asking for __attribute__((aligned()) clarification
All,
this is my first post on these lists, so please bear with me.
My question is about gcc's __attribute__((aligned()). Please consider the
following code:
#include
typedef uint32_t uuint32_t __attribute__((aligned(1)));
uint32_t getuuint32(uint8_t p[]) {
return *(uuint32_t*)p;
}
This is meant to prevent gcc to produce hard faults/address errors on
architectures that do not support unaligned access to shorts/ints (e.g some
ARMs, some m68k). On these architectures, gcc is supposed to replace the 32 bit
access with a series of 8 or 16 bit accesses.
I originally came from gcc 4.6.4 (yes, pretty old) where this did not work and
gcc does not respect the aligned(1) attribute for its code generation (i.e. it
generates a 'normal' pointer dereference, consequently crashing when the code
executes). To be fair, it is my understanding that the gcc manuals never
promised this *would* work.
As - at least as far as I can tell - documentation didn't really change
regarding lowering alignment for variables and does not appear to say anything
specific regarding pointer dereference on single, misaligned variables, I was
pretty astonished to see this working on newer gcc versions (tried 6.2 and
7.4), however. gcc appears to even know the differences within an architecture
(68000 generates a bytewise copy while ColdFire - that supports unaligned
access - copies a 32 bit value).
My question: is this now intended behaviour we can rely on? If yes, can we have
documentation upgraded to clearly state that this use case is valid?
Thank you.
Markus
Aw: Re: asking for __attribute__((aligned()) clarification
Thank you (and others) for your answers. Now I'm just as smart as before, however. Is it a supported, documented, 'long term' feature we can rely on or not? If yes, I would expect it to be properly documented. If not, never mind. > Gesendet: Montag, 19. August 2019 um 16:08 Uhr > Von: "Alexander Monakov" > An: "Richard Earnshaw (lists)" > Cc: "Paul Koning" , "Markus Fröschle" > , gcc@gcc.gnu.org > Betreff: Re: asking for __attribute__((aligned()) clarification > > On Mon, 19 Aug 2019, Richard Earnshaw (lists) wrote: > > > Correct, but note that you can only pack structs and unions, not basic > > types. > > there is no way of under-aligning a basic type except by wrapping it in a > > struct. > > I don't think that's true. In GCC-9 the doc for 'aligned' attribute has been > significantly revised, and now ends with > > When used as part of a typedef, the aligned attribute can both increase and > decrease alignment, and specifying the packed attribute generates a > warning. > > (but I'm sure defacto behavior of accepting and honoring reduced alignment on > a typedef'ed scalar type goes way earlier than gcc-9) > > Alexander >
