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

[Mehdi AMINI via llvm-dev]

On Thu, May 16, 2019 at 11:40 PM Kell Maresh via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> 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.
>

If you already go the llvm-extract route, you may want to look at llvm-cat
and/or llvm-link which should accomplish what you're trying to do manually
below.

-- 
Mehdi





>
> *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,
> Context));
>         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
> using:
>
>         std::unique_ptr<Module> ParsedMod2(parseIRFile("server.ll", Err,
> Context));
>         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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