) that extension point uses a Function Pass Manager

whereas all the others look like they use a Module Pass Manager.

John

3

Thanks John,
All clear now. Is it worthy to open a bug issue?

Thanks