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to reflect the new license. We understand that people may be surprised that we're moving the header entirely to discuss the new license. We checked this carefully with the Foundation's lawyer and we believe this is the correct approach. Essentially, all code in the project is now made available by the LLVM project under our new license, so you will see that the license headers include that license only. Some of our contributors have contributed code under our old license, and accordingly, we have retained a copy of our old license notice in the top-level files in each project and repository. llvm-svn: 351636
86 lines
3.1 KiB
C++
86 lines
3.1 KiB
C++
//===- Filesystem.cpp -----------------------------------------------------===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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//
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// This file contains a few utility functions to handle files.
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//
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//===----------------------------------------------------------------------===//
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#include "Filesystem.h"
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#include "Config.h"
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#include "lld/Common/Threads.h"
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#include "llvm/Config/llvm-config.h"
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#include "llvm/Support/FileOutputBuffer.h"
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#include "llvm/Support/FileSystem.h"
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#if LLVM_ON_UNIX
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#include <unistd.h>
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#endif
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#include <thread>
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using namespace llvm;
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using namespace lld;
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using namespace lld::elf;
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// Removes a given file asynchronously. This is a performance hack,
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// so remove this when operating systems are improved.
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//
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// On Linux (and probably on other Unix-like systems), unlink(2) is a
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// noticeably slow system call. As of 2016, unlink takes 250
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// milliseconds to remove a 1 GB file on ext4 filesystem on my machine.
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//
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// To create a new result file, we first remove existing file. So, if
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// you repeatedly link a 1 GB program in a regular compile-link-debug
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// cycle, every cycle wastes 250 milliseconds only to remove a file.
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// Since LLD can link a 1 GB binary in about 5 seconds, that waste
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// actually counts.
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//
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// This function spawns a background thread to remove the file.
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// The calling thread returns almost immediately.
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void elf::unlinkAsync(StringRef Path) {
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// Removing a file is async on windows.
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#if defined(_WIN32)
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sys::fs::remove(Path);
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#else
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if (!ThreadsEnabled || !sys::fs::exists(Path) ||
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!sys::fs::is_regular_file(Path))
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return;
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// We cannot just remove path from a different thread because we are now going
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// to create path as a new file.
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// Instead we open the file and unlink it on this thread. The unlink is fast
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// since the open fd guarantees that it is not removing the last reference.
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int FD;
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std::error_code EC = sys::fs::openFileForRead(Path, FD);
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sys::fs::remove(Path);
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// close and therefore remove TempPath in background.
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if (!EC)
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std::thread([=] { ::close(FD); }).detach();
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#endif
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}
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// Simulate file creation to see if Path is writable.
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//
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// Determining whether a file is writable or not is amazingly hard,
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// and after all the only reliable way of doing that is to actually
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// create a file. But we don't want to do that in this function
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// because LLD shouldn't update any file if it will end in a failure.
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// We also don't want to reimplement heuristics to determine if a
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// file is writable. So we'll let FileOutputBuffer do the work.
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//
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// FileOutputBuffer doesn't touch a desitnation file until commit()
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// is called. We use that class without calling commit() to predict
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// if the given file is writable.
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std::error_code elf::tryCreateFile(StringRef Path) {
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if (Path.empty())
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return std::error_code();
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if (Path == "-")
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return std::error_code();
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return errorToErrorCode(FileOutputBuffer::create(Path, 1).takeError());
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}
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