1 //===- Filesystem.cpp -----------------------------------------------------===//
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This file contains a few utility functions to handle files.
12 //===----------------------------------------------------------------------===//
14 #include "Filesystem.h"
16 #include "lld/Common/Threads.h"
17 #include "llvm/Config/llvm-config.h"
18 #include "llvm/Support/FileOutputBuffer.h"
19 #include "llvm/Support/FileSystem.h"
28 using namespace lld::elf;
30 // Removes a given file asynchronously. This is a performance hack,
31 // so remove this when operating systems are improved.
33 // On Linux (and probably on other Unix-like systems), unlink(2) is a
34 // noticeably slow system call. As of 2016, unlink takes 250
35 // milliseconds to remove a 1 GB file on ext4 filesystem on my machine.
37 // To create a new result file, we first remove existing file. So, if
38 // you repeatedly link a 1 GB program in a regular compile-link-debug
39 // cycle, every cycle wastes 250 milliseconds only to remove a file.
40 // Since LLD can link a 1 GB binary in about 5 seconds, that waste
43 // This function spawns a background thread to remove the file.
44 // The calling thread returns almost immediately.
45 void elf::unlinkAsync(StringRef Path) {
46 // Removing a file is async on windows.
48 sys::fs::remove(Path);
50 if (!ThreadsEnabled || !sys::fs::exists(Path) ||
51 !sys::fs::is_regular_file(Path))
54 // We cannot just remove path from a different thread because we are now going
55 // to create path as a new file.
56 // Instead we open the file and unlink it on this thread. The unlink is fast
57 // since the open fd guarantees that it is not removing the last reference.
59 std::error_code EC = sys::fs::openFileForRead(Path, FD);
60 sys::fs::remove(Path);
62 // close and therefore remove TempPath in background.
64 std::thread([=] { ::close(FD); }).detach();
68 // Simulate file creation to see if Path is writable.
70 // Determining whether a file is writable or not is amazingly hard,
71 // and after all the only reliable way of doing that is to actually
72 // create a file. But we don't want to do that in this function
73 // because LLD shouldn't update any file if it will end in a failure.
74 // We also don't want to reimplement heuristics to determine if a
75 // file is writable. So we'll let FileOutputBuffer do the work.
77 // FileOutputBuffer doesn't touch a desitnation file until commit()
78 // is called. We use that class without calling commit() to predict
79 // if the given file is writable.
80 std::error_code elf::tryCreateFile(StringRef Path) {
82 return std::error_code();
84 return std::error_code();
85 return errorToErrorCode(FileOutputBuffer::create(Path, 1).takeError());