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

[John Holdsworth via llvm-commits]

================
@@ -282,11 +284,85 @@ 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);
+class DeferredFile {
+public:
+  StringRef path;
+  bool isLazy;
+  MemoryBufferRef buffer;
+};
+using DeferredFiles = std::vector<DeferredFile>;
+
+// 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(const DeferredFiles &deferred) {
+  static size_t pageSize = Process::getPageSizeEstimate(), totalBytes;
+  static std::mutex mutex;
+  size_t index = 0;
+
+  parallelFor(0, config->readThreads, [&](size_t I) {
+    while (true) {
+      mutex.lock();
+      if (index >= deferred.size()) {
+        mutex.unlock();
+        return;
+      }
+      const StringRef &buff = deferred[index].buffer.getBuffer();
+      totalBytes += buff.size();
+      index += 1;
+      mutex.unlock();
+
+      // Reference each page to load it into memory.
+      for (const char *page = buff.data(), *end = page + buff.size();
+           page < end; page += pageSize)
+        volatile char t = *page;
+    }
+  });
+
+  if (getenv("LLD_MULTI_THREAD_PAGE"))
+    llvm::dbgs() << "multiThreadedPageIn " << totalBytes << "/"
+                 << deferred.size() << "\n";
+}
+
+static void multiThreadedPageIn(const DeferredFiles &deferred) {
+  static std::deque<DeferredFiles *> queue;
+  static std::thread *running;
+  static std::mutex mutex;
+
+  mutex.lock();
+  if (running && (queue.empty() || deferred.empty())) {
+    running->join();
+    delete running;
+    running = nullptr;
+  }
+
+  if (!deferred.empty()) {
+    queue.emplace_back(new DeferredFiles(deferred));
+    if (!running)
+      running = new std::thread([&]() {
+        while (true) {
+          mutex.lock();
+          if (queue.empty()) {
+            mutex.unlock();
+            return;
+          }
+          DeferredFiles *deferred = queue.front();
+          queue.pop_front();
+          mutex.unlock();
+          multiThreadedPageInBackground(*deferred);
+          delete deferred;
+        }
+      });
+  }
+  mutex.unlock();
+}
----------------
johnno1962 wrote:

Sorry I was a bit slow on the uptake there, I can see what you mean now. I've committed unlocking around the thread join, this seemed to be the simplest fix.

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