158 lines · cpp
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-exception6//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_UNIX22#include <unistd.h>23#endif24#include <thread>25 26using namespace llvm;27using 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 a33// noticeably slow system call. As of 2016, unlink takes 25034// 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, if37// you repeatedly link a 1 GB program in a regular compile-link-debug38// 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 waste40// actually counts.41//42// This function spawns a background thread to remove the file.43// The calling thread returns almost immediately.44void 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's51 // output in FILE_SHARE_DELETE mode. This allows us to delete the52 // file (by moving it to a temporary filename and then deleting53 // it) so that we can link another output file that overwrites54 // 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 the57 // operation fails. The consequence is merely that the user58 // 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 of62 // Windows has changed, so that we could simply delete the output63 // file. This code should be simplified once support for older64 // versions of Windows is dropped.65 //66 // Warning: It seems that the WINVER and _WIN32_WINNT preprocessor67 // defines affect the behavior of the Windows versions of the calls68 // we are using here. If this code stops working this is worth69 // 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 else75 sys::fs::remove(tmpName);76 }77 sys::fs::remove(path);78#else79 if (parallel::strategy.ThreadsRequested == 1)80 return;81 82 // We cannot just remove path from a different thread because we are now going83 // to create path as a new file.84 // Instead we open the file and unlink it on this thread. The unlink is fast85 // 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 process107 // 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#endif111}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 actually117// create a file. But we don't want to do that in this function118// 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 a120// 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 predict124// if the given file is writable.125std::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.135std::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 that147// supports reading, seeking and writing. Such a file allows BitcodeWriter to148// flush buffered data to reduce memory consumption. If this fails, open a file149// stream that supports only write.150std::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