有关更多详细信息，请阅读 官方文件资料.

需要修改 <llvm project source dir>\include\llvm\Support\Compiler.h, 使生成pass的dll能正常导出接口.

设置以下几个值：

点击Configure->Generate, 等待生成Visual Studio 2019 Project的项目(默认是x64工程), 如果想生成x86工程需要在CMake GUI配置Configure的时候配置项为Win32.

打开Cmake Gui配置生成项目build所在路径,双击打开LLVM.sln.
在Visual Studio 2019中开始编译LLVM，编译时间根据你PC的配置情况(i7-8700k 16g)前后编译时间长达5个小时，最后生成成功的build目录大小(Debug)152G.

生成成功,到此完成了LLVM的构建.

以下代码为例：

有关更多详细信息，请阅读 官方文件资料.

设置以下几个值：

打开Cmake Gui配置生成项目build所在路径,双击打开llvm-tutor.sln编译,成功后在build\bin\Debug目录下生HelloWorld.dll

到此完成了LLVM Pass HelloWorld.dll的构建.

HelloWorld.dll不会修改input_for_hello.ll模块,使用-disable-output是为了防止输出bitcode文件.

我们通过Visual Studio 2019来调试HelloWorld.dll的模块,方便了解LLVM Pass使用过程.
打开llvm-tutor.sln项目, 配置HelloWorld为设为启动项目

设置以下几个值：

有问题可以关注下我的 Github

<llvm project source dir>/
    |
    <projects>/
        |
        <compiler-rt>
        <libcxx>
        <libcxxabi>
        <libunwind>
        <openmp>
        ...
    <lib>
        |
        ...
    <tools>
        |
        <clang>
            |
            <tools>
                |
                <clang-tools-extra>
                ...
            ...
        <lld>
        <lldb>
        <polly>
        ...

// FIXME: Provide this for PE/COFF targets.
#if (__has_attribute(weak) || LLVM_GNUC_PREREQ(4, 0, 0)) &&                    \
    (!defined(__MINGW32__) && !defined(__CYGWIN__) && !defined(_WIN32))
#define LLVM_ATTRIBUTE_WEAK __attribute__((__weak__))
#else
#define LLVM_ATTRIBUTE_WEAK __declspec(dllexport)
#endif

<llvm-tutor source dir>/
    |
    CMakeLists.txt
    <HelloWorld>/
        |
        HelloWorld.cpp
        CMakeLists.txt

cmake_minimum_required(VERSION 3.4.3)
project(llvm-tutor)

#===============================================================================
# 1. VERIFY LLVM INSTALLATION DIR
#===============================================================================
# Set this to a valid LLVM installation dir
set(LT_LLVM_INSTALL_DIR "" CACHE PATH "LLVM installation directory")

# A bit of a sanity checking
set(LT_LLVM_INCLUDE_DIR "${LT_LLVM_INSTALL_DIR}/include/llvm")
if(NOT EXISTS "${LT_LLVM_INCLUDE_DIR}")
message(FATAL_ERROR
  " LT_LLVM_INSTALL_DIR (${LT_LLVM_INCLUDE_DIR}) is invalid.")
endif()

set(LT_LLVM_CMAKE_FILE "${LT_LLVM_INSTALL_DIR}/lib/cmake/llvm/LLVMConfig.cmake")
if(NOT EXISTS "${LT_LLVM_CMAKE_FILE}")
message(FATAL_ERROR
    " LT_LLVM_CMAKE_FILE (${LT_LLVM_CMAKE_FILE}) is invalid.")
endif()

#===============================================================================
# 2. LOAD LLVM CONFIGURATION
#    For more: http://llvm.org/docs/CMake.html#embedding-llvm-in-your-project
#===============================================================================
# Add the location of LLVMConfig.cmake to CMake search paths (so that
# find_package can locate it)
list(APPEND CMAKE_PREFIX_PATH "${LT_LLVM_INSTALL_DIR}/lib/cmake/llvm/")

find_package(LLVM 10.0.0 REQUIRED CONFIG)
message(STATUS "Found LLVM ${LLVM_PACKAGE_VERSION}")
message(STATUS "Using LLVMConfig.cmake in: ${LT_LLVM_INSTALL_DIR}")

message("LLVM STATUS:
  Definitions ${LLVM_DEFINITIONS}
  Includes    ${LLVM_INCLUDE_DIRS}
  Libraries   ${LLVM_LIBRARY_DIRS}
  Targets     ${LLVM_TARGETS_TO_BUILD}"
)

