[clang][modules][deps] Optimize in-process timestamping of PCMs (#137363)

In the past, timestamps used for
`-fmodules-validate-once-per-build-session` were found to be a source of
contention in the dependency scanner
([D149802](https://reviews.llvm.org/D149802),
https://github.com/llvm/llvm-project/pull/112452). This PR is yet
another attempt to optimize these. We now make use of the new
`ModuleCache` interface to implement the in-process version in terms of
atomic `std::time_t` variables rather the mtime attribute on
`.timestamp` files.

1 files changed, 3 insertions, 9 deletions

diff --git a/clang/lib/Serialization/ModuleManager.cpp b/clang/lib/Serialization/ModuleManager.cpp
index d466ea0..e3d7ff4 100644
--- a/clang/lib/Serialization/ModuleManager.cpp
+++ b/clang/lib/Serialization/ModuleManager.cpp
@@ -174,15 +174,9 @@ ModuleManager::addModule(StringRef FileName, ModuleKind Type,
   NewModule->ImportLoc = ImportLoc;
   NewModule->InputFilesValidationTimestamp = 0;
 
-  if (NewModule->Kind == MK_ImplicitModule) {
-    std::string TimestampFilename =
-        ModuleFile::getTimestampFilename(NewModule->FileName);
-    llvm::vfs::Status Status;
-    // A cached stat value would be fine as well.
-    if (!FileMgr.getNoncachedStatValue(TimestampFilename, Status))
-      NewModule->InputFilesValidationTimestamp =
-          llvm::sys::toTimeT(Status.getLastModificationTime());
-  }
+  if (NewModule->Kind == MK_ImplicitModule)
+    NewModule->InputFilesValidationTimestamp =
+        ModCache->getModuleTimestamp(NewModule->FileName);
 
   // Load the contents of the module
   if (std::unique_ptr<llvm::MemoryBuffer> Buffer = lookupBuffer(FileName)) {