[clang] -ftime-report: reorganize timers (PR #122225)

[Fangrui Song via cfe-commits]

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@@ -1346,14 +1344,30 @@ static void runThinLTOBackend(
   }
 }
 
-void clang::EmitBackendOutput(
-    DiagnosticsEngine &Diags, const HeaderSearchOptions &HeaderOpts,
-    const CodeGenOptions &CGOpts, const clang::TargetOptions &TOpts,
-    const LangOptions &LOpts, StringRef TDesc, llvm::Module *M,
-    BackendAction Action, IntrusiveRefCntPtr<llvm::vfs::FileSystem> VFS,
-    std::unique_ptr<raw_pwrite_stream> OS, BackendConsumer *BC) {
-
+void clang::emitBackendOutput(CompilerInstance &CI, StringRef TDesc,
+                              llvm::Module *M, BackendAction Action,
+                              IntrusiveRefCntPtr<llvm::vfs::FileSystem> VFS,
+                              std::unique_ptr<raw_pwrite_stream> OS,
+                              BackendConsumer *BC) {
   llvm::TimeTraceScope TimeScope("Backend");
+  DiagnosticsEngine &Diags = CI.getDiagnostics();
+  const auto &HeaderOpts = CI.getHeaderSearchOpts();
+  const auto &CGOpts = CI.getCodeGenOpts();
+  const auto &TOpts = CI.getTargetOpts();
+  const auto &LOpts = CI.getLangOpts();
+
+  Timer timer;
+  if (CGOpts.TimePasses) {
+    CI.getFrontendTimer().stopTimer();
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MaskRay wrote:

"Clang front-end" has a `TimeRegion` in `clang/lib/Frontend/FrontendAction.cpp`. 

This PR repurposes the "Clang front-end timer" (which included backend time) to remove the backend part. If we don't stop "Clang front-end", "Clang front-end" and "Backend" in the same group would double count the time...

I recalled another complaint  https://aras-p.info/blog/2019/01/12/Investigating-compile-times-and-Clang-ftime-report/

> “Clang front-end time report” seems to be time for “everything”, not just the frontend (Clang frontend, LLVM backend, and whatever else it did).

https://github.com/llvm/llvm-project/pull/122225