On 07/08/20 10:55 +0200, Jakub Jelinek wrote:
On Fri, Aug 07, 2020 at 09:34:38AM +0100, Jonathan Wakely via Gcc-patches wrote:
> Now that you say it, vec has a T[1] member so depending on T
> there might be a side-effect as invoking its CTOR? Or it might
> even not compile if there is no default CTOR available... Ick :/
Right.
Does the GTY stuff add members to the struct, or otherwise alter its
layout?
Or perhaps use offsetof on an alternate structure that should have the same
layout.
Yes that's what I was going to suggest next.
So instead of
typedef vec<T, A, vl_embed> vec_embedded;
return offsetof (vec_embedded, m_vecdata) + alloc * sizeof (T);
use
struct alignas (T) U { char data[sizeof (T)]; };
typedef vec<U, A, vl_embed> vec_embedded;
static_assert(sizeof(vec_embedded) == sizeof(vec), "");
static_assert(alignof(vec_embedded) == alignof(vec), "");
return offsetof (vec_embedded, m_vecdata) + alloc * sizeof (T);
where vec_embedded should have the same offset of m_vecdata as vec.
