vendor/lld/lld-trunk-r300422

1 files changed, 79 insertions, 0 deletions

diff --git a/ELF/Filesystem.cpp b/ELF/Filesystem.cpp
new file mode 100644
index 000000000000..75f7bda75a23
--- /dev/null
+++ b/ELF/Filesystem.cpp
@@ -0,0 +1,79 @@
+//===- Filesystem.cpp -----------------------------------------------------===//
+//
+//                             The LLVM Linker
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file contains a few utility functions to handle files.
+//
+//===----------------------------------------------------------------------===//
+
+#include "Filesystem.h"
+#include "Config.h"
+#include "Error.h"
+#include "llvm/Support/FileSystem.h"
+#include "llvm/Support/FileOutputBuffer.h"
+#include <thread>
+
+using namespace llvm;
+
+using namespace lld;
+using namespace lld::elf;
+
+// 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 call unlink.
+// The calling thread returns almost immediately.
+void elf::unlinkAsync(StringRef Path) {
+  if (!Config->Threads || !sys::fs::exists(Config->OutputFile))
+    return;
+
+  // First, rename Path to avoid race condition. We cannot remove
+  // Path from a different thread because we are now going to create
+  // Path as a new file. If we do that in a different thread, the new
+  // thread can remove the new file.
+  SmallString<128> TempPath;
+  if (sys::fs::createUniqueFile(Path + "tmp%%%%%%%%", TempPath))
+    return;
+  if (sys::fs::rename(Path, TempPath)) {
+    sys::fs::remove(TempPath);
+    return;
+  }
+
+  // Remove TempPath in background.
+  std::thread([=] { ::remove(TempPath.str().str().c_str()); }).detach();
+}
+
+// Returns true if a given file seems to be 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. 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.
+bool elf::isFileWritable(StringRef Path, StringRef Desc) {
+  if (auto EC = FileOutputBuffer::create(Path, 1).getError()) {
+    error("cannot open " + Desc + " " + Path + ": " + EC.message());
+    return false;
+  }
+  return true;
+}