[PATCH] amd64/pcpu.h: Use Clang builtins for clarity when referencing thread's pcpu

[John Baldwin]

On Friday, March 14, 2014 2:31:08 pm Meyer, Conrad wrote:
> We can efficiently reference thread-local pcpu members via the %gs
> register with Clang-annotated C code, in place of inline GNU assembly.
> 
> Motivations:
>   - Use C in leiu of inline assembly for clarity
>   - Clang's static analyser may be better able to understand PCPU_*
>     macros using the C constructs rather than inline assembly
>     (unverified)
> 
> Sponsored by: EMC/Isilon storage division
> Signed-off-by: Conrad Meyer <conrad.meyer at isilon.com>
> Reviewed-by: Max Laier <mlaier at FreeBSD.org>
> ---
> This is more of a "what do you think?" than a pull request. It seems like using
> annotated C instead of asm is nice (in particular, Clang detects casts from
> pointers typed with one segment to another, or unsegmented type). On the other
> hand, this is code that doesn't change frequently, and we may still need to
> support GCC for some time. So adding a second, parallel implementation just
> doubles room for bugs.

I think this is neat and wanted to look at doing this when I first noticed the
address_space() attribute in the clang docs.
 
> Open questions:
>   - How long is GCC intended to be supported as a compiler?

That I don't know.

>   - How atomic does PCPU_INC() need to be? It looks like it updates cpu-local
>     counters; as long as it's a single asm instruction, should it be fine
>     w.r.t. interrupts? The existing implementation does NOT use the 'lock; ' prefix.

I think a single instruction is fine.

> diff --git a/sys/amd64/include/pcpu.h b/sys/amd64/include/pcpu.h
> index fe898e9..68892fc 100644
> --- a/sys/amd64/include/pcpu.h
> +++ b/sys/amd64/include/pcpu.h
> +#define	curthread __extension__ ({					\
> +	*((volatile __pcpu_type(pc_curthread) __GS_RELATIVE *)		\
> +		__pcpu_offset(pc_curthread));				\
> +})

Would be nice to not lose the __pure2 attribute for curthread (you
might need it to still be an inline function to keep that)

-- 
John Baldwin