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185b739cf1
Function types aren't always trivially copyable, e.g. with address-discriminated signed pointers on ARM64e. Introduce a function_cast helper and use that instead.
52 lines
2.0 KiB
C++
52 lines
2.0 KiB
C++
//===--- Casting.h - Helpers for casting ------------------------*- C++ -*-===//
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//
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// This source file is part of the Swift.org open source project
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//
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// Copyright (c) 2014 - 2025 Apple Inc. and the Swift project authors
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// Licensed under Apache License v2.0 with Runtime Library Exception
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//
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// See https://swift.org/LICENSE.txt for license information
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// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
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//
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//===----------------------------------------------------------------------===//
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#ifndef SWIFT_BASIC_CASTING_H
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#define SWIFT_BASIC_CASTING_H
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#include <type_traits>
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namespace swift {
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/// Cast between two function types. Use in place of std::bit_cast, which
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/// doesn't work on ARM64e with address-discriminated signed function types.
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///
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/// Address-discriminated ptrauth attributes can only be applied to values with
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/// a defined storage location, such as struct fields, since their value is
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/// inherently tied to their address. They can't be applied to function
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/// paremeters. When passing such a value to a templated function, the ptrauth
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/// attribute disappears from the inferred type.
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///
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/// bit_cast takes the source by reference, which means that the ptrauth
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/// attribute remains on the inferred type, and the value is not trivially
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/// copyable.
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///
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/// function_cast instead takes the source by value, avoiding that issue and
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/// ensuring that passed-in function pointers are always trivially copyable.
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template <typename Destination, typename Source>
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Destination function_cast(Source source) {
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static_assert(sizeof(Destination) == sizeof(Source),
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"Source and destination must be the same size");
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static_assert(std::is_trivially_copyable_v<Source>,
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"The source type must be trivially constructible");
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static_assert(std::is_trivially_copyable_v<Destination>,
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"The destination type must be trivially constructible");
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Destination destination;
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memcpy(&destination, &source, sizeof(source));
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return destination;
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}
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}
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#endif
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