[PATCH] powerpc/ppc64: remove __volatile__ in get_current()

[James Yang]

On Sat, 10 Aug 2013, James Yang wrote:

> Uses of get_current() that normally get optimized away still result in
> a load instruction of the current pointer in 64-bit because the inline
> asm uses __volatile__.  This patch removes __volatile__ so that nop-ed
> uses of get_current() don't actually result in a load of the pointer.
> 
> Signed-off-by: James Yang <James.Yang at freescale.com>
> ---
>  arch/powerpc/include/asm/current.h |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/arch/powerpc/include/asm/current.h b/arch/powerpc/include/asm/current.h
> index e2c7f06..bb250c8 100644
> --- a/arch/powerpc/include/asm/current.h
> +++ b/arch/powerpc/include/asm/current.h
> @@ -19,7 +19,7 @@ static inline struct task_struct *get_current(void)
>  {
>  	struct task_struct *task;
>  
> -	__asm__ __volatile__("ld %0,%1(13)"
> +	__asm__ ("ld %0,%1(13)"
>  	: "=r" (task)
>  	: "i" (offsetof(struct paca_struct, __current)));


Hello, 

Scott's been able to put enough doubt in me to think that this is not 
entirely safe, even though the testing and code generation show it to 
work.  Please reject this patch.

I think there is still value in getting the unnecessary loads to be 
removed since it would also allow unnecessary conditional branches to 
be removed.  I'll think about alternate ways to do this.

Regards,

--James