src/tint/utils/memory/bitcast.h

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#ifndef SRC_TINT_UTILS_MEMORY_BITCAST_H_
#define SRC_TINT_UTILS_MEMORY_BITCAST_H_

#include <cstddef>
#include <cstring>
#include <type_traits>

#include "src/utils/compiler.h"

namespace tint {

/// Bitcast performs a cast of `from` to the `TO` type using a memcpy.
/// This unsafe cast avoids triggering Clang's Control Flow Integrity checks.
/// See: crbug.com/dawn/1406
/// See: https://clang.llvm.org/docs/ControlFlowIntegrity.html#bad-cast-checking
/// @param from the value to cast
/// @tparam TO the value to cast to
/// @returns the cast value
template <typename TO, typename FROM>
inline TO Bitcast(FROM&& from) {
    static_assert(sizeof(FROM) == sizeof(TO));
    // gcc warns in cases where either TO or FROM are classes, even if they are trivially
    // copyable, with for example:
    //
    // error: ‘void* memcpy(void*, const void*, size_t)’ copying an object of
    // non-trivial type ‘struct tint::Number<unsigned int>’ from an array of ‘float’
    // [-Werror=class-memaccess]
    //
    // We avoid this by asserting that both types are indeed trivially copyable, and casting both
    // args to std::byte*.
    static_assert(std::is_trivially_copyable_v<std::decay_t<FROM>>);
    static_assert(std::is_trivially_copyable_v<std::decay_t<TO>>);
    TO to;
    // SAFETY: The source and destination are guaranteed to be of identical size (sizeof(TO) ==
    // sizeof(FROM)) by the static_assert above, and they are trivially copyable.
    DAWN_UNSAFE_BUFFERS(memcpy(reinterpret_cast<std::byte*>(&to),
                               reinterpret_cast<const std::byte*>(&from), sizeof(TO)));
    return to;
}

}  // namespace tint

#endif  // SRC_TINT_UTILS_MEMORY_BITCAST_H_