https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77330
--- Comment #8 from joseph at codesourcery dot com <joseph at codesourcery dot com> --- On Tue, 23 Aug 2016, bernd.edlinger at hotmail dot de wrote: > gcc assumes that malloc, calloc, realloc, strdup, strndup > and anything with the __attribute__((__malloc__)) > returns a pointer that is aligned to MALLOC_ABI_ALIGNMENT bits. That it makes this more conservative assumption for some optimization purposes is separate from the assumption that if you dereference a pointer to a type, that pointer, however you got it, meets the alignment requirements for the type. __float128 is 16-byte-aligned, so GCC assumes any dereferenced pointer to it is also 16-byte-aligned, and it's the user's job to meet that requirement (by not using malloc for that allocation if it doesn't return sufficiently aligned memory). There is no difference between __float128 and __m512 in this regard: a malloc that doesn't return suitably aligned memory isn't suitable for allocating storage for such objects, and if you use malloc to do so then GCC will assume that, at least for that particular allocation, the return value is suitably aligned.
