#include "llvm/ADT/StringMap.h"
#include "llvm/ADT/StringSwitch.h"
#include "llvm/Support/FileOutputBuffer.h"
+#include "llvm/Support/FileSystem.h"
#include "llvm/Support/raw_ostream.h"
#include <climits>
+#include <thread>
using namespace llvm;
using namespace llvm::ELF;
Sec->writeHeaderTo<ELFT>(++SHdrs);
}
+// 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.
+static void unlinkAsync(StringRef Path) {
+ if (!Config->Threads || !sys::fs::exists(Config->OutputFile))
+ return;
+
+ // First, rename Path to avoid race condition. We cannot remomve
+ // 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 (auto EC = sys::fs::createUniqueFile(Path + "tmp%%%%%%%%", TempPath))
+ fatal(EC, "createUniqueFile failed");
+ if (auto EC = sys::fs::rename(Path, TempPath))
+ fatal(EC, "rename failed");
+
+ // Remove TempPath in background.
+ std::thread([=] { ::remove(TempPath.str().str().c_str()); }).detach();
+}
+
+// Open a result file.
template <class ELFT> void Writer<ELFT>::openFile() {
+ unlinkAsync(Config->OutputFile);
ErrorOr<std::unique_ptr<FileOutputBuffer>> BufferOrErr =
FileOutputBuffer::create(Config->OutputFile, FileSize,
FileOutputBuffer::F_executable);
+
if (auto EC = BufferOrErr.getError())
error(EC, "failed to open " + Config->OutputFile);
else