RE: [RFC 12/12] asm-generic: simplify asm/unaligned.h

From: David Laight
Date: Sat May 08 2021 - 07:03:33 EST


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

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