337 lines
12 KiB
C++
337 lines
12 KiB
C++
/*
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* Copyright 2017-present Facebook, Inc.
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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#pragma once
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#include <string>
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#include <type_traits>
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#include <vector>
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namespace {
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// This struct is different in every translation unit. We use template
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// instantiations to define inline freestanding methods. Since the
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// methods are inline it is fine to define them in multiple translation
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// units, but the instantiation itself would be an ODR violation if it is
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// present in the program more than once. By tagging the instantiations
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// with this struct, we avoid ODR problems for the instantiation while
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// allowing the resulting methods to be inline-able. If you think that
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// seems hacky keep reading...
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struct FollyMemoryDetailTranslationUnitTag {};
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} // namespace
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namespace folly {
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namespace detail {
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void unsafeStringSetLargerSize(std::string& s, std::size_t n);
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template <typename T>
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void unsafeVectorSetLargerSize(std::vector<T>& v, std::size_t n);
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} // namespace detail
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/*
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* This file provides helper functions resizeWithoutInitialization()
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* that can resize std::string or std::vector without constructing or
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* initializing new elements.
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*
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* IMPORTANT: These functions can be unsafe if used improperly. If you
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* don't write to an element with index >= oldSize and < newSize, reading
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* the element can expose arbitrary memory contents to the world, including
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* the contents of old strings. If you're lucky you'll get a segfault,
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* because the kernel is only required to fault in new pages on write
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* access. MSAN should be able to catch problems in the common case that
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* the string or vector wasn't previously shrunk.
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*
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* Pay extra attention to your failure paths. For example, if you try
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* to read directly into a caller-provided string, make sure to clear
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* the string when you get an I/O error.
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*
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* You should only use this if you have profiling data from production
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* that shows that this is not a premature optimization. This code is
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* designed for retroactively optimizing code where touching every element
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* twice (or touching never-used elements once) shows up in profiling,
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* and where restructuring the code to use fixed-length arrays or IOBuf-s
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* would be difficult.
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*
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* NOTE: Just because .resize() shows up in your profile (probably
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* via one of the intrinsic memset implementations) doesn't mean that
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* these functions will make your program faster. A lot of the cost
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* of memset comes from cache misses, so avoiding the memset can mean
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* that the cache miss cost just gets pushed to the following code.
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* resizeWithoutInitialization can be a win when the contents are bigger
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* than a cache level, because the second access isn't free in that case.
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* It can be a win when the memory is already cached, so touching it
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* doesn't help later code. It can also be a win if the final length
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* of the string or vector isn't actually known, so the suffix will be
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* chopped off with a second call to .resize().
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*/
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/**
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* Like calling s.resize(n), but when growing the string does not
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* initialize new elements. It is undefined behavior to read from
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* any element added to the string by this method unless it has been
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* written to by an operation that follows this call.
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*
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* IMPORTANT: Read the warning at the top of this header file.
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*/
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inline void resizeWithoutInitialization(std::string& s, std::size_t n) {
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if (n <= s.size()) {
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s.resize(n);
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} else {
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// careful not to call reserve unless necessary, as it causes
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// shrink_to_fit on many platforms
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if (n > s.capacity()) {
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s.reserve(n);
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}
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detail::unsafeStringSetLargerSize(s, n);
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}
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}
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/**
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* Like calling v.resize(n), but when growing the vector does not construct
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* or initialize new elements. It is undefined behavior to read from any
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* element added to the vector by this method unless it has been written
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* to by an operation that follows this call.
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*
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* Use the FOLLY_DECLARE_VECTOR_RESIZE_WITHOUT_INIT(T) macro to
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* declare (and inline define) the internals required to call
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* resizeWithoutInitialization for a std::vector<T>. This must
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* be done exactly once in each translation unit that wants to call
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* resizeWithoutInitialization(std::vector<T>&,size_t). char and unsigned
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* char are provided by default. If you don't do this you will get linker
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* errors about folly::detail::unsafeVectorSetLargerSize. Requiring that
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* T be trivially_destructible is only an approximation of the property
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* required of T. In fact what is required is that any random sequence of
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* bytes may be safely reinterpreted as a T and passed to T's destructor.
