1 //===- Filesystem.cpp -----------------------------------------------------===// 2 // 3 // The LLVM Linker 4 // 5 // This file is distributed under the University of Illinois Open Source 6 // License. See LICENSE.TXT for details. 7 // 8 //===----------------------------------------------------------------------===// 9 // 10 // This file contains a few utility functions to handle files. 11 // 12 //===----------------------------------------------------------------------===// 13 14 #include "Filesystem.h" 15 #include "Config.h" 16 #include "llvm/Support/FileSystem.h" 17 #include "llvm/Support/FileOutputBuffer.h" 18 #include <thread> 19 20 using namespace llvm; 21 22 using namespace lld; 23 using namespace lld::elf; 24 25 // Removes a given file asynchronously. This is a performance hack, 26 // so remove this when operating systems are improved. 27 // 28 // On Linux (and probably on other Unix-like systems), unlink(2) is a 29 // noticeably slow system call. As of 2016, unlink takes 250 30 // milliseconds to remove a 1 GB file on ext4 filesystem on my machine. 31 // 32 // To create a new result file, we first remove existing file. So, if 33 // you repeatedly link a 1 GB program in a regular compile-link-debug 34 // cycle, every cycle wastes 250 milliseconds only to remove a file. 35 // Since LLD can link a 1 GB binary in about 5 seconds, that waste 36 // actually counts. 37 // 38 // This function spawns a background thread to call unlink. 39 // The calling thread returns almost immediately. 40 void elf::unlinkAsync(StringRef Path) { 41 if (!Config->Threads || !sys::fs::exists(Config->OutputFile) || 42 !sys::fs::is_regular_file(Config->OutputFile)) 43 return; 44 45 // First, rename Path to avoid race condition. We cannot remove 46 // Path from a different thread because we are now going to create 47 // Path as a new file. If we do that in a different thread, the new 48 // thread can remove the new file. 49 SmallString<128> TempPath; 50 if (sys::fs::createUniqueFile(Path + "tmp%%%%%%%%", TempPath)) 51 return; 52 if (sys::fs::rename(Path, TempPath)) { 53 sys::fs::remove(TempPath); 54 return; 55 } 56 57 // Remove TempPath in background. 58 std::thread([=] { ::remove(TempPath.str().str().c_str()); }).detach(); 59 } 60 61 // Simulate file creation to see if Path is writable. 62 // 63 // Determining whether a file is writable or not is amazingly hard, 64 // and after all the only reliable way of doing that is to actually 65 // create a file. But we don't want to do that in this function 66 // because LLD shouldn't update any file if it will end in a failure. 67 // We also don't want to reimplement heuristics to determine if a 68 // file is writable. So we'll let FileOutputBuffer do the work. 69 // 70 // FileOutputBuffer doesn't touch a desitnation file until commit() 71 // is called. We use that class without calling commit() to predict 72 // if the given file is writable. 73 std::error_code elf::tryCreateFile(StringRef Path) { 74 if (Path.empty()) 75 return std::error_code(); 76 return FileOutputBuffer::create(Path, 1).getError(); 77 } 78