2 files changed, 100 insertions, 0 deletions
diff --git a/lld/Common/CMakeLists.txt b/lld/Common/CMakeLists.txt
index 29f38d0e7e6..70849cc7b94 100644
--- a/lld/Common/CMakeLists.txt
+++ b/lld/Common/CMakeLists.txt
@@ -30,6 +30,7 @@ set_property(SOURCE Version.cpp APPEND PROPERTY
 add_lld_library(lldCommon
   Args.cpp
   ErrorHandler.cpp
+  Filesystem.cpp
   Memory.cpp
   Reproduce.cpp
   Strings.cpp
diff --git a/lld/Common/Filesystem.cpp b/lld/Common/Filesystem.cpp
new file mode 100644
index 00000000000..6484286f14b
--- /dev/null
+++ b/lld/Common/Filesystem.cpp
@@ -0,0 +1,99 @@
+//===- Filesystem.cpp -----------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This file contains a few utility functions to handle files.
+//
+//===----------------------------------------------------------------------===//
+
+#include "lld/Common/Filesystem.h"
+#include "lld/Common/Threads.h"
+#include "llvm/Config/llvm-config.h"
+#include "llvm/Support/FileOutputBuffer.h"
+#include "llvm/Support/FileSystem.h"
+#if LLVM_ON_UNIX
+#include <unistd.h>
+#endif
+#include <thread>
+
+using namespace llvm;
+using namespace lld;
+
+// Removes a given file asynchronously. This is a performance hack,
+// so remove this when operating systems are improved.
+//
+// On Linux (and probably on other Unix-like systems), unlink(2) is a
+// noticeably slow system call. As of 2016, unlink takes 250
+// milliseconds to remove a 1 GB file on ext4 filesystem on my machine.
+//
+// To create a new result file, we first remove existing file. So, if
+// you repeatedly link a 1 GB program in a regular compile-link-debug
+// cycle, every cycle wastes 250 milliseconds only to remove a file.
+// Since LLD can link a 1 GB binary in about 5 seconds, that waste
+// actually counts.
+//
+// This function spawns a background thread to remove the file.
+// The calling thread returns almost immediately.
+void lld::unlinkAsync(StringRef Path) {
+// Removing a file is async on windows.
+#if defined(_WIN32)
+  sys::fs::remove(Path);
+#else
+  if (!ThreadsEnabled || !sys::fs::exists(Path) ||
+      !sys::fs::is_regular_file(Path))
+    return;
+
+  // We cannot just remove path from a different thread because we are now going
+  // to create path as a new file.
+  // Instead we open the file and unlink it on this thread. The unlink is fast
+  // since the open fd guarantees that it is not removing the last reference.
+  int FD;
+  std::error_code EC = sys::fs::openFileForRead(Path, FD);
+  sys::fs::remove(Path);
+
+  if (EC)
+    return;
+
+  // close and therefore remove TempPath in background.
+  std::mutex M;
+  std::condition_variable CV;
+  bool Started = false;
+  std::thread([&, FD] {
+    {
+      std::lock_guard<std::mutex> L(M);
+      Started = true;
+      CV.notify_all();
+    }
+    ::close(FD);
+  }).detach();
+
+  // GLIBC 2.26 and earlier have race condition that crashes an entire process
+  // if the main thread calls exit(2) while other thread is starting up.
+  std::unique_lock<std::mutex> L(M);
+  CV.wait(L, [&] { return Started; });
+#endif
+}
+
+// Simulate file creation to see if Path is writable.
+//
+// Determining whether a file is writable or not is amazingly hard,
+// and after all the only reliable way of doing that is to actually
+// create a file. But we don't want to do that in this function
+// because LLD shouldn't update any file if it will end in a failure.
+// We also don't want to reimplement heuristics to determine if a
+// file is writable. So we'll let FileOutputBuffer do the work.
+//
+// FileOutputBuffer doesn't touch a desitnation file until commit()
+// is called. We use that class without calling commit() to predict
+// if the given file is writable.
+std::error_code lld::tryCreateFile(StringRef Path) {
+  if (Path.empty())
+    return std::error_code();
+  if (Path == "-")
+    return std::error_code();
+  return errorToErrorCode(FileOutputBuffer::create(Path, 1).takeError());
+}