llvm-pass-practice

the road to write llvm pass

notice

• path should not include Chinese character

install llvm

./install.sh

practices

• my first pass:show function name

how to write a simplest pass

take FuncName.cpp in my first pass as an example

setup env

1. add library add

   add_llvm_library( LLVMFuncName MODULE
   YOUR_PASS.cpp
   
   PLUGIN_TOOL
   opt
   )
   

   into PASS_DIR_PATH/CMakeLists.txt

2. add subdirectory add add_subdirectory(PASS_DIR_PATH) into lib/Transforms/CMakeLists.txt

coding

1. the necessary header

   #include "llvm/Pass.h"
   #include "llvm/IR/Function.h"
   #include "llvm/Support/raw_ostream.h"

2. add namespace

   using namespace llvm;

3. write your own pass in an anonymous namespace

   namespace {
       
   }

build

⇒  doublemice@DoubleMice-MBP:~/Documents/graduate/llvm-pass-practice|master⚡ pwd
/Users/doublemice/Documents/graduate/llvm-pass-practice
pushd llvm/lib/Transforms
ln -s ../../../my\ first\ pass/FuncName .
popd
pushd build
make
popd

load the pass

the output lib

doublemice@DoubleMice-MBP:~/Documents/graduate/llvm-pass-practice|master⚡ 
⇒  ls build/lib/LLVMFuncName.dylib 
build/lib/LLVMFuncName.dylib

as we see,the output dylib name is exact the name we define in PASS_DIR_PATH/CMakeLists.txt(see: setup env -> 1)

load it with opt

1. prepare a test file in test/FuncName_test.c

   #include <stdio.h>
   void sayHello() {
       printf("hello\n");
   }
   void sayGoodbye() {
       printf("goodbye\n");
   }
   int main() {
       sayHello();
       sayGoodbye();
       return 0;
   }

2. compile it into a llvm bitcode

   $LLVM_BIN/clang -O3 -emit-llvm test/FuncName_test.c -c -o test/FuncName_test.bc

3. run the pass with opt

   doublemice@DoubleMice-MBP:~/Documents/graduate/llvm-pass-practice|master⚡ 
   ⇒  $LLVM_BIN/opt -load build/lib/LLVMFuncName.dylib -funcName < test/FuncName_test.bc > /dev/null
   sayHello
   sayGoodbye
   main

a skeleton

#include "llvm/Pass.h"
#include "llvm/IR/Function.h"
#include "llvm/Support/raw_ostream.h"
using namespace llvm;
namespace {
    struct Example:public FunctionPass {
        static char ID;
        Example():FunctionPass(ID) {}

        //tells llvm which other pass we need
        void getAnalysisUsage(AnalysisUsage &AU) const {
            ...
        }
        virtual bool runOnFunction(Function &F) {
            ...
            //true:this pass will change our program
            //false:won't change
            return false;
        }
    };
}

char Example::ID = 0;
static RegisterPass<Example> X("PASS_NAME","PASS_DESCRIPTION")