Aligning arrays

When developing stack-based containers for RDESTL I’ve encountered the following problem - how to get block of uninitialized memory that’s aligned properly for type T. Consider fixed_vector class:

template
class fixed_array
{
    ...
    char    m_data[N * sizeof(T)];

template
struct alignof_helper
{
    char    x;
    T       y;
};
template
struct alignof
{
    enum
    {
        res = offsetof(alignof_helper, y)
    };
};

CHECK_EQUAL(16, rde::alignof<__m128>::res);

First part of a problem solved. However, “res” cannot be simply used as an argument to __declspec(align). Instead, we have to create basic types with most common alignments.

__declspec(align(16)) struct aligned16 { uint64_t member[2]; };
template struct type_with_alignment
{
    typedef char err_invalid_alignment[N > 0 ? -1 : 1];
};
template<> struct type_with_alignment<0> {};
template<> struct type_with_alignment<1> { uint8_t member; };
template<> struct type_with_alignment<2> { uint16_t member; };
template<> struct type_with_alignment<4> { uint32_t member; };
template<> struct type_with_alignment<8> { uint64_t member; };
template<> struct type_with_alignment<16> { aligned16 member; };

...

template
struct aligned_as
{
    typedef typename internal::type_with_alignment<alignof::res> res;
};

Finally, we create array of properly aligned types instead of chars. We also have to modify array size, as element size is no longer one byte, it needs to be corrected:

typedef typename aligned_as::res    etype_t;
etype_t                    m_data[(N * sizeof(T)) / sizeof(etype_t)];

Old comments

Reg 2009-03-08 11:26:37

I suppose you could use __alignof instead of this alignof_helper struct with offsetof.

admin 2009-03-08 11:59:45

Sadly, __alignof isn’t really reliable: http://www.nabble.com/-Format--Asserts-with-non-default-structure-packing-in-1.37-(MSVC)-td21215959.html

jason gregory 2010-04-04 23:40:33

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