1 //===- Filesystem.cpp -----------------------------------------------------===// 2 // 3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. 4 // See https://llvm.org/LICENSE.txt for license information. 5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception 6 // 7 //===----------------------------------------------------------------------===// 8 // 9 // This file contains a few utility functions to handle files. 10 // 11 //===----------------------------------------------------------------------===// 12 13 #include "lld/Common/Filesystem.h" 14 #include "llvm/Config/llvm-config.h" 15 #include "llvm/Support/FileOutputBuffer.h" 16 #include "llvm/Support/FileSystem.h" 17 #include "llvm/Support/Parallel.h" 18 #include "llvm/Support/Path.h" 19 #if LLVM_ON_UNIX 20 #include <unistd.h> 21 #endif 22 #include <thread> 23 24 using namespace llvm; 25 using namespace lld; 26 27 // Removes a given file asynchronously. This is a performance hack, 28 // so remove this when operating systems are improved. 29 // 30 // On Linux (and probably on other Unix-like systems), unlink(2) is a 31 // noticeably slow system call. As of 2016, unlink takes 250 32 // milliseconds to remove a 1 GB file on ext4 filesystem on my machine. 33 // 34 // To create a new result file, we first remove existing file. So, if 35 // you repeatedly link a 1 GB program in a regular compile-link-debug 36 // cycle, every cycle wastes 250 milliseconds only to remove a file. 37 // Since LLD can link a 1 GB binary in about 5 seconds, that waste 38 // actually counts. 39 // 40 // This function spawns a background thread to remove the file. 41 // The calling thread returns almost immediately. 42 void lld::unlinkAsync(StringRef path) { 43 if (!sys::fs::exists(path) || !sys::fs::is_regular_file(path)) 44 return; 45 46 // Removing a file is async on windows. 47 #if defined(_WIN32) 48 // On Windows co-operative programs can be expected to open LLD's 49 // output in FILE_SHARE_DELETE mode. This allows us to delete the 50 // file (by moving it to a temporary filename and then deleting 51 // it) so that we can link another output file that overwrites 52 // the existing file, even if the current file is in use. 53 // 54 // This is done on a best effort basis - we do not error if the 55 // operation fails. The consequence is merely that the user 56 // experiences an inconvenient work-flow. 57 // 58 // The code here allows LLD to work on all versions of Windows. 59 // However, at Windows 10 1903 it seems that the behavior of 60 // Windows has changed, so that we could simply delete the output 61 // file. This code should be simplified once support for older 62 // versions of Windows is dropped. 63 // 64 // Warning: It seems that the WINVER and _WIN32_WINNT preprocessor 65 // defines affect the behavior of the Windows versions of the calls 66 // we are using here. If this code stops working this is worth 67 // bearing in mind. 68 SmallString<128> tmpName; 69 if (!sys::fs::createUniqueFile(path + "%%%%%%%%.tmp", tmpName)) { 70 if (!sys::fs::rename(path, tmpName)) 71 path = tmpName; 72 else 73 sys::fs::remove(tmpName); 74 } 75 sys::fs::remove(path); 76 #else 77 if (parallel::strategy.ThreadsRequested == 1) 78 return; 79 80 // We cannot just remove path from a different thread because we are now going 81 // to create path as a new file. 82 // Instead we open the file and unlink it on this thread. The unlink is fast 83 // since the open fd guarantees that it is not removing the last reference. 84 int fd; 85 std::error_code ec = sys::fs::openFileForRead(path, fd); 86 sys::fs::remove(path); 87 88 if (ec) 89 return; 90 91 // close and therefore remove TempPath in background. 92 std::mutex m; 93 std::condition_variable cv; 94 bool started = false; 95 std::thread([&, fd] { 96 { 97 std::lock_guard<std::mutex> l(m); 98 started = true; 99 cv.notify_all(); 100 } 101 ::close(fd); 102 }).detach(); 103 104 // GLIBC 2.26 and earlier have race condition that crashes an entire process 105 // if the main thread calls exit(2) while other thread is starting up. 106 std::unique_lock<std::mutex> l(m); 107 cv.wait(l, [&] { return started; }); 108 #endif 109 } 110 111 // Simulate file creation to see if Path is writable. 112 // 113 // Determining whether a file is writable or not is amazingly hard, 114 // and after all the only reliable way of doing that is to actually 115 // create a file. But we don't want to do that in this function 116 // because LLD shouldn't update any file if it will end in a failure. 117 // We also don't want to reimplement heuristics to determine if a 118 // file is writable. So we'll let FileOutputBuffer do the work. 119 // 120 // FileOutputBuffer doesn't touch a destination file until commit() 121 // is called. We use that class without calling commit() to predict 122 // if the given file is writable. 123 std::error_code lld::tryCreateFile(StringRef path) { 124 if (path.empty()) 125 return std::error_code(); 126 if (path == "-") 127 return std::error_code(); 128 return errorToErrorCode(FileOutputBuffer::create(path, 1).takeError()); 129 } 130