GC overhead just because of aligning long double?

[Richard Guenther]

We're waisting 8 bytes for every gimple_seq_node_d on x86_64 just
because we might be allocating a structure with a long double
element (16 byte aligned).  I grepped and didn't find traces of
such a use, so - can we just document that callers need to
round up allocation sizes to multiples of the required alignment
(and just enforce that for alignment requirements bigger than pointers)?

A gimple_seq_node_d should be allocated from 24 byte page sizes
but are allocated from 32 byte page sizes because of the above
issue - wasting 25%.

Thanks,
Richard.