[llvm-dev] Copy Function from one LLVM IR file to another

Kell Maresh via llvm-dev llvm-dev at lists.llvm.org
Thu May 16 23:39:54 PDT 2019

Hello everyone,

I wanted to copy a function from one file to another. The file that I
wanted to copy the function into contains a function with the same name and
same number of instructions. I decided to just replace the instructions
with those of the other function. I am doing all of this from within a
function pass.

*1.* I created a function pass in which I extract a function using
"llvm-extract" from within the pass.

*2.* Then I read the (.ll) file created by the "llvm-extract" tool using:

        SMDiagnostic Err;
        LLVMContext Context;

        std::unique_ptr<Module> ParsedMod(parseIRFile("add.ll", Err,
        Module &M = dynamic_cast<Module&>(*ParsedMod);
        Module* Mod_ptr = &M;

*3.* Then I read the (.ll) file into which I want to copy the function

        std::unique_ptr<Module> ParsedMod2(parseIRFile("server.ll", Err,
        Module &M2 = dynamic_cast<Module&>(*ParsedMod2);
        Module* Mod_ptr2 = &M2;

*4.* I keep two iterators to save the points from where i have to start
replacing instructions as:

    Module::iterator F1;
    Module::iterator F2;

*5.* Assign them values so that F1 points to the first function and F2 to
the second function:

    for(Module::iterator func = Mod_ptr->begin(), Lfunc = Mod_ptr->end();
func!=Lfunc; ++func)
          F1 = func;

    for(Module::iterator func2 = Mod_ptr2->begin(), Lfunc2 =
Mod_ptr2->end(); func2!=Lfunc2; ++func2)
          if(func2->getName() == F.getName()) //F.getName() gives the same
name as the function that was extracted
            F2 = func2;

*6.* Replacing each instruction of F2 with instruction from F1 using:

    for(Function::iterator bb = F1->begin(), Lbb = F1->end(), bb2 =
F2->begin(), Lbb2 = F2->end(); bb2!=Lbb2; ++bb, ++bb2)
          for(BasicBlock::iterator inst = bb->begin(), Linst = bb->end(),
inst2 = bb2->begin(), Linst2 = bb2->end(); inst2 != Linst2; ++inst, ++inst2)
            Instruction* I = &*inst;
            Instruction* I2 = &*inst2;

            //errs() << "F1's instruction: " << *I << "\n";
            //errs() << "F2's instruction: " << *I2 << "\n";

            ReplaceInstWithInst(I, I2);

If I comment the ReplaceInstWithInst instruction and uncomment the
instructions printing the instructions from two functions, correct
instructions are printed.

I get *Segmentation Fault* if I use ReplaceInstWithInst instruction.
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