[CodeGen] Use reentrant methods to time IR gen

[modocache]

Summary

Use the reentrant timer methods from
https://reviews.llvm.org/D36847 instead of manually keeping track of the
number of times startTimer() and stopTimer() should be called.

Test plan:
Run clang -ftime-report and confirm the time measurements are roughly
the same before and after applying this patch.

[modocache]

Use startReentrantTimer and stopReentrantTimer

lib/CodeGen/CodeGenAction.cpp

class BackendConsumer : public ASTConsumer {
     const HeaderSearchOptions &HeaderSearchOpts;
     const CodeGenOptions &CodeGenOpts;
     const TargetOptions &TargetOpts;
     const LangOptions &LangOpts;
     std::unique_ptr<raw_pwrite_stream> AsmOutStream;
     ASTContext *Context;
 
     Timer LLVMIRGeneration;
-    unsigned LLVMIRGenerationRefCount;
 
     /// True if we've finished generating IR. This prevents us from generating
     /// additional LLVM IR after emitting output in HandleTranslationUnit. This
     /// can happen when Clang plugins trigger additional AST deserialization.
     bool IRGenFinished = false;
 
     std::unique_ptr<CodeGenerator> Gen;
 
BackendConsumer(BackendAction Action, DiagnosticsEngine &Diags,
                     const std::string &InFile,
                     SmallVector<LinkModule, 4> LinkModules,
                     std::unique_ptr<raw_pwrite_stream> OS, LLVMContext &C,
                     CoverageSourceInfo *CoverageInfo = nullptr)
         : Diags(Diags), Action(Action), HeaderSearchOpts(HeaderSearchOpts),
           CodeGenOpts(CodeGenOpts), TargetOpts(TargetOpts), LangOpts(LangOpts),
           AsmOutStream(std::move(OS)), Context(nullptr),
           LLVMIRGeneration("irgen", "LLVM IR Generation Time"),
-          LLVMIRGenerationRefCount(0),
           Gen(CreateLLVMCodeGen(Diags, InFile, HeaderSearchOpts, PPOpts,
                                 CodeGenOpts, C, CoverageInfo)),
           LinkModules(std::move(LinkModules)) {
       llvm::TimePassesIsEnabled = TimePasses;
     }
     llvm::Module *getModule() const { return Gen->GetModule(); }
     std::unique_ptr<llvm::Module> takeModule() {
       return std::unique_ptr<llvm::Module>(Gen->ReleaseModule());
     }
 
     CodeGenerator *getCodeGenerator() { return Gen.get(); }
 
     void HandleCXXStaticMemberVarInstantiation(VarDecl *VD) override {
       Gen->HandleCXXStaticMemberVarInstantiation(VD);
     }
 
     void Initialize(ASTContext &Ctx) override {
       assert(!Context && "initialized multiple times");
 
       Context = &Ctx;
 
       if (llvm::TimePassesIsEnabled)
-        LLVMIRGeneration.startTimer();
+        LLVMIRGeneration.startReentrantTimer();
 
       Gen->Initialize(Ctx);
 
       if (llvm::TimePassesIsEnabled)
-        LLVMIRGeneration.stopTimer();
+        LLVMIRGeneration.stopReentrantTimer();
     }
 
     bool HandleTopLevelDecl(DeclGroupRef D) override {
       PrettyStackTraceDecl CrashInfo(*D.begin(), SourceLocation(),
                                      Context->getSourceManager(),
                                      "LLVM IR generation of declaration");
 
       // Recurse.
-      if (llvm::TimePassesIsEnabled) {
-        LLVMIRGenerationRefCount += 1;
-        if (LLVMIRGenerationRefCount == 1)
-          LLVMIRGeneration.startTimer();
-      }
+      if (llvm::TimePassesIsEnabled)
+        LLVMIRGeneration.startReentrantTimer();
 
       Gen->HandleTopLevelDecl(D);
 
-      if (llvm::TimePassesIsEnabled) {
-        LLVMIRGenerationRefCount -= 1;
-        if (LLVMIRGenerationRefCount == 0)
-          LLVMIRGeneration.stopTimer();
-      }
+      if (llvm::TimePassesIsEnabled)
+        LLVMIRGeneration.stopReentrantTimer();
 
       return true;
     }
 
     void HandleInlineFunctionDefinition(FunctionDecl *D) override {
       PrettyStackTraceDecl CrashInfo(D, SourceLocation(),
                                      Context->getSourceManager(),
                                      "LLVM IR generation of inline function");
       if (llvm::TimePassesIsEnabled)
-        LLVMIRGeneration.startTimer();
+        LLVMIRGeneration.startReentrantTimer();
 
       Gen->HandleInlineFunctionDefinition(D);
 
       if (llvm::TimePassesIsEnabled)
-        LLVMIRGeneration.stopTimer();
+        LLVMIRGeneration.stopReentrantTimer();
     }
 
     void HandleInterestingDecl(DeclGroupRef D) override {
       // Ignore interesting decls from the AST reader after IRGen is finished.
       if (!IRGenFinished)
         HandleTopLevelDecl(D);
     }
 
bool LinkInModules() {
           return true;
       }
       return false; // success
     }
 
     void HandleTranslationUnit(ASTContext &C) override {
       {
         PrettyStackTraceString CrashInfo("Per-file LLVM IR generation");
-        if (llvm::TimePassesIsEnabled) {
-          LLVMIRGenerationRefCount += 1;
-          if (LLVMIRGenerationRefCount == 1)
-            LLVMIRGeneration.startTimer();
-        }
+        if (llvm::TimePassesIsEnabled)
+          LLVMIRGeneration.startReentrantTimer();
 
         Gen->HandleTranslationUnit(C);
 
-        if (llvm::TimePassesIsEnabled) {
-          LLVMIRGenerationRefCount -= 1;
-          if (LLVMIRGenerationRefCount == 0)
-            LLVMIRGeneration.stopTimer();
-        }
+        if (llvm::TimePassesIsEnabled)
+          LLVMIRGeneration.stopReentrantTimer();
 
   IRGenFinished = true;
       }
 
       // Silently ignore if we weren't initialized for some reason.
       if (!getModule())
         return;