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[libc++] Increase atomic_ref's required alignment for small types #99654

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Merged
merged 7 commits into from
Aug 1, 2024
Merged

[libc++] Increase atomic_ref's required alignment for small types #99654

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8d1fc92
[libc++] Increase atomic_ref's required alignment for small types
dalg24 Jul 19, 2024
8545aeb
fixup! [libc++] Increase atomic_ref's required alignment for small types
dalg24 Jul 19, 2024
6027a62
Fix typo and follow the suggestion from the review
dalg24 Jul 19, 2024
056d4af
Fixup bool -> size_t
dalg24 Jul 23, 2024
e3b95d4
Merge branch 'main' of https://github.com/llvm/llvm-project into incr…
dalg24 Jul 26, 2024
edeef46
Follow James' suggestion and add aligned attribute to the pointer dat…
dalg24 Jul 26, 2024
976dbd9
Use alignas to make objects satisfy atomic_ref's alignement requirement
dalg24 Aug 1, 2024
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17 changes: 11 additions & 6 deletions libcxx/include/__atomic/atomic_ref.h
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Original file line number Diff line number Diff line change
Expand Up @@ -57,11 +57,6 @@ struct __get_aligner_instance {

template <class _Tp>
struct __atomic_ref_base {
protected:
_Tp* __ptr_;

_LIBCPP_HIDE_FROM_ABI __atomic_ref_base(_Tp& __obj) : __ptr_(std::addressof(__obj)) {}

private:
_LIBCPP_HIDE_FROM_ABI static _Tp* __clear_padding(_Tp& __val) noexcept {
_Tp* __ptr = std::addressof(__val);
Expand Down Expand Up @@ -108,10 +103,14 @@ struct __atomic_ref_base {

friend struct __atomic_waitable_traits<__atomic_ref_base<_Tp>>;

// require types that are 1, 2, 4, 8, or 16 bytes in length to be aligned to at least their size to be potentially
// used lock-free
static constexpr size_t __min_alignment = (sizeof(_Tp) & (sizeof(_Tp) - 1)) || (sizeof(_Tp) > 16) ? 0 : sizeof(_Tp);

public:
using value_type = _Tp;

static constexpr size_t required_alignment = alignof(_Tp);
static constexpr size_t required_alignment = alignof(_Tp) > __min_alignment ? alignof(_Tp) : __min_alignment;
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I think we should calculate the alignment using something like this?

template <class T>
constexpr size_t calc_alignment() {
    size_t align = alignof(T);
    while (align <= alignof(max_align_t)) {
        if (__atomic_always_lock_free(sizeof(T), align))
            return align;
        align *= 2;
    }
    return alignof(T);
}

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I really like this, I think this directly translates to what we are actually trying to achieve (which is to calculate the smallest alignment that allows us to be lockfree).

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That is not a valid implementation, because it causes the ABI of the type to change depending on the microarchitecture selection.

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TIL


// The __atomic_always_lock_free builtin takes into account the alignment of the pointer if provided,
// so we create a fake pointer with a suitable alignment when querying it. Note that we are guaranteed
Expand Down Expand Up @@ -218,6 +217,12 @@ struct __atomic_ref_base {
}
_LIBCPP_HIDE_FROM_ABI void notify_one() const noexcept { std::__atomic_notify_one(*this); }
_LIBCPP_HIDE_FROM_ABI void notify_all() const noexcept { std::__atomic_notify_all(*this); }

protected:
typedef _Tp _Aligned_Tp __attribute__((aligned(required_alignment)));
_Aligned_Tp* __ptr_;

_LIBCPP_HIDE_FROM_ABI __atomic_ref_base(_Tp& __obj) : __ptr_(std::addressof(__obj)) {}
};

template <class _Tp>
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2 changes: 1 addition & 1 deletion libcxx/test/std/atomics/atomics.ref/is_always_lock_free.pass.cpp
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Expand Up @@ -54,7 +54,7 @@ void check_always_lock_free(std::atomic_ref<T> const& a) {
#define CHECK_ALWAYS_LOCK_FREE(T) \
do { \
typedef T type; \
type obj{}; \
alignas(std::atomic_ref<type>::required_alignment) type obj{}; \
std::atomic_ref<type> a(obj); \
check_always_lock_free(a); \
} while (0)
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