From: Arnd Bergmann
> Sent: 07 May 2021 23:08
>
> The get_unaligned()/put_unaligned() implementations are much more complex
> than necessary, now that all architectures use the same code.
>
...
> +#define __get_unaligned_t(type, ptr) ({ \
> + const struct { type x; } __packed *__pptr = (typeof(__pptr))(ptr); \
> + __pptr->x; \
> +})
I thought gcc was likely to track through the alignment of the
variable holding the source pointer (through any (void *) casts
implied by inlined function calls) through to the pointer used
for the actual access - so it would tend to issue a single
instruction that assumed an aligned address.
I know that has caused grief trying to copy unaligned data
to an aligned structure.
Possibly adding:
asm ("" :: "+r" (__pptr)) );
in the middle stops that assumption without generating any code.
David
-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)
