[LLVMdev] code-altering Passes for llc

[Artjom Kochtchi]

Greetinigs,

I am extending llc to include runtime checks for calls (in X86). So a call
'call target' is altered to look like this:

  [some check]
  jne error_function
  call target

I've done this by implementing a MachineFunctionPass that is instantiated
and added to the PassManager in X86TargetMachine::addPreRegAlloc.

In order to create the jne-instruction I need some BasicBlock that contains
the error routine. So I tried to create a ModulePass to insert some basic
block e. g. in the beginning of the program, to use it for the
jne-instruction. Unfortunately, llc then produces the error

  llc: PassManager.cpp:1597:
    virtual void llvm::ModulePass::assignPassManager(llvm::PMStack&,
llvm::PassManagerType):
    Assertion `!PMS.empty() && "Unable to find appropriate Pass Manager"'
failed.

Adding a MachineFunctionPass the same way works fine. I've read here on the
mailing list that some Pass dependencies cause this error, but I have no
dependencies specified.

How do I get the ModulePass to run? Is a ModulePass the right way to
accomplish my aim, altogether?

One more question concerning passes:
Clearly, inserting a jne-instruction changes control flow, so the CFG is not
up to date after the pass. Do I set AnalysisUsage in getAnalysisUsage to
preserve nothing? In that case, is the CFG updated automatically by LLVM or
do I need to apply the changes during the Pass not only the the
MachineFunction, but also the the CFG?

-Artjom
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