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

[Michael Spencer via cfe-commits]

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@@ -59,19 +60,46 @@ class InProcessModuleCache : public ModuleCache {
   InMemoryModuleCache InMemory;
 
 public:
-  InProcessModuleCache(ModuleCacheMutexes &Mutexes) : Mutexes(Mutexes) {}
+  InProcessModuleCache(ModuleCacheEntries &Entries) : Entries(Entries) {}
 
   void prepareForGetLock(StringRef Filename) override {}
 
   std::unique_ptr<llvm::AdvisoryLock> getLock(StringRef Filename) override {
-    auto &Mtx = [&]() -> std::shared_mutex & {
-      std::lock_guard<std::mutex> Lock(Mutexes.Mutex);
-      auto &Mutex = Mutexes.Map[Filename];
-      if (!Mutex)
-        Mutex = std::make_unique<std::shared_mutex>();
-      return *Mutex;
+    auto &CompilationMutex = [&]() -> std::shared_mutex & {
+      std::lock_guard Lock(Entries.Mutex);
+      auto &Entry = Entries.Map[Filename];
+      if (!Entry)
+        Entry = std::make_unique<ModuleCacheEntry>();
+      return Entry->CompilationMutex;
     }();
-    return std::make_unique<ReaderWriterLock>(Mtx);
+    return std::make_unique<ReaderWriterLock>(CompilationMutex);
+  }
+
+  std::time_t getModuleTimestamp(StringRef Filename) override {
+    auto &Timestamp = [&]() -> std::atomic<std::time_t> & {
+      std::lock_guard Lock(Entries.Mutex);
+      auto &Entry = Entries.Map[Filename];
+      if (!Entry)
+        Entry = std::make_unique<ModuleCacheEntry>();
+      return Entry->Timestamp;
+    }();
+
+    return Timestamp.load();
+  }
+
+  void updateModuleTimestamp(StringRef Filename) override {
+    // Note: This essentially replaces FS contention with mutex contention.
+    auto &Timestamp = [&]() -> std::atomic<std::time_t> & {
+      std::lock_guard Lock(Entries.Mutex);
+      auto &Entry = Entries.Map[Filename];
+      if (!Entry)
+        Entry = std::make_unique<ModuleCacheEntry>();
+      return Entry->Timestamp;
+    }();
+
+    std::time_t Expected = 0;
+    std::time_t Now = llvm::sys::toTimeT(std::chrono::system_clock::now());
+    Timestamp.compare_exchange_weak(Expected, Now);
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Bigcheese wrote:

This has different semantics than the file system version. This will keep the oldest time, and the filesystem version keeps the newest time. Not sure if that's a problem though.

https://github.com/llvm/llvm-project/pull/137363