LLVM_pass应用1 读取输入文件的函数

蔚落 2023-02-24 14:25 101阅读 0赞

### 文章目录 ###

*  1、pass
 *  2、具体操作
 *   *  1.工程目录
     *  2.测试文件hello.c
     *  3.CMake说明文件CMakeList.txt
     *  4.主要代码 FirstPass.cpp
     *  5.编译生成动态链接so文件
     *  6.测试使用动态链接
 *  总结:

*  平台: Ubuntu18.04
 *  版本 llvm+clang 5.0.0 [编译安装见链接六-24][-24]

# 1、pass #

[官方介绍与例子][Link 1]  
LLVM pass是编译器对代码进行转化和优化。所有pass都是Pass类的子类，通过覆盖Pass类的虚函数来实现功能，可继承的类有ModulePass , CallGraphSCCPass, FunctionPass , LoopPass, RegionPass, BasicBlockPass。  
通俗的讲,由于llvm分为前端-优化-后端,因此pass部分就是将代码经过前端预编译后, 进行下一步代码优化策略,同时包含了很多对代码处理的技巧,如提取cfg,dfg,函数名称,节点类型等.

# 2、具体操作 #

**本文主要编写第一个llvm pass, 实现的功能是读取源码中所有的函数名**  
即  
`in_file = hello.bc`  
`out: func, func2, main` \# 因为hello.bc里面有此三个函数定义

## 1.工程目录 ##

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzMyNDYwODE5_size_16_color_FFFFFF_t_70]

*  CMakeLists.txt 使用CMake编译工程的说明文件
 *  FirstPass.cpp 主文件
 *  build 文件夹用于保存CMake编译后的动态链接文件
 *  hello.c 测试文件, 即输入文件的原始文件
 *  hello.bc .c文件的字节码文件, 生成命令是`clang -c -emit-llvm hello.c -o hello.bc`
 *  hello.ll .c文件的IR文件,指令集,生成命令`clang -O0 -S -emit-llvm input.c -o input.ll`

## 2.测试文件hello.c ##

#include <stdio.h>
    
    int fun1(){ 
        printf("%s\n", "hello");
        return 25;
    }
    int fun2(){ 
        printf("%s\n", "fun2");
        return 23;
    }
    int main(int argc, char const *argv[]){ 
        int b=0;
        int i = 0;
        for (i = 0; i < 23; ++i){ 
            b+=2;
        }
        printf("%d\n", b);
        return 0;
    }

## 3.CMake说明文件CMakeList.txt ##

该文件将说明如何编译链接一个pass, 即FirstPass.cpp文件

add_library(FirstPass MODULE
        # List your source files here.
        FirstPass.cpp
    )
    
    # Use C++11 to compile our pass (i.e., supply -std=c++11).
    target_compile_features(FirstPass PRIVATE cxx_range_for cxx_auto_type)
    
    # LLVM is (typically) built with no C++ RTTI. We need to match that;
    # otherwise, we'll get linker errors about missing RTTI data.
    set_target_properties(FirstPass PROPERTIES
        COMPILE_FLAGS "-fno-rtti"
    )
    
    # Get proper shared-library behavior (where symbols are not necessarily
    # resolved when the shared library is linked) on OS X.
    # 设置目标属性 链接标志“未定义的动态查找”
    if(APPLE)
        set_target_properties(FirstPass PROPERTIES
            LINK_FLAGS "-undefined dynamic_lookup"
        )
    endif(APPLE)

## 4.主要代码 FirstPass.cpp ##

// https://blog.csdn.net/zhang14916/article/details/89295573
    //导入头文件
    #include "llvm/ADT/Statistic.h"
    #include "llvm/IR/Function.h" //因为这里使用IR的Function处理,因此导入
    #include "llvm/Pass.h"
    #include "llvm/Support/raw_ostream.h"
     
    using namespace llvm;  //指定所使用的 LLVM 命名空间
     
    namespace { 
     
      //定义 First 类继承自 FunctionPass
      struct First : public FunctionPass { 
        static char ID; // Pass ID
        First() : FunctionPass(ID) { }
     
        //定义 runOnFunction 重载继承父类的抽象虚函数
        bool runOnFunction(Function &F) override {  //这里将输入的文件里面的函数F一个一个的传入
     
