[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 +284,83 @@ 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:
+  DeferredFile(StringRef path, bool isLazy, MemoryBufferRef buffer)
+      : path(path), isLazy(isLazy), buffer(buffer) {}
+  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 const size_t pageSize = Process::getPageSizeEstimate();
+  static size_t totalBytes = 0;
+  std::atomic_int index = 0;
+
+  parallelFor(0, config->readThreads, [&](size_t I) {
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johnno1962 wrote:

Where are we on this PR? I've removed my benchmarking statements for a release build and looked for pre-existing abstraction for queuing work and found [BackgroundQueue](https://clang.llvm.org/extra//doxygen/classclang_1_1clangd_1_1BackgroundQueue.html) which looked promising but it's part of clangd. What are the blockers now?

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