[lld] [lld][MachO]Multi-threaded i/o. Twice as fast linking a large project. (PR #147134)

[John Holdsworth via llvm-commits]

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@@ -282,11 +284,117 @@ static void saveThinArchiveToRepro(ArchiveFile const *file) {
           ": Archive::children failed: " + toString(std::move(e)));
 }
 
-static InputFile *addFile(StringRef path, LoadType loadType,
-                          bool isLazy = false, bool isExplicit = true,
-                          bool isBundleLoader = false,
-                          bool isForceHidden = false) {
-  std::optional<MemoryBufferRef> buffer = readFile(path);
+struct DeferredFile {
+  StringRef path;
+  bool isLazy;
+  MemoryBufferRef buffer;
+};
+using DeferredFiles = std::vector<DeferredFile>;
+
+class SerialBackgroundQueue {
+  std::deque<std::function<void()>> queue;
+  std::thread *running;
+  std::mutex mutex;
+
+public:
+  void queueWork(std::function<void()> work) {
+    mutex.lock();
+    if (running && queue.empty()) {
+      mutex.unlock();
+      running->join();
+      mutex.lock();
+      delete running;
+      running = nullptr;
+    }
+
+    if (work) {
+      queue.emplace_back(std::move(work));
+      if (!running)
+        running = new std::thread([&]() {
+          while (true) {
+            mutex.lock();
+            if (queue.empty()) {
+              mutex.unlock();
+              break;
+            }
+            auto work = std::move(queue.front());
+            mutex.unlock();
+            work();
+            mutex.lock();
+            queue.pop_front();
+            mutex.unlock();
+          }
+        });
+    }
+    mutex.unlock();
+  }
+};
+
+// Most input files have been mapped but not yet paged in.
+// This code forces the page-ins on multiple threads so
+// the process is not stalled waiting on disk buffer i/o.
+void multiThreadedPageInBackground(DeferredFiles &deferred) {
----------------
johnno1962 wrote:

Hi, I've only evaluated the benefit of this approach on macOS so if there are other approaches that achieve the same on other platforms feel free to pick up the baton (I'd be interested to know if there was an improvement as-is on Linux though). This PR is more an observation that mmaping input files can be slow than a definitive solution.

https://github.com/llvm/llvm-project/pull/147134