Must have been supported for a while because this stuff is even used in Linux:
#define __packed2__ __attribute__((packed, aligned(2)))
/*
* SystemV FS comes in two variants:
* sysv2: System V Release 2 (e.g. Microport), structure elements aligned(2).
* sysv4: System V Release 4 (e.g. Consensys), structure elements aligned(4).
*/
struct sysv2_super_block {
__fs16 s_isize; /* index of first data zone */
__fs32 s_fsize __packed2__; /* total number of zones of this fs */
Note the use of `packed`. `packed` causes the alignment to be set as small as possible (1 byte). The aligned(2) that follows then increases the alignment from 1 byte to 2 bytes.
> "The aligned attribute can only increase the alignment; but you can decrease it by specifying packed as well. See below."
but gcc-6.2 documentation adds:
> "When used as part of a typedef, the aligned attribute can both increase and decrease alignment, and specifying the packed attribute generates a warning."
FWIW, Clang has supported reducing alignment in this fashion for a few years now.
[1] it's possible (likely, even) that it has been working for a while but the documentation was only recently brought up to date; I haven't investigated too carefully.