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
1 int foo(int x) {
2 x = x + 1;
3 if (x > 2)
4 x++;
5 else {
6 x += 2;
7 x *= 2;
8 }
9
10 return x;
11 }
View Code
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：
1 //------------------------------------------------------------------------------
2 // Tooling sample. Demonstrates:
3 //
4 // CFG Demo to show how to use CFG
5 //
6 // jourluohua (jourluohua@sina.com)
7 // This code is in the public domain
8 //------------------------------------------------------------------------------
9 #include <sstream>
10 #include <string>
11 #include <map>
12
13 #include "clang/AST/AST.h"
14 #include "clang/AST/ASTConsumer.h"
15 #include "clang/AST/RecursiveASTVisitor.h"
16 #include "clang/Frontend/ASTConsumers.h"
17 #include "clang/Frontend/CompilerInstance.h"
18 #include "clang/Frontend/FrontendActions.h"
19 #include "clang/Rewrite/Core/Rewriter.h"
20 #include "clang/Tooling/CommonOptionsParser.h"
21 #include "clang/Tooling/Tooling.h"
22 #include "llvm/Support/raw_ostream.h"
23 #include "clang/Analysis/CFG.h"
24 #include "clang/Analysis/CFGStmtMap.h"
25 #include "clang/Analysis/Analyses/CFGReachabilityAnalysis.h"
26 #include "clang/Analysis/Analyses/CalledOnceCheck.h"
27 #include "clang/Analysis/Analyses/Consumed.h"
28 #include "clang/Analysis/Analyses/ReachableCode.h"
29 #include "clang/Analysis/Analyses/ThreadSafety.h"
30 #include "clang/Analysis/Analyses/UninitializedValues.h"
31 #include "clang/Analysis/AnalysisDeclContext.h"
32
33 using namespace clang;
34 using namespace clang::driver;
35 using namespace clang::tooling;
36
37
38 static llvm::cl::OptionCategory ToolingSampleCategory("Tooling Sample");
39
40 // Implementation of the ASTConsumer interface for reading an AST produced
41 // by the Clang parser.
42 class MyASTConsumer : public ASTConsumer {
43 public:
44 MyASTConsumer(Rewriter &R) {
45 }
46
47 // Override the method that gets called for each parsed top-level
48 // declaration.
49 bool HandleTopLevelDecl(DeclGroupRef DR) override {
50 for (DeclGroupRef::iterator b = DR.begin(), e = DR.end(); b != e; ++b) {
51 // Traverse the declaration using our AST visitor.
52 (*b)->dump();
53 // Construct the analysis context with the specified CFG build options.
54 AnalysisDeclContext AC(/* AnalysisDeclContextManager */ nullptr, *b);
55 // Don't generate EH edges for CallExprs as we'd like to avoid the n^2
56 // explosion for destructors that can result and the compile time hit.
57 if(&AC == nullptr){
58 continue;
59 }
60 AC.getCFGBuildOptions().PruneTriviallyFalseEdges = true;
61 AC.getCFGBuildOptions().AddEHEdges = false;
62 AC.getCFGBuildOptions().AddInitializers = true;
63 AC.getCFGBuildOptions().AddImplicitDtors = true;
64 AC.getCFGBuildOptions().AddTemporaryDtors = true;
65 AC.getCFGBuildOptions().AddCXXNewAllocator = false;
66 AC.getCFGBuildOptions().AddCXXDefaultInitExprInCtors = true;
67
68 AC.getCFGBuildOptions().setAllAlwaysAdd();
69
70 if(CFG *cfg = AC.getCFG()){
71 cfg->dump(LangOptions(), false);
72 }
73 }
74 return true;
75 }
76
77 private:
78 };
79
80 // For each source file provided to the tool, a new FrontendAction is created.
81 class MyFrontendAction : public ASTFrontendAction {
82 public:
83 MyFrontendAction() {}
84 void EndSourceFileAction() override {
85 SourceManager &SM = TheRewriter.getSourceMgr();
86 llvm::errs() << "** EndSourceFileAction for: "
87 << SM.getFileEntryForID(SM.getMainFileID())->getName() << "\n";
88
89 // Now emit the rewritten buffer.
90 TheRewriter.getEditBuffer(SM.getMainFileID()).write(llvm::outs());
91 }
92
93 std::unique_ptr<ASTConsumer> CreateASTConsumer(CompilerInstance &CI,
94 StringRef file) override {
95 llvm::errs() << "** Creating AST consumer for: " << file << "\n";
96 TheRewriter.setSourceMgr(CI.getSourceManager(), CI.getLangOpts());
97 return std::make_unique<MyASTConsumer>(TheRewriter);
98 }
99
100 private:
101 Rewriter TheRewriter;
102 };
103
104 int main(int argc, const char **argv) {
105 llvm::Expected<CommonOptionsParser> op=CommonOptionsParser::create(argc, argv, ToolingSampleCategory);
106
107 ClangTool Tool(op.get().getCompilations(), op.get().getSourcePathList());
108
109 // ClangTool::run accepts a FrontendActionFactory, which is then used to
110 // create new objects implementing the FrontendAction interface. Here we use
111 // the helper newFrontendActionFactory to create a default factory that will
112 // return a new MyFrontendAction object every time.
113 // To further customize this, we could create our own factory class.
114 return Tool.run(newFrontendActionFactory<MyFrontendAction>().get());
115 }
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：