Inserting trace information during opt transformations

Hi all,

In several of the transformations I'm working on I need to embed the logical equivalent of puts/printf/etc. calls inline with code. I am having some difficulties getting the transformation code right. As I understand it, I should be creating a ConstantArray based on the string (in this case I'm taking an instruction, 'disassembling' it to an ostringstream then turning it into a constant array):

  std::ostringstream instr;
  (*wp)->print(instr); Constant *itext = ConstantArray::get(instr.str());

This bit seems OK. I am calling a function in my run time system with the signature

    void mcp_trace_instruction(const char *)

by first declaring it inside doInitialization():

  virtual bool doInitialization(Module &M)
    std::vector<const Type*> args;
    FunctionType *FT = FunctionType::get(Type::VoidTy, args, false);
    TraceFunc = M.getOrInsertFunction("mcp_trace_instruction", FT);

    return true;

then inserting a suitable CallInst instruction

  new CallInst(TraceFunc, gepinst, "", *wp);

Looking at disassembly output from similar code, I can't just pass the constant array directly to the call because I need a GetElementPtrInst conversion. This is the thing I'm having trouble with. My current attempt is:

            GetElementPtrInst *gepinst = new GetElementPtrInst(itext, ConstantInt::get(Type::Int32Ty, 0), ConstantInt::get(Type::Int32Ty, 0), "", *wp);

but it fails as follows:

opt -load ~/llvm-tools/mcp-build/Debug/lib/ -trace-instructions -lowerselect -lowerallocs -lowergc -lowerinvoke -lowerswitch -reg2mem -insertyieldpoints -trace-memory -o Abort.fbc Abort.bc
opt: /usr/local/rse/llvm/src/lib/VMCore/Instructions.cpp:872: const llvm::Type* checkType(const llvm::Type*): Assertion `Ty && "Invalid GetElementPtrInst indices for type!"' failed.
opt((anonymous namespace)::PrintStackTrace()+0x15)[0x8346cf5]
make: *** [Abort.fbc] Aborted

The pass that is blowing up is -trace-instructions, which is intended basically just to go through the module inserting mcp_trace_instruction calls before each instruction, before any other modifications are made to the code (I have to trace instructions this way because by the time the other transformations run, I end up with something not very much like the original code).

I've tried a few other obvious-looking alternatives, but I seem to be stabbing in the dark. If anyone could suggest the correct GetElementPtrInst configuration I would be very grateful -- I've already wasted a couple of days on this.

Thank you in advance,

Right. The problem here is that you need to create a GlobalVariable to hold the constant array. Something like this should work:

itext = new GlobalVariable(itext->getType(), true, GlobalValue::InternalLinkage, itext, "", &M);