          //打印每个函数的名字
          errs() << "First: ";
          errs().write_escaped(F.getName()) << '\n'; //得到函数的名字,
          return false;
        }
      };
    }
     
    char First::ID = 0;  //初始化 Pass ID
     
    //最后注册 FirstPass， 指定命令行参数为 First
    static RegisterPass <First>X("First", "First Pass");

## 5.编译生成动态链接so文件 ##

`mkdir build && cd build` build作为保存编译文件夹  
`cmake ../` 进行预编译生成makefile文件用于编译cpp文件

# 输出
    zjq@DESKTOP-O28RVV3:build$ rm -rf * && cmake ../
    -- The C compiler identification is GNU 7.5.0
    -- The CXX compiler identification is GNU 7.5.0
    -- Check for working C compiler: /usr/bin/cc
    -- Check for working C compiler: /usr/bin/cc -- works
    -- Detecting C compiler ABI info
    -- Detecting C compiler ABI info - done
    -- Detecting C compile features
    -- Detecting C compile features - done
    -- Check for working CXX compiler: /usr/bin/c++
    -- Check for working CXX compiler: /usr/bin/c++ -- works
    -- Detecting CXX compiler ABI info
    -- Detecting CXX compiler ABI info - done
    -- Detecting CXX compile features
    -- Detecting CXX compile features - done
    CMake Warning (dev) in CMakeLists.txt:
      No cmake_minimum_required command is present.  A line of code such as
    
        cmake_minimum_required(VERSION 3.10)
    
      should be added at the top of the file.  The version specified may be lower
      if you wish to support older CMake versions for this project.  For more
      information run "cmake --help-policy CMP0000".
    This warning is for project developers.  Use -Wno-dev to suppress it.
    
    -- Configuring done
    -- Generating done
    -- Build files have been written to: /mnt/e/21_llvm_pass/test1/build
    
    zjq@DESKTOP-O28RVV3:build$ ls
    CMakeCache.txt  CMakeFiles  Makefile  cmake_install.cmake

如果上面生成了`Makefile`即可以进行编译了, 因为后面的make是基于`Makefile`的

zjq@DESKTOP-O28RVV3:build$ make
    Scanning dependencies of target FirstPass
    [ 50%] Building CXX object CMakeFiles/FirstPass.dir/FirstPass.o
    [100%] Linking CXX shared module libFirstPass.so
    [100%] Built target FirstPass
    
    zjq@DESKTOP-O28RVV3:build$ ls
    CMakeCache.txt  CMakeFiles  Makefile  cmake_install.cmake  libFirstPass.so

如果上面生成了`libFirstPass.so`即完成了第一个pass,接下来进行测试

## 6.测试使用动态链接 ##

zjq@DESKTOP-O28RVV3:build$ cd .. && ls
    CMakeLists.txt  FirstPass.cpp  build  hello.bc  hello.c
    
    zjq@DESKTOP-O28RVV3:test1$ opt -load build/libFirstPass.so -First hello.bc
    WARNING: You're attempting to print out a bitcode file. This is inadvisable as it may cause display problems. If you REALLY want to taste LLVM bitcode first-hand, you can force output with the `-f' option.
    
    First: fun1
    First: fun2
    First: main
    zjq@DESKTOP-O28RVV3:test1$

`opt -load build/libFirstPass.so -First hello.bc`

*  `opt`是llvm的工具
 *  `-load build/libFirstPass.so` 加载动态链接
 *  `-First hello.bc` cpp文件中定义了调用链接库中方法的标识是`-First`, 后面的hello.bc是输入文件用于处理

# 总结: #

*  文本主要介绍pass文件结构由`CMakeList.txt` `.cpp` `build/.so`三个文件构成
 *  其中`CMakeList.txt`格式不变, 以后的pass皆可参考
 *  `build/.so`生成格式固定并无难点
 *  最重要的就在于`.cpp` 文件的继承和处理
 *  如果需要修改cpp文件重新编译, 只需要`cd build && make && cd ..`即可

还需要注意一点的是`CMakeList.txt` 内容是文件级的  
`.cpp`内容是代码级的

[-24]: https://blog.csdn.net/qq_32460819/article/details/89409599
[Link 1]: https://llvm.org/docs/WritingAnLLVMPass.html#quick-start-writing-hello-world
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