llvm/lld/Common/Filesystem.cpp

//===- Filesystem.cpp -----------------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This file contains a few utility functions to handle files.
//
//===----------------------------------------------------------------------===//

#include "lld/Common/Filesystem.h"
#include "lld/Common/ErrorHandler.h"
#include "llvm/Config/llvm-config.h"
#include "llvm/Support/FileOutputBuffer.h"
#include "llvm/Support/FileSystem.h"
#include "llvm/Support/Parallel.h"
#include "llvm/Support/Path.h"
#include "llvm/Support/TimeProfiler.h"
#if LLVM_ON_UNIX
#include <unistd.h>
#endif
#include <thread>

usingnamespacellvm;
usingnamespacelld;

// Removes a given file asynchronously. This is a performance hack,
// so remove this when operating systems are improved.
//
// On Linux (and probably on other Unix-like systems), unlink(2) is a
// noticeably slow system call. As of 2016, unlink takes 250
// milliseconds to remove a 1 GB file on ext4 filesystem on my machine.
//
// To create a new result file, we first remove existing file. So, if
// you repeatedly link a 1 GB program in a regular compile-link-debug
// cycle, every cycle wastes 250 milliseconds only to remove a file.
// Since LLD can link a 1 GB binary in about 5 seconds, that waste
// actually counts.
//
// This function spawns a background thread to remove the file.
// The calling thread returns almost immediately.
void lld::unlinkAsync(StringRef path) { … }

// Simulate file creation to see if Path is writable.
//
// Determining whether a file is writable or not is amazingly hard,
// and after all the only reliable way of doing that is to actually
// create a file. But we don't want to do that in this function
// because LLD shouldn't update any file if it will end in a failure.
// We also don't want to reimplement heuristics to determine if a
// file is writable. So we'll let FileOutputBuffer do the work.
//
// FileOutputBuffer doesn't touch a destination file until commit()
// is called. We use that class without calling commit() to predict
// if the given file is writable.
std::error_code lld::tryCreateFile(StringRef path) { … }

// Creates an empty file to and returns a raw_fd_ostream to write to it.
std::unique_ptr<raw_fd_ostream> lld::openFile(StringRef file) { … }

// The merged bitcode after LTO is large. Try opening a file stream that
// supports reading, seeking and writing. Such a file allows BitcodeWriter to
// flush buffered data to reduce memory consumption. If this fails, open a file
// stream that supports only write.
std::unique_ptr<raw_fd_ostream> lld::openLTOOutputFile(StringRef file) { … }