# Set the LLVM header and library paths
include_directories(${LLVM_INCLUDE_DIRS})
link_directories(${LLVM_LIBRARY_DIRS})
add_definitions(${LLVM_DEFINITIONS})

#===============================================================================
# 3. LLVM-TUTOR BUILD CONFIGURATION
#===============================================================================
# Use the same C++ standard as LLVM does
set(CMAKE_CXX_STANDARD 14 CACHE STRING "")

# Build type
if (NOT CMAKE_BUILD_TYPE)
  set(CMAKE_BUILD_TYPE Debug CACHE
      STRING "Build type (default Debug):" FORCE)
endif()
if(NOT MSVC)
    # Compiler flags
    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall\
        -fdiagnostics-color=always")

    # LLVM is normally built without RTTI. Be consistent with that.
    if(NOT LLVM_ENABLE_RTTI)
      set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fno-rtti")
    endif()

    # -fvisibility-inlines-hidden is set when building LLVM and on Darwin warnings
    # are triggered if llvm-tutor is built without this flag (though otherwise it
    # builds fine). For consistency, add it here too.
    include(CheckCXXCompilerFlag)
    check_cxx_compiler_flag("-fvisibility-inlines-hidden" SUPPORTS_FVISIBILITY_INLINES_HIDDEN_FLAG)
    if (${SUPPORTS_FVISIBILITY_INLINES_HIDDEN_FLAG} EQUAL "1")
      set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fvisibility-inlines-hidden")
    endif()
endif()
# Set the build directories
set(CMAKE_RUNTIME_OUTPUT_DIRECTORY "${PROJECT_BINARY_DIR}/bin")
set(CMAKE_LIBRARY_OUTPUT_DIRECTORY "${PROJECT_BINARY_DIR}/lib")

#===============================================================================
# 4. ADD SUB-TARGETS
# Doing this at the end so that all definitions and link/include paths are
# available for the sub-projects.
#===============================================================================

add_subdirectory(HelloWorld)

#include "llvm/IR/LegacyPassManager.h"
#include "llvm/Passes/PassBuilder.h"
#include "llvm/Passes/PassPlugin.h"
#include "llvm/Support/raw_ostream.h"

using namespace llvm;

//-----------------------------------------------------------------------------
// HelloWorld implementation
//-----------------------------------------------------------------------------
// No need to expose the internals of the pass to the outside world - keep
// everything in an anonymous namespace.
namespace {

// This method implements what the pass does
void visitor(Function &F) {
  errs() << "(llvm-tutor) Hello from: " << F.getName() << "\n";
  errs() << "(llvm-tutor)   number of arguments: " << F.arg_size() << "\n";
}

// New PM implementation
struct HelloWorld : PassInfoMixin<HelloWorld> {
  // Main entry point, takes IR unit to run the pass on (&F) and the
  // corresponding pass manager (to be queried if need be)
  PreservedAnalyses run(Function &F, FunctionAnalysisManager &) {
    visitor(F);
    return PreservedAnalyses::all();
  }
};

// Legacy PM implementation
struct LegacyHelloWorld : public FunctionPass {
  static char ID;
  LegacyHelloWorld() : FunctionPass(ID) {}
  // Main entry point - the name conveys what unit of IR this is to be run on.
  bool runOnFunction(Function &F) override {
    visitor(F);
    // Doesn't modify the input unit of IR, hence 'false'
    return false;
  }
};
} // namespace

//-----------------------------------------------------------------------------
// New PM Registration
//-----------------------------------------------------------------------------
llvm::PassPluginLibraryInfo getHelloWorldPluginInfo() {
  return {LLVM_PLUGIN_API_VERSION, "HelloWorld", LLVM_VERSION_STRING,
          [](PassBuilder &PB) {
            PB.registerPipelineParsingCallback(
                [](StringRef Name, FunctionPassManager &FPM,
                   ArrayRef<PassBuilder::PipelineElement>) {
                  if (Name == "hello-world") {
                    FPM.addPass(HelloWorld());
                    return true;
                  }
                  return false;
                });
          }};
}

// This is the core interface for pass plugins. It guarantees that 'opt' will
// be able to recognize HelloWorld when added to the pass pipeline on the
// command line, i.e. via '-passes=hello-world'
extern "C" LLVM_ATTRIBUTE_WEAK ::llvm::PassPluginLibraryInfo
llvmGetPassPluginInfo() {
  return getHelloWorldPluginInfo();
}

//-----------------------------------------------------------------------------
// Legacy PM Registration
//-----------------------------------------------------------------------------
// The address of this variable is used to uniquely identify the pass. The
// actual value doesn't matter.
char LegacyHelloWorld::ID = 0;

// This is the core interface for pass plugins. It guarantees that 'opt' will
// recognize LegacyHelloWorld when added to the pass pipeline on the command
// line, i.e.  via '--legacy-hello-world'
static RegisterPass<LegacyHelloWorld>
    X("legacy-hello-world", "Hello World Pass",
      true, // This pass doesn't modify the CFG => true
      false // This pass is not a pure analysis pass => false
    );

cmake_minimum_required(VERSION 3.4.3)
project(llvm-tutor-hello-world)

#===============================================================================
# 1. LOAD LLVM CONFIGURATION
#===============================================================================
# Set this to a valid LLVM installation dir
set(LT_LLVM_INSTALL_DIR "" CACHE PATH "LLVM installation directory")

# Add the location of LLVMConfig.cmake to CMake search paths (so that
# find_package can locate it)
list(APPEND CMAKE_PREFIX_PATH "${LT_LLVM_INSTALL_DIR}/lib/cmake/llvm/")

find_package(LLVM 10.0.0 REQUIRED CONFIG)

# HelloWorld includes headers from LLVM - update the include paths accordingly
include_directories(${LLVM_INCLUDE_DIRS})

#===============================================================================
# 2. LLVM-TUTOR BUILD CONFIGURATION
#===============================================================================
# Use the same C++ standard as LLVM does
set(CMAKE_CXX_STANDARD 14 CACHE STRING "")

if(NOT MSVC)
    # LLVM is normally built without RTTI. Be consistent with that.
    if(NOT LLVM_ENABLE_RTTI)
      set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fno-rtti")
    endif()
endif()
#===============================================================================
# 3. ADD THE TARGET
#===============================================================================
add_library(HelloWorld SHARED HelloWorld.cpp)

if(NOT MSVC)
    # Allow undefined symbols in shared objects on Darwin (this is the default
    # behaviour on Linux)
    target_link_libraries(HelloWorld
      "$<$<PLATFORM_ID:Darwin>:-undefined dynamic_lookup>")
else()
    llvm_map_components_to_libnames(llvm_libs Support Core IRReader TransformUtils)
    target_link_libraries(HelloWorld PRIVATE ${llvm_libs})
endif()

long f(long a, long b)
{
    long x = 0;
    if (a > b) {
        x = 1;
    } 
    else {
        x = 2;
    }

    return x;
}

int main() {

    f(1, 2);

    return 0;
}

# Generate an LLVM test file
$LLVM_DIR/bin/clang -S -emit-llvm input_for_hello.c -o input_for_hello.ll

# Run the pass
$LLVM_DIR/bin/opt -load-pass-plugin HelloWorld.dll -passes=hello-world -disable-output input_for_hello.ll
# Expected output
(llvm-tutor) Hello from: f
(llvm-tutor)   number of arguments: 2
(llvm-tutor) Hello from: main
(llvm-tutor)   number of arguments: 0

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