[clang][modules] Avoid unnecessary writes of .timestamp files

[jansvoboda11]

Summary

Clang currently updates the mtime of .timestamp files on each load of the corresponding .pcm file. This is not necessary. In a given build session, Clang only needs to write the .timestamp file once, when we first validate the input files. This patch makes it so that we only touch the .timestamp file when it's older than the build session, alleviating some filesystem contention in clang-scan-deps.

clang/lib/Serialization/ASTReader.cpp

ASTReader::ASTReadResult ASTReader::ReadAST(StringRef FileName,
   if (ContextObj) {
     for (unsigned I = 0, N = ObjCClassesLoaded.size(); I != N; ++I) {
       loadObjCCategories(ObjCClassesLoaded[I]->getGlobalID(),
                          ObjCClassesLoaded[I],
                          PreviousGeneration);
     }
   }
 
-  if (PP.getHeaderSearchInfo()
-          .getHeaderSearchOpts()
-          .ModulesValidateOncePerBuildSession) {
+  HeaderSearchOptions &HSOpts = PP.getHeaderSearchInfo().getHeaderSearchOpts();
+  if (HSOpts.ModulesValidateOncePerBuildSession) {
     // Now we are certain that the module and all modules it depends on are
-    // up to date.  Create or update timestamp files for modules that are
-    // located in the module cache (not for PCH files that could be anywhere
-    // in the filesystem).
+    // up-to-date. For implicitly-built module files, ensure the corresponding
+    // timestamp files are up-to-date in this build session.
     for (unsigned I = 0, N = Loaded.size(); I != N; ++I) {
       ImportedModule &M = Loaded[I];
-      if (M.Mod->Kind == MK_ImplicitModule) {
+      if (M.Mod->Kind == MK_ImplicitModule &&
+          M.Mod->InputFilesValidationTimestamp < HSOpts.BuildSessionTimestamp)
         updateModuleTimestamp(*M.Mod);
-      }
-    }
-  }
 
   return Success;
 }
 
 static ASTFileSignature readASTFileSignature(StringRef PCH);
 
 /// Whether \p Stream doesn't start with the AST/PCH file magic number 'CPCH'.
 static llvm::Error doesntStartWithASTFileMagic(BitstreamCursor &Stream) {