Clang教程之实现源源变化

这篇博客是之前就写过的，因为最近在制作一个新的系列教程，又重新下载了最新的clang/llvm，因此准备更新一下，因此本篇中仅描述新（13.0）旧（4.0）的不同，原文地址：https://www.cnblogs.com/jourluohua/p/9526663.html

基本可以这么说，实现思路没有变化，只有部分函数API发生了变化，我也会额外强调下。 从4.0到13.0，clang前端丰富了非常多，在现在的clang/tools下可以看到非常多的Tooling，因此整个文件夹被我提升到了clang/tools/extra下，没有了原来的loop-convert一层，现在的Cmake版本也需要3.10版本以上，同时现在的cmake要求文件夹名字和库名字不能相同，CMakeLists.txt文件也发生了变化，需要增加attr进行标识。关键的CMakeLists.txt变成了：

set(LLVM_LINK_COMPONENTS support)

add_clang_executable(loop-convert
  LoopConvert.cpp
  )
target_link_libraries(loop-convert
  PRIVATE
  clangTooling
  clangBasic
  clangASTMatchers
  )

同时，LoopConveret.cpp中代码变为了：

//------------------------------------------------------------------------------
// Tooling sample. Demonstrates:
//
// * How to write a simple source tool using libTooling.
// * How to use RecursiveASTVisitor to find interesting AST nodes.
// * How to use the Rewriter API to rewrite the source code.
//
// Eli Bendersky (eliben@gmail.com)
// This code is in the public domain
//------------------------------------------------------------------------------
#include <sstream>
#include <string>

#include "clang/AST/AST.h"
#include "clang/AST/ASTConsumer.h"
#include "clang/AST/RecursiveASTVisitor.h"
#include "clang/Frontend/ASTConsumers.h"
#include "clang/Frontend/CompilerInstance.h"
#include "clang/Frontend/FrontendActions.h"
#include "clang/Rewrite/Core/Rewriter.h"
#include "clang/Tooling/CommonOptionsParser.h"
#include "clang/Tooling/Tooling.h"
#include "llvm/Support/raw_ostream.h"

using namespace clang;
using namespace clang::driver;
using namespace clang::tooling;

static llvm::cl::OptionCategory ToolingSampleCategory("Tooling Sample");

// By implementing RecursiveASTVisitor, we can specify which AST nodes
// we're interested in by overriding relevant methods.
class MyASTVisitor : public RecursiveASTVisitor<MyASTVisitor> {
public:
  MyASTVisitor(Rewriter &R) : TheRewriter(R) {}

  bool VisitStmt(Stmt *s) {
    // Only care about If statements.
    if (isa<IfStmt>(s)) {
      IfStmt *IfStatement = cast<IfStmt>(s);
      Stmt *Then = IfStatement->getThen();

      TheRewriter.InsertText(Then->getBeginLoc(), "// the 'if' part\n", true,
                             true);

      Stmt *Else = IfStatement->getElse();
      if (Else)
        TheRewriter.InsertText(Else->getBeginLoc(), "// the 'else' part\n",
                               true, true);
    }

    return true;
  }

  bool VisitFunctionDecl(FunctionDecl *f) {
    // Only function definitions (with bodies), not declarations.
    if (f->hasBody()) {
      Stmt *FuncBody = f->getBody();

      // Type name as string
      QualType QT = f->getReturnType();
      std::string TypeStr = QT.getAsString();

      // Function name
      DeclarationName DeclName = f->getNameInfo().getName();
      std::string FuncName = DeclName.getAsString();

      // Add comment before
      std::stringstream SSBefore;
      SSBefore << "// Begin function " << FuncName << " returning " << TypeStr
               << "\n";
      SourceLocation ST = f->getSourceRange().getBegin();
      TheRewriter.InsertText(ST, SSBefore.str(), true, true);

      // And after
      std::stringstream SSAfter;
      SSAfter << "\n// End function " << FuncName;
      ST = FuncBody->getEndLoc().getLocWithOffset(1);
      TheRewriter.InsertText(ST, SSAfter.str(), true, true);
    }

    return true;
  }

private:
  Rewriter &TheRewriter;
};

// Implementation of the ASTConsumer interface for reading an AST produced
// by the Clang parser.
class MyASTConsumer : public ASTConsumer {
public:
  MyASTConsumer(Rewriter &R) : Visitor(R) {}

  // Override the method that gets called for each parsed top-level
  // declaration.
  bool HandleTopLevelDecl(DeclGroupRef DR) override {
    for (DeclGroupRef::iterator b = DR.begin(), e = DR.end(); b != e; ++b) {
      // Traverse the declaration using our AST visitor.
      Visitor.TraverseDecl(*b);
      (*b)->dump();
    }
    return true;
  }

private:
  MyASTVisitor Visitor;
};

// For each source file provided to the tool, a new FrontendAction is created.
class MyFrontendAction : public ASTFrontendAction {
public:
  MyFrontendAction() {}
  void EndSourceFileAction() override {
    SourceManager &SM = TheRewriter.getSourceMgr();
    llvm::errs() << "** EndSourceFileAction for: "
                 << SM.getFileEntryForID(SM.getMainFileID())->getName() << "\n";

    // Now emit the rewritten buffer.
    TheRewriter.getEditBuffer(SM.getMainFileID()).write(llvm::outs());
  }

  std::unique_ptr<ASTConsumer> CreateASTConsumer(CompilerInstance &CI,
                                                 StringRef file) override {
    llvm::errs() << "** Creating AST consumer for: " << file << "\n";
    TheRewriter.setSourceMgr(CI.getSourceManager(), CI.getLangOpts());
    return std::make_unique<MyASTConsumer>(TheRewriter);
  }

private:
  Rewriter TheRewriter;
};

int main(int argc, const char **argv) {
  llvm::Expected<CommonOptionsParser> op=CommonOptionsParser::create(argc, argv, ToolingSampleCategory);
  
  ClangTool Tool(op.get().getCompilations(), op.get().getSourcePathList());

  // ClangTool::run accepts a FrontendActionFactory, which is then used to
  // create new objects implementing the FrontendAction interface. Here we use
  // the helper newFrontendActionFactory to create a default factory that will
  // return a new MyFrontendAction object every time.
  // To further customize this, we could create our own factory class.
  return Tool.run(newFrontendActionFactory<MyFrontendAction>().get());
}

主要是原来是用getLocStart()->getBeginLoc()
getLocEnd() ->getEndLoc()
还有一个是新的llvm中添加了很多错误判断和状态控制的机制，返回值通常被包在Expected中，需要使用get()函数获取对象，也可以使用takeError()来获取错误返回值。