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

[Geert Uytterhoeven]

Hi Arnd,

On Sat, May 8, 2021 at 12:12 AM Arnd Bergmann <arnd@xxxxxxxxxx> wrote:
> The get_unaligned()/put_unaligned() implementations are much more complex
> than necessary, now that all architectures use the same code.
>
> Move everything into one file and use a much more compact way to express
> the same logic.
>
> I've compared the binary output using gcc-11 across defconfig builds for
> all architectures and found this patch to make no difference, except for
> a single function on powerpc that needs two additional register moves
> because of random differences in register allocation.
>
> There are a handful of callers of the low-level __get_unaligned_cpu32,
> so leave that in place for the time being even though the common code
> no longer uses it.
>
> This adds a warning for any caller of get_unaligned()/put_unaligned()
> that passes in a single-byte pointer, but I've sent patches for all
> instances that show up in x86 and randconfig builds. It would be nice
> to change the arguments of the endian-specific accessors to take the
> matching __be16/__be32/__be64/__le16/__le32/__le64 arguments instead of
> a void pointer, but that requires more changes to the rest of the kernel.
>
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>

Thanks for your patch!

> @@ -6,20 +6,132 @@
>   * This is the most generic implementation of unaligned accesses
>   * and should work almost anywhere.
>   */
> +#include <linux/unaligned/packed_struct.h>
>  #include <asm/byteorder.h>
>
> -#if defined(__LITTLE_ENDIAN)
> -# include <linux/unaligned/le_struct.h>
> -# include <linux/unaligned/generic.h>
> -# define get_unaligned __get_unaligned_le
> -# define put_unaligned __put_unaligned_le
> -#elif defined(__BIG_ENDIAN)
> -# include <linux/unaligned/be_struct.h>
> -# include <linux/unaligned/generic.h>
> -# define get_unaligned __get_unaligned_be
> -# define put_unaligned __put_unaligned_be
> -#else
> -# error need to define endianess
> -#endif
> +#define __get_unaligned_t(type, ptr) ({                                                \
> +       const struct { type x; } __packed *__pptr = (typeof(__pptr))(ptr);      \
> +       __pptr->x;                                                              \

Space before tab (cfr. checkpatch).

> +})
> +
> +#define get_unaligned(ptr) ({                                                  \
> +       __auto_type __ptr = (ptr);                                              \
> +       __get_unaligned_t(typeof(*__ptr), __ptr);                               \
> +})
> +
> +#define __put_unaligned_t(type, val, ptr) ({                                   \
> +       struct { type x; } __packed *__pptr = (typeof(__pptr))(ptr);            \
> +       __pptr->x = (val);                                                      \

Likewise

> +})
>
Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds