該方法仍然有效，只不過還需要一些修改，遇到的錯誤需要解決。

錯誤1

CMake Error at CMakeLists.txt:658 (message):
  Unexpected failure executing llvm-build: llvm-build: fatal error: missing
  LLVMBuild.txt file at:
  'F:\\svnlocal\\temp\\llvm\\llvm6.0/lib\\Transforms\\Hello\\LLVMBuild.txt'

這樣的錯誤是說明缺少LLVMBuild.txt檔案，只要在CMakeLists.txt的同目錄下配置一個LLVMBuild.txt

錯誤2

CMake Error at CMakeLists.txt:658 (message):
  Unexpected failure executing llvm-build: Traceback (most recent call last):

    File "F:/svnlocal/temp/llvm/llvm6.0/utils/llvm-build/llvm-build", line 6, in
 <module>
      llvmbuild.main()
    File "F:\svnlocal\temp\llvm\llvm6.0\utils\llvm-build\llvmbuild
 
\main.py", lin
e 952, in main
      opts.optional_components)
    File "F:\svnlocal\temp\llvm\llvm6.0\utils\llvm-build\llvmbuild\main.py", lin
e 332, in write_library_table
      tg = c.get_parent_target_group()
    File "F:\svnlocal\temp\llvm\llvm6.0\utils\llvm-build\llvmbuild\componentinfo
.py", line 87, in get_parent_target_group
      return self.parent_instance.get_parent_target_group()
    File "F:\svnlocal
 
\temp\llvm\llvm6.0\utils\llvm-build\llvmbuild\componentinfo
.py", line 87, in get_parent_target_group
      return self.parent_instance.get_parent_target_group()
    File "F:\svnlocal\temp\llvm\llvm6.0\utils\llvm-build\llvmbuild\componentinfo
.py", line 87, in get_parent_target_group
      return self.parent_instance.get_parent_target_group()
    [Previous line repeated 992 more times]
    File "F:\svnlocal\temp\llvm\llvm6.0\utils\llvm-build\llvmbuild\componentinfo
.py", line 82, in get_parent_target_group
      if self.type_name == 'TargetGroup':

  RecursionError: maximum recursion depth exceeded in comparison

最初看到這個錯誤還以為LLVM不支援Python3，於是安裝了Python2，結果發現問題依然存在。最後確定下來原因是：LLVMBuild.txt內容的parent配置不正確，正確的配置是：LLVMBuild.txt檔案的父目錄的父目錄，例如llvm/lib/Transforms/Custom（或者llvm/lib/Transforms/Hello）下的LLVMBuild.txt內容中parent應該是Transforms，這裡放一個完整的llvm/lib/Transforms/Custom/LLVMBuild.txt內容：

[component_0]
type = Library
name = Custom
parent = Transforms

這一點上，原參考文章裡是沒有介紹的，需要自己摸索。

錯誤3

-- Clang version: 6.0.0
-- SampleAnalyzerPlugin ignored -- Loadable modules not supported on this platform.
-- PrintFunctionNames ignored -- Loadable modules not supported on this platform.
-- AnnotateFunctions ignored -- Loadable modules not supported on this platform.
CMake Error at cmake/modules/LLVM-Config.cmake:105 (target_link_libraries):
  Target "LLVMCustom" of type UTILITY may not be linked into another target.
  One may link only to INTERFACE, STATIC or SHARED libraries, or to
  executables with the ENABLE_EXPORTS property set.
Call Stack (most recent call first):
  cmake/modules/LLVM-Config.cmake:93 (explicit_llvm_config)
  cmake/modules/AddLLVM.cmake:759 (llvm_config)
  cmake/modules/AddLLVM.cmake:860 (add_llvm_executable)
  tools/bugpoint/CMakeLists.txt:24 (add_llvm_tool)


-- BugpointPasses ignored -- Loadable modules not supported on this platform.
CMake Error at cmake/modules/LLVM-Config.cmake:105 (target_link_libraries):
  Target "LLVMCustom" of type UTILITY may not be linked into another target.
  One may link only to INTERFACE, STATIC or SHARED libraries, or to
  executables with the ENABLE_EXPORTS property set.
Call Stack (most recent call first):
  cmake/modules/LLVM-Config.cmake:93 (explicit_llvm_config)
  cmake/modules/AddLLVM.cmake:759 (llvm_config)
  cmake/modules/AddLLVM.cmake:860 (add_llvm_executable)
  tools/opt/CMakeLists.txt:26 (add_llvm_tool)


-- Configuring incomplete, errors occurred!
See also "F:/svnlocal/temp/llvm/llvm6.0/build/CMakeFiles/CMakeOutput.log".
See also "F:/svnlocal/temp/llvm/llvm6.0/build/CMakeFiles/CMakeError.log".

這個錯誤是因為我偷懶導致，直接複製了Hello資料夾下的CMakeLists.txt，然後只修改了LLVMHello為：LLVMCustom

add_llvm_loadable_module( LLVMCustom
  MyPass.cpp
  Custom.cpp

  DEPENDS
  intrinsics_gen
  PLUGIN_TOOL
  opt
  )

實際上應該是add_llvm_library：

add_llvm_library( LLVMCustom
    MyPass.cpp
    Custom.cpp
)

這一點參考文章是正確的，只是因為直接複製了Hello的CMakeLists.txt而沒有完全修改正確而已，add_llvm_loadable_module也要修改掉，修改為add_llvm_library。

錯誤4

CMake Error at cmake/modules/AddLLVM.cmake:1333 (add_dependencies):
  The dependency target "LLVMHello" of target "check-all" does not exist.
Call Stack (most recent call first):
  CMakeLists.txt:919 (add_lit_target)


CMake Error at cmake/modules/AddLLVM.cmake:1333 (add_dependencies):
  The dependency target "LLVMHello" of target "check-llvm" does not exist.
Call Stack (most recent call first):
  cmake/modules/AddLLVM.cmake:1354 (add_lit_target)
  test/CMakeLists.txt:157 (add_lit_testsuite)

llvm/test/CMakeLists.txt中刪除LLVMHello，如果出現類似的錯誤，就找到所有有關Hello工程的CMakeLists.txt和LLVMBuild.txt，把關於對Hello的行註釋掉或去掉。

錯誤5

not found unwrap

參考文章的CPP檔案程式碼中少了標頭檔案引用，導致unwrap找不到，正確的Custom.cpp程式碼如下：

#include "llvm/InitializePasses.h"
#include "llvm-c/Initialization.h"
#include "llvm/InitializePasses.h"
#include "llvm/PassRegistry.h"


using namespace llvm;

/// initializeCustom - Initialize all passes in the Custom library
void llvm::initializeCustom(PassRegistry &Registry) {
  initializeMyPassPass(Registry);
}

/// LLVMInitializeCustom - C binding for initializeCustom.
void LLVMInitializeCustom(LLVMPassRegistryRef R) {
    initializeCustom(*unwrap(R));
}

梳理一遍完整的步驟

基於llvm6.0介紹

步驟1：建立模組及原始碼檔案

在llvm/lib/Transforms目錄下建立一個Custom資料夾，這裡存放要自定義的pass，建立一個MyPass.cpp，內容如下：

#define DEBUG_TYPE "mypass"
#include "llvm/IR/Function.h"
#include "llvm/Pass.h"
#include "llvm/Transforms/Custom.h"
#include "llvm/Support/raw_ostream.h"
using namespace llvm;

namespace {
  struct MyPass : public FunctionPass {
      bool m_flag;
    static char ID;
    MyPass() : MyPass(ID) { m_flag = false; }
    MyPass(bool flag) : FunctionPass(ID) {
        this->m_flag = flag;
      initializeMyPassPass(*PassRegistry::getPassRegistry());
    }

    /*
     * Just print the function name 
     */
    bool runOnFunction(Function &F) {
        bool Changed = false;
        if (m_flag == false) {
            return Changed;
        }

        errs().write_escaped(F.getName()) << "\n";
        return Changed;
    }
  };
}

char MyPass::ID = 0;
INITIALIZE_PASS(MyPass, "mypass", "Print all function names", false, false)

FunctionPass *llvm::createMyPassPass(bool flag) {
  return new MyPass(flag);
}

再建立一個Custom.cpp，內容如下：

#include "llvm/InitializePasses.h"
#include "llvm-c/Initialization.h"
#include "llvm/InitializePasses.h"
#include "llvm/PassRegistry.h"


using namespace llvm;

// initializeCustom - Initialize all passes in the Custom library
void llvm::initializeCustom(PassRegistry &Registry) {
  initializeMyPassPass(Registry);
}

// LLVMInitializeCustom - C binding for initializeCustom.
void LLVMInitializeCustom(LLVMPassRegistryRef R) {
    initializeCustom(*unwrap(R));
}

步驟2 配置LLVMBuild.txt和CMakeLists.txt

在Custom資料夾下建立LLVMBuild.txt，內容如下：

[component_0]
type = Library
name = Custom
parent = Transforms

簡單解釋下：
- type：是該模組的編譯後的檔案形式，這裡是Library，也就是編譯作為庫檔案。
- name：是該模組的名稱，這裡就填Custom，一般跟當前所在的資料夾同名。
- parent：當前資料夾的父目錄，一般是Transforms。

在Custom資料夾下建立CMakeLists.txt，內容如下：

add_llvm_library( LLVMCustom
    MyPass.cpp
    Custom.cpp
)

