Clang教程之实现源源变化(5)

Clang教程之实现源源变化(5)

其实我也没想到会有这一节。一直有人在说AST上只有抽象的语法结构，没有CFG信息，不能实现某某功能等等，但就实际来说，目前的clang上边，通过AST的Anslysis也能实现一些控制流相关的东西，确实没有IR上进行比较方便和功能丰富。

先介绍下这一节要用到的一个库CFG，在clang/lib/Analysis文件夹下边，在这个文件夹下，除了CFG还有可达分析（ReachabilityAnalysis）、活跃变量（LiveVariable）、ThreadSafety和未初始化（UninitizedValues）分析等工具，本版本（13.0）功能相比之前4.0版本有了非常大的提升。

先介绍下使用的测试代码，其实是从Clang的UserManual上拷过来的。https://clang.llvm.org/docs/InternalsManual.html#basic-blocks

int foo(int x) {
  x = x + 1;
  if (x > 2)
    x++;
  else {
    x += 2;
    x *= 2;
  }

  return x;
}

其对应AST为

 从CFG（这里提供了一个CFGBlock的功能）的角度来说，因为是foo函数，可以大胆猜一下，肯定会有一个函数入口，一个函数出口，然后函数体里边有一个“x = x + 1;”的Block块，然后if/else的TrueBody和FalseBody又是两个Block，可能还有一个条件判断的Block（也可能是条件判断和“x = x +1;”这个Block合并到了一起）。

下边给出主要的实现代码：

//------------------------------------------------------------------------------
// Tooling sample. Demonstrates:
//
// CFG Demo to show how to use CFG
//
// jourluohua (jourluohua@sina.com)
// This code is in the public domain
//------------------------------------------------------------------------------
#include <sstream>
#include <string>
#include <map>

#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"
#include "clang/Analysis/CFG.h"
#include "clang/Analysis/CFGStmtMap.h"
#include "clang/Analysis/Analyses/CFGReachabilityAnalysis.h"
#include "clang/Analysis/Analyses/CalledOnceCheck.h"
#include "clang/Analysis/Analyses/Consumed.h"
#include "clang/Analysis/Analyses/ReachableCode.h"
#include "clang/Analysis/Analyses/ThreadSafety.h"
#include "clang/Analysis/Analyses/UninitializedValues.h"
#include "clang/Analysis/AnalysisDeclContext.h"

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


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

// Implementation of the ASTConsumer interface for reading an AST produced
// by the Clang parser.
class MyASTConsumer : public ASTConsumer {
public:
  MyASTConsumer(Rewriter &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.
      (*b)->dump();
      // Construct the analysis context with the specified CFG build options.
      AnalysisDeclContext AC(/* AnalysisDeclContextManager */ nullptr, *b);
      // Don't generate EH edges for CallExprs as we'd like to avoid the n^2
      // explosion for destructors that can result and the compile time hit.
      if(&AC == nullptr){
        continue;
      }
      AC.getCFGBuildOptions().PruneTriviallyFalseEdges = true;
      AC.getCFGBuildOptions().AddEHEdges = false;
      AC.getCFGBuildOptions().AddInitializers = true;
      AC.getCFGBuildOptions().AddImplicitDtors = true;
      AC.getCFGBuildOptions().AddTemporaryDtors = true;
      AC.getCFGBuildOptions().AddCXXNewAllocator = false;
      AC.getCFGBuildOptions().AddCXXDefaultInitExprInCtors = true;

      AC.getCFGBuildOptions().setAllAlwaysAdd();

      if(CFG *cfg = AC.getCFG()){
        cfg->dump(LangOptions(), false);
      }
    }
    return true;
  }

private:
};

// 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() << "
";

    // 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 << "
";
    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());
}

从代码中可以看到主要修改的是DeclRef的循环处理部分，大概流程分为以下几步：

1. 新建一个针对Decl*的AnalysisDeclContext

2. 添加针对分析的Options（尤其是setAllAlwaysAdd）

3. 得到针对这个Decl的CFG

最后得到的CFG如下：