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🐛 Fix stack word alignment on x86_64 builds #98

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Merged
kammce merged 2 commits into from
Apr 24, 2026
Merged

🐛 Fix stack word alignment on x86_64 builds #98

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2ca2cf2
:bug: Fix stack word alignment on x86_64 builds
kammce Apr 24, 2026
8e8361d
[skip ci] remove extra newline between docs of stack_wrod
kammce Apr 24, 2026
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31 changes: 19 additions & 12 deletions modules/coroutine.cppm
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Original file line number Diff line number Diff line change
Expand Up @@ -50,7 +50,10 @@ namespace async::inline v0 {
* satisfy the alignment contract of operator new. See initialize_stack_memory()
* for details.
*/
export using stack_word = std::uintptr_t;
export struct alignas(__STDCPP_DEFAULT_NEW_ALIGNMENT__) stack_word
{
uintptr_t data = 0;
};

/**
* @brief Bit mask for pointer alignment checking
Expand Down Expand Up @@ -403,7 +406,7 @@ public:
// NOTE: subtract 1 because we use the end of the stack for holding the
// length of the stack.
auto const capacity = p_stack_memory.size() - 1uz;
p_stack_memory.back() = capacity;
p_stack_memory.back().data = capacity;
m_stack_pointer = &p_stack_memory.front();
m_stack_end = &p_stack_memory.back();
}
Expand Down Expand Up @@ -663,7 +666,7 @@ public:
*/
[[nodiscard]] constexpr auto capacity() const noexcept
{
return *m_stack_end;
return m_stack_end->data;
}

/**
Expand Down Expand Up @@ -850,7 +853,8 @@ private:
// Put the address of the stack pointer member on the stack, before the
// coroutine frame, such that the delete operation can find the address and
// update it.
*m_stack_pointer = std::bit_cast<stack_word>(&m_stack_pointer);
m_stack_pointer->data =
std::bit_cast<decltype(m_stack_pointer->data)>(&m_stack_pointer);

// Address of the coroutine frame will be the current position of the stack
// pointer + 1 to avoid overwriting the stack pointer address.
Expand Down Expand Up @@ -1031,7 +1035,7 @@ public:
}

private:
alignas(std::max_align_t) std::array<stack_word, StackSizeInWords> m_stack{};
std::array<stack_word, StackSizeInWords> m_stack{};
};

// =============================================================================
Expand Down Expand Up @@ -1084,13 +1088,16 @@ public:
*/
static constexpr void operator delete(void* p_promise) noexcept
{
// Acquire the pointer to the context stack memory from behind the coroutine
// frame's memory.
auto** stack_pointer_address = *(static_cast<stack_word***>(p_promise) - 1);

// Update the stack pointer's address to be equal where it was before the
// promise was allocated. Or said another way, the
*stack_pointer_address = (static_cast<stack_word*>(p_promise) - 1);
// The stack that this promise is allocated on is in stack_word chunks.
// Behind the promise's address is a stack_word that has the stack pointer's
// address within its `data` member.
auto* frame_header = static_cast<stack_word*>(p_promise) - 1uz;
// The data field contains the address of the stack pointer, making it a
// double pointer.
auto** stack_pointer_ptr = std::bit_cast<stack_word**>(frame_header->data);
// De-reference the stack_pointer_ptr to modify the stack_pointer_ptr and
// assign it to the address of the frame_header
*stack_pointer_ptr = frame_header;
}

/**
Expand Down