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*
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* std::vector<bool> has specialized internals and is not supported.
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*
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* IMPORTANT: Read the warning at the top of this header file.
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*/
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template <
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typename T,
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typename = typename std::enable_if<
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std::is_trivially_destructible<T>::value &&
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!std::is_same<T, bool>::value>::type>
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void resizeWithoutInitialization(std::vector<T>& v, std::size_t n) {
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if (n <= v.size()) {
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v.resize(n);
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} else {
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if (n > v.capacity()) {
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v.reserve(n);
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}
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detail::unsafeVectorSetLargerSize(v, n);
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}
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}
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namespace detail {
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#if defined(_LIBCPP_STRING)
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// libc++
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} // namespace detail
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} // namespace folly
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template void std::string::__set_size(std::size_t);
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namespace folly {
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namespace detail {
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template <typename Tag, typename T, typename A, A Ptr__set_size>
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struct MakeUnsafeStringSetLargerSize {
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friend void unsafeStringSetLargerSize(
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std::basic_string<T>& s,
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std::size_t n) {
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// s.__set_size(n);
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(s.*Ptr__set_size)(n);
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(&s[0])[n] = '\0';
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}
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};
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template struct MakeUnsafeStringSetLargerSize<
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FollyMemoryDetailTranslationUnitTag,
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char,
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void (std::string::*)(std::size_t),
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&std::string::__set_size>;
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#elif defined(_GLIBCXX_USE_FB)
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// FBString
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template <typename Tag, typename T, typename A, A Ptrstore_>
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struct MakeUnsafeStringSetLargerSize {
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friend void unsafeStringSetLargerSize(
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std::basic_string<T>& s,
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std::size_t n) {
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// s.store_.expandNoinit(n - s.size(), false);
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(s.*Ptrstore_).expandNoinit(n - s.size(), false);
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}
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};
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template struct MakeUnsafeStringSetLargerSize<
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FollyMemoryDetailTranslationUnitTag,
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char,
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std::fbstring_core<char>(std::string::*),
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&std::string::store_>;
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#elif defined(_GLIBCXX_STRING) && _GLIBCXX_USE_CXX11_ABI
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// libstdc++ new implementation with SSO
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} // namespace detail
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} // namespace folly
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template void std::string::_M_set_length(std::size_t);
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namespace folly {
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namespace detail {
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template <typename Tag, typename T, typename A, A Ptr_M_set_length>
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struct MakeUnsafeStringSetLargerSize {
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friend void unsafeStringSetLargerSize(
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std::basic_string<T>& s,
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std::size_t n) {
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// s._M_set_length(n);
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(s.*Ptr_M_set_length)(n);
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}
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};
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template struct MakeUnsafeStringSetLargerSize<
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FollyMemoryDetailTranslationUnitTag,
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char,
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void (std::string::*)(std::size_t),
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&std::string::_M_set_length>;
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#elif defined(_GLIBCXX_STRING)
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// libstdc++ old implementation
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} // namespace detail
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} // namespace folly
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template std::string::_Rep* std::string::_M_rep() const;
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template void std::string::_Rep::_M_set_length_and_sharable(std::size_t);
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namespace folly {
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namespace detail {
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template <
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typename Tag,
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typename T,
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typename A,
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A Ptr_M_rep,
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typename B,
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B Ptr_M_set_length_and_sharable>
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struct MakeUnsafeStringSetLargerSize {
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friend void unsafeStringSetLargerSize(
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std::basic_string<T>& s,
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std::size_t n) {
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// s._M_rep()->_M_set_length_and_sharable(n);
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auto rep = (s.*Ptr_M_rep)();
