forked from OSchip/llvm-project
77 lines
2.8 KiB
C++
77 lines
2.8 KiB
C++
//===- Filesystem.cpp -----------------------------------------------------===//
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//
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// The LLVM Linker
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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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 "llvm/Support/FileSystem.h"
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#include "llvm/Support/FileOutputBuffer.h"
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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 call unlink.
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// The calling thread returns almost immediately.
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void elf::unlinkAsync(StringRef Path) {
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if (!Config->Threads || !sys::fs::exists(Config->OutputFile))
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return;
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// First, rename Path to avoid race condition. We cannot remove
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// Path from a different thread because we are now going to create
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// Path as a new file. If we do that in a different thread, the new
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// thread can remove the new file.
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SmallString<128> TempPath;
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if (sys::fs::createUniqueFile(Path + "tmp%%%%%%%%", TempPath))
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return;
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if (sys::fs::rename(Path, TempPath)) {
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sys::fs::remove(TempPath);
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return;
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}
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// Remove TempPath in background.
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std::thread([=] { ::remove(TempPath.str().str().c_str()); }).detach();
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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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return FileOutputBuffer::create(Path, 1).getError();
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}
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