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 "lld/Common/ErrorHandler.h" 15 #include "llvm/Config/llvm-config.h" 16 #include "llvm/Support/FileOutputBuffer.h" 17 #include "llvm/Support/FileSystem.h" 18 #include "llvm/Support/Parallel.h" 19 #include "llvm/Support/Path.h" 20 #include "llvm/Support/TimeProfiler.h" 21 #if LLVM_ON_UNIX 22 #include <unistd.h> 23 #endif 24 #include <thread> 25 26 using namespace llvm; 27 using namespace lld; 28 29 // Removes a given file asynchronously. This is a performance hack, 30 // so remove this when operating systems are improved. 31 // 32 // On Linux (and probably on other Unix-like systems), unlink(2) is a 33 // noticeably slow system call. As of 2016, unlink takes 250 34 // milliseconds to remove a 1 GB file on ext4 filesystem on my machine. 35 // 36 // To create a new result file, we first remove existing file. So, if 37 // you repeatedly link a 1 GB program in a regular compile-link-debug 38 // cycle, every cycle wastes 250 milliseconds only to remove a file. 39 // Since LLD can link a 1 GB binary in about 5 seconds, that waste 40 // actually counts. 41 // 42 // This function spawns a background thread to remove the file. 43 // The calling thread returns almost immediately. 44 void lld::unlinkAsync(StringRef path) { 45 if (!sys::fs::exists(path) || !sys::fs::is_regular_file(path)) 46 return; 47 48 // Removing a file is async on windows. 49 #if defined(_WIN32) 50 // On Windows co-operative programs can be expected to open LLD's 51 // output in FILE_SHARE_DELETE mode. This allows us to delete the 52 // file (by moving it to a temporary filename and then deleting 53 // it) so that we can link another output file that overwrites 54 // the existing file, even if the current file is in use. 55 // 56 // This is done on a best effort basis - we do not error if the 57 // operation fails. The consequence is merely that the user 58 // experiences an inconvenient work-flow. 59 // 60 // The code here allows LLD to work on all versions of Windows. 61 // However, at Windows 10 1903 it seems that the behavior of 62 // Windows has changed, so that we could simply delete the output 63 // file. This code should be simplified once support for older 64 // versions of Windows is dropped. 65 // 66 // Warning: It seems that the WINVER and _WIN32_WINNT preprocessor 67 // defines affect the behavior of the Windows versions of the calls 68 // we are using here. If this code stops working this is worth 69 // bearing in mind. 70 SmallString<128> tmpName; 71 if (!sys::fs::createUniqueFile(path + "%%%%%%%%.tmp", tmpName)) { 72 if (!sys::fs::rename(path, tmpName)) 73 path = tmpName; 74 else 75 sys::fs::remove(tmpName); 76 } 77 sys::fs::remove(path); 78 #else 79 if (parallel::strategy.ThreadsRequested == 1) 80 return; 81 82 // We cannot just remove path from a different thread because we are now going 83 // to create path as a new file. 84 // Instead we open the file and unlink it on this thread. The unlink is fast 85 // since the open fd guarantees that it is not removing the last reference. 86 int fd; 87 std::error_code ec = sys::fs::openFileForRead(path, fd); 88 sys::fs::remove(path); 89 90 if (ec) 91 return; 92 93 // close and therefore remove TempPath in background. 94 std::mutex m; 95 std::condition_variable cv; 96 bool started = false; 97 std::thread([&, fd] { 98 { 99 std::lock_guard<std::mutex> l(m); 100 started = true; 101 cv.notify_all(); 102 } 103 ::close(fd); 104 }).detach(); 105 106 // GLIBC 2.26 and earlier have race condition that crashes an entire process 107 // if the main thread calls exit(2) while other thread is starting up. 108 std::unique_lock<std::mutex> l(m); 109 cv.wait(l, [&] { return started; }); 110 #endif 111 } 112 113 // Simulate file creation to see if Path is writable. 114 // 115 // Determining whether a file is writable or not is amazingly hard, 116 // and after all the only reliable way of doing that is to actually 117 // create a file. But we don't want to do that in this function 118 // because LLD shouldn't update any file if it will end in a failure. 119 // We also don't want to reimplement heuristics to determine if a 120 // file is writable. So we'll let FileOutputBuffer do the work. 121 // 122 // FileOutputBuffer doesn't touch a destination file until commit() 123 // is called. We use that class without calling commit() to predict 124 // if the given file is writable. 125 std::error_code lld::tryCreateFile(StringRef path) { 126 llvm::TimeTraceScope timeScope("Try create output file"); 127 if (path.empty()) 128 return std::error_code(); 129 if (path == "-") 130 return std::error_code(); 131 return errorToErrorCode(FileOutputBuffer::create(path, 1).takeError()); 132 } 133 134 // Creates an empty file to and returns a raw_fd_ostream to write to it. 135 std::unique_ptr<raw_fd_ostream> lld::openFile(StringRef file) { 136 std::error_code ec; 137 auto ret = 138 std::make_unique<raw_fd_ostream>(file, ec, sys::fs::OpenFlags::OF_None); 139 if (ec) { 140 error("cannot open " + file + ": " + ec.message()); 141 return nullptr; 142 } 143 return ret; 144 } 145 146 // The merged bitcode after LTO is large. Try opening a file stream that 147 // supports reading, seeking and writing. Such a file allows BitcodeWriter to 148 // flush buffered data to reduce memory consumption. If this fails, open a file 149 // stream that supports only write. 150 std::unique_ptr<raw_fd_ostream> lld::openLTOOutputFile(StringRef file) { 151 std::error_code ec; 152 std::unique_ptr<raw_fd_ostream> fs = 153 std::make_unique<raw_fd_stream>(file, ec); 154 if (!ec) 155 return fs; 156 return openFile(file); 157 } 158