步驟3 建立模組包含標頭檔案

在llvm/include/llvm/Transforms/目錄下建立Custom.h，內容如下：

#ifndef LLVM_CUSTOM_H
#define LLVM_CUSTOM_H

#include "llvm/Pass.h"


namespace llvm {

FunctionPass *createMyPassPass(bool flag);

}

#endif

開啟編輯檔案llvm/include/llvm/InitializePasses.h，在名稱空間llvm中的程式碼塊末尾新增程式碼：

void initializeCustom(PassRegistry&);
void initializeMyPassPass(PassRegistry&);

開啟編輯檔案llvm/include/llvm/LinkAllPasses.h，在ForcePassLinking建構函式中新增程式碼：

#include "llvm/Transforms/Custom.h"

// This part must reside in the constructor of struct ForcePassLinking
(void) llvm::createMyPassPass(true);

步驟4 修改各種LLVMBuild.txt和CMakeLists.txt

• 修改llvm/lib/Transforms/LLVMBuild.txt，common節的subdirectories內容新增：Custom
• 修改llvm/lib/Transforms/CMakeLists.txt，末尾新增：

add_subdirectory(Custom)

• 修改llvm/tools/opt/LLVMBuild.txt，required_libraries內容新增：Custom
• 修改llvm/tools/opt/CMakeLists.txt，在set(LLVM_LINK_COMPONENTS中新增：Custom
• 修改llvm/tools/bugpoint/LLVMBuild.txt，required_libraries內容新增：Custom
• 修改llvm/tools/bugpoint/CMakeLists.txt在set(LLVM_LINK_COMPONENTS中新增：Custom

步驟5 修改opt.cpp

修改llvm/tools/opt/opt.cpp，在main函式中新增程式碼：

initializeCustom(Registry);

步驟6 CMake GUI重新生成下sln

因為是直接修改的CMakeLists.txt和LLVMBuild.txt，VisualStudio是識別不了的，如果直接編譯opt專案最終會出現連結錯誤，提示找不到Custom中新增的函式。

這個時候再次開啟CMakeGUI再次生成下sln工程，由於CMakeGUI有快取機制，不會對其他檔案造成修改，速度是很快的。

目的就是要讓llvm/lib/Transforms/CMakeLists.txt中的這句生效：

add_subdirectory(Custom)

然後重新開啟VisualStudio，直接定位到opt專案，單獨編譯即可。

編譯執行

VisualStudio2017開啟專案工程，找到Tools分組下的opt，單獨編譯，比全部編譯時間會少一些。連結的過程略長，編譯的opt.exe在llvm/build/Debug/bin目錄下，debug版本的有100多MB！總共耗時30多分鐘吧，也夠長的了。

命令列下執行

opt.exe -help

輸出的內容：

……
-mypass                                         - Print all function names
……

說明自定義的pass已經被編譯進opt裡啦。

原理解讀

上面程式碼中並沒有看到函式initializeMyPassPass的定義，這個是由巨集INITIALIZE_PASS自動生成的。

INITIALIZE_PASS(MyPass, "mypass", "Print all function names", false, false)

可以參見INITIALIZE_PASS巨集定義：

#define INITIALIZE_PASS(passName, arg, name, cfg, analysis)                    \
  static void *initialize##passName##PassOnce(PassRegistry &Registry) {        \
    PassInfo *PI = new PassInfo(                                               \
        name, arg, &passName::ID,                                              \
        PassInfo::NormalCtor_t(callDefaultCtor<passName>), cfg, analysis);     \
    Registry.registerPass(*PI, true);                                          \
    return PI;                                                                 \
  }                                                                            \
  static llvm::once_flag Initialize##passName##PassFlag;                       \
  void llvm::initialize##passName##Pass(PassRegistry &Registry) {              \
    llvm::call_once(Initialize##passName##PassFlag,                            \
                    initialize##passName##PassOnce, std::ref(Registry));       \
  }

整合到CLANG

完成上面的步驟，可以把pass整合到opt中，但是如果使用clang的話是沒有這些功能的，所以仍需要做些處理。
- llvm/lib/Transforms/IPO/PassManagerBuilder.cpp

#include "llvm/Transforms/Custom.h"

static cl::opt<bool>
MyPass("mypass", cl::init(false), cl::Hidden,
    cl::ZeroOrMore, cl::desc("Print all function names"));

在PassManagerBuilder::populateModulePassManager函式中新增：

MPM.add(createMyPassPass(MyPass));

這裡設計的是一個開關，當用戶使用了-mllvm -mypass引數的時候，MyPass為true，相當於啟用了該pass。

• llvm/lib/Transforms/IPO/LLVMBuild.txt新增Custom
• llvm/lib/Target/X86/LLVMBuild.txt新增Custom

最後記住用CMkaeGUI重新生成一下解決方案，然後VisualStudio只編譯clang模組即可。

使用clang時開啟該pass：

clang.exe -mllvm -mypass example.c -o example.exe