Incorrect optimization: std::byte representation swap not observed

Mingxin_Wang — Tue, 16 Sep 2025 08:55:59 GMT

A trivially copyable object (struct Foo) is stored inside a std::byte buffer via placement new. Swapping two such objects by swapping their std::byte arrays (std::swap on the arrays) should exchange their object representations. The program prints the original value instead of the swapped value only with icx; other compilers (GCC, Clang, MSVC, NVHPC) produce the expected output. This suggests icx fails to treat writes through std::byte as potentially modifying the overlapped object, violating the aliasing rules that grant std::byte access to any object's representation.

Compiler Explorer link

#include <cstddef> /* For std::byte */ #include <cstdio> /* For printf */ #include <memory> /* For std::construct_at */ #include <new> /* For std::launder */ #include <type_traits> /* For std::is_trivially_copyable_v */ #include <utility> /* For std::swap */ struct Erased { template <class T> Erased(T val) requires(std::is_trivially_copyable_v<T> && sizeof(T) <= sizeof(void*) && alignof(T) <= alignof(void*)) { std::construct_at(reinterpret_cast<T*>(data), val); } void Swap(Erased& rhs) { std::swap(data, rhs.data); std::swap(padding, rhs.padding); } alignas(alignof(void*)) std::byte data[sizeof(void*)]; void* padding = nullptr; }; int main() { struct Foo { const char* msg; }; Erased a{Foo{"bug"}}; Erased b{Foo{"ok"}}; a.Swap(b); Foo* p = std::launder(reinterpret_cast<Foo*>(a.data)); puts(p->msg); // Expect: "ok"; Actual: "bug" }

Expected Result: Program outputs: ok

Actual Result: Program outputs: bug

Impact: Causes silent logic errors in type‑erasure / small buffer optimization code that relies on valid object representation swaps.

topic Incorrect optimization: std::byte representation swap not observed in Intel® oneAPI DPC++/C++ Compiler

Incorrect optimization: std::byte representation swap not observed