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(rep->*Ptr_M_set_length_and_sharable)(n);
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}
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};
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template struct MakeUnsafeStringSetLargerSize<
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FollyMemoryDetailTranslationUnitTag,
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char,
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std::string::_Rep* (std::string::*)() const,
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&std::string::_M_rep,
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void (std::string::_Rep::*)(std::size_t),
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&std::string::_Rep::_M_set_length_and_sharable>;
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#elif defined(_MSC_VER)
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// MSVC
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inline void unsafeStringSetLargerSize(std::string& s, std::size_t n) {
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s._Eos(n);
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}
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#else
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#warning "No implementation for resizeWithoutInitialization of std::string"
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#endif
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// This machinery bridges template expansion and macro expansion
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#define FOLLY_DECLARE_VECTOR_RESIZE_WITHOUT_INIT_IMPL(TYPE) \
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namespace folly { \
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namespace detail { \
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void unsafeVectorSetLargerSizeImpl(std::vector<TYPE>& v, std::size_t); \
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template <> \
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inline void unsafeVectorSetLargerSize<TYPE>( \
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std::vector<TYPE> & v, \
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std::size_t n) { \
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unsafeVectorSetLargerSizeImpl(v, n); \
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} \
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} \
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}
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#if defined(_LIBCPP_VECTOR)
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// libc++
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template <typename Tag, typename T, typename A, A Ptr__end_>
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struct MakeUnsafeVectorSetLargerSize {
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friend void unsafeVectorSetLargerSizeImpl(std::vector<T>& v, std::size_t n) {
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// v.__end_ += (n - v.size());
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using Base = std::__vector_base<T, std::allocator<T>>;
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static_assert(
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std::is_standard_layout<std::vector<T>>::value &&
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sizeof(std::vector<T>) == sizeof(Base),
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"reinterpret_cast safety conditions not met");
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reinterpret_cast<Base&>(v).*Ptr__end_ += (n - v.size());
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}
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};
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#define FOLLY_DECLARE_VECTOR_RESIZE_WITHOUT_INIT(TYPE) \
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template struct folly::detail::MakeUnsafeVectorSetLargerSize< \
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FollyMemoryDetailTranslationUnitTag, \
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TYPE, \
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TYPE*(std::__vector_base<TYPE, std::allocator<TYPE>>::*), \
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&std::vector<TYPE>::__end_>; \
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FOLLY_DECLARE_VECTOR_RESIZE_WITHOUT_INIT_IMPL(TYPE)
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#elif defined(_GLIBCXX_VECTOR)
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// libstdc++
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template <
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typename Tag,
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typename T,
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typename A,
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A Ptr_M_impl,
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typename B,
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B Ptr_M_finish>
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struct MakeUnsafeVectorSetLargerSize : std::vector<T> {
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friend void unsafeVectorSetLargerSizeImpl(std::vector<T>& v, std::size_t n) {
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// v._M_impl._M_finish += (n - v.size());
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(v.*Ptr_M_impl).*Ptr_M_finish += (n - v.size());
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}
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};
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#define FOLLY_DECLARE_VECTOR_RESIZE_WITHOUT_INIT(TYPE) \
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template struct folly::detail::MakeUnsafeVectorSetLargerSize< \
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FollyMemoryDetailTranslationUnitTag, \
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TYPE, \
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std::vector<TYPE>::_Vector_impl( \
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std::_Vector_base<TYPE, std::allocator<TYPE>>::*), \
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&std::vector<TYPE>::_M_impl, \
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TYPE*(std::vector<TYPE>::_Vector_impl::*), \
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&std::vector<TYPE>::_Vector_impl::_M_finish>; \
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FOLLY_DECLARE_VECTOR_RESIZE_WITHOUT_INIT_IMPL(TYPE)
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#elif defined(_MSC_VER)
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// MSVC
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#define FOLLY_DECLARE_VECTOR_RESIZE_WITHOUT_INIT(TYPE) \
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extern inline void unsafeVectorSetLargerSizeImpl( \
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std::vector<TYPE>& v, std::size_t n) { \
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v._Mylast() += (n - v.size()); \
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} \
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FOLLY_DECLARE_VECTOR_RESIZE_WITHOUT_INIT_IMPL(TYPE)
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#else
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#warning "No implementation for resizeWithoutInitialization of std::vector"
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#endif
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} // namespace detail
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} // namespace folly
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#if defined(FOLLY_DECLARE_VECTOR_RESIZE_WITHOUT_INIT)
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FOLLY_DECLARE_VECTOR_RESIZE_WITHOUT_INIT(char)
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FOLLY_DECLARE_VECTOR_RESIZE_WITHOUT_INIT(unsigned char)
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#endif
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