4f4690530e
Currently std::expected can have some padding bytes in its tail due to
[[no_unique_address]]. Those padding bytes can be used by other objects.
For example, in the current implementation:
sizeof(std::expected<std::optional<int>, bool>) ==
sizeof(std::expected<std::expected<std::optional<int>, bool>, bool>)
As a result, the data layout of an
std::expected<std::expected<std::optional<int>, bool>, bool>
can look like this:
+-- optional "has value" flag
| +--padding
/---int---\ | |
00 00 00 00 01 00 00 00
| |
| +- "outer" expected "has value" flag
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+- expected "has value" flag
This is problematic because `emplace()`ing the "inner" expected can not
only overwrite the "inner" expected "has value" flag (issue #68552) but
also the tail padding where other objects might live.
This patch fixes the problem by ensuring that std::expected has no tail
padding, which is achieved by conditional usage of [[no_unique_address]]
based on the tail padding that this would create.
This is an ABI breaking change because the following property changes:
sizeof(std::expected<std::optional<int>, bool>) <
sizeof(std::expected<std::expected<std::optional<int>, bool>, bool>)
Before the change, this relation didn't hold. After the change, the relation
does hold, which means that the size of std::expected in these cases increases
after this patch. The data layout will change in the following cases where
tail padding can be reused by other objects:
class foo : std::expected<std::optional<int>, bool> {
bool b;
};
or using [[no_unique_address]]:
struct foo {
[[no_unique_address]] std::expected<std::optional<int>, bool> e;
bool b;
};
The vendor communication is handled in #70820.
Fixes: #70494
Co-authored-by: philnik777 <nikolasklauser@berlin.de>
Co-authored-by: Louis Dionne <ldionne.2@gmail.com>
105 lines
2.7 KiB
C++
105 lines
2.7 KiB
C++
//===----------------------------------------------------------------------===//
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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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// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
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// template<class... Args>
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// constexpr T& emplace(Args&&... args) noexcept;
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// Constraints: is_nothrow_constructible_v<T, Args...> is true.
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//
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// Effects: Equivalent to:
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// if (has_value()) {
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// destroy_at(addressof(val));
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// } else {
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// destroy_at(addressof(unex));
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// has_val = true;
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// }
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// return *construct_at(addressof(val), std::forward<Args>(args)...);
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#include <cassert>
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#include <concepts>
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#include <expected>
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#include <type_traits>
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#include <utility>
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#include "../../types.h"
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#include "test_macros.h"
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template <class T, class... Args>
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concept CanEmplace = requires(T t, Args&&... args) { t.emplace(std::forward<Args>(args)...); };
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static_assert(CanEmplace<std::expected<int, int>, int>);
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template <bool Noexcept>
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struct CtorFromInt {
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CtorFromInt(int) noexcept(Noexcept);
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CtorFromInt(int, int) noexcept(Noexcept);
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};
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static_assert(CanEmplace<std::expected<CtorFromInt<true>, int>, int>);
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static_assert(CanEmplace<std::expected<CtorFromInt<true>, int>, int, int>);
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static_assert(!CanEmplace<std::expected<CtorFromInt<false>, int>, int>);
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static_assert(!CanEmplace<std::expected<CtorFromInt<false>, int>, int, int>);
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constexpr bool test() {
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// has_value
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{
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BothNoexcept::state oldState{};
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BothNoexcept::state newState{};
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std::expected<BothNoexcept, int> e(std::in_place, oldState, 5);
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decltype(auto) x = e.emplace(newState, 10);
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static_assert(std::same_as<decltype(x), BothNoexcept&>);
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assert(&x == &(*e));
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assert(oldState.dtorCalled);
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assert(e.has_value());
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assert(e.value().data_ == 10);
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}
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// !has_value
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{
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BothMayThrow::state oldState{};
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std::expected<int, BothMayThrow> e(std::unexpect, oldState, 5);
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decltype(auto) x = e.emplace(10);
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static_assert(std::same_as<decltype(x), int&>);
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assert(&x == &(*e));
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assert(oldState.dtorCalled);
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assert(e.has_value());
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assert(e.value() == 10);
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}
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// TailClobberer
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{
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std::expected<TailClobberer<0>, bool> e(std::unexpect);
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e.emplace();
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assert(e.has_value());
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}
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// CheckForInvalidWrites
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{
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{
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CheckForInvalidWrites<true> e;
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e.emplace();
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assert(e.check());
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}
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{
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CheckForInvalidWrites<false> e;
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e.emplace();
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assert(e.check());
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}
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}
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return true;
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}
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int main(int, char**) {
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test();
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static_assert(test());
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return 0;
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}
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