[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, 23 insertions, 0 deletions

diff --git a/clang/lib/Serialization/ModuleCache.cpp b/clang/lib/Serialization/ModuleCache.cpp
index 955e5f3..4ae49c4 100644
--- a/clang/lib/Serialization/ModuleCache.cpp
+++ b/clang/lib/Serialization/ModuleCache.cpp
@@ -9,6 +9,7 @@
 #include "clang/Serialization/ModuleCache.h"
 
 #include "clang/Serialization/InMemoryModuleCache.h"
+#include "clang/Serialization/ModuleFile.h"
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/LockFileManager.h"
 #include "llvm/Support/Path.h"
@@ -32,6 +33,28 @@ public:
     return std::make_unique<llvm::LockFileManager>(ModuleFilename);
   }
 
+  std::time_t getModuleTimestamp(StringRef ModuleFilename) override {
+    std::string TimestampFilename =
+        serialization::ModuleFile::getTimestampFilename(ModuleFilename);
+    llvm::sys::fs::file_status Status;
+    if (llvm::sys::fs::status(ModuleFilename, Status) != std::error_code{})
+      return 0;
+    return llvm::sys::toTimeT(Status.getLastModificationTime());
+  }
+
+  void updateModuleTimestamp(StringRef ModuleFilename) override {
+    // Overwrite the timestamp file contents so that file's mtime changes.
+    std::error_code EC;
+    llvm::raw_fd_ostream OS(
+        serialization::ModuleFile::getTimestampFilename(ModuleFilename), EC,
+        llvm::sys::fs::OF_TextWithCRLF);
+    if (EC)
+      return;
+    OS << "Timestamp file\n";
+    OS.close();
+    OS.clear_error(); // Avoid triggering a fatal error.
+  }
+
   InMemoryModuleCache &getInMemoryModuleCache() override { return InMemory; }
   const InMemoryModuleCache &getInMemoryModuleCache() const override {
     return InMemory;