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 <stdint.h>
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
