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

[Ellis Hoag via llvm-commits]

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@@ -282,11 +284,87 @@ 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;
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ellishg wrote:

Can we use `std::atomic_int` for `index` and `totalBytes` instead of a mutex? That way we can guarantee that each thread gets their own `index` and threads don't have to fight over the mutex.

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