Subject: Re: [PATCH v5 20/22] powerpc/syscall: Avoid storing 'current' in another pointer

From: Segher Boessenkool <[email-masked]>
Date: Tue, 9 Feb 2021 07:50:53 -0600

On Tue, Feb 09, 2021 at 12:36:20PM +1000, Nicholas Piggin wrote:
> What if you did this?

> +static inline struct task_struct *get_current(void)
> +{
> +	register struct task_struct *task asm ("r2");
> +
> +	return task;
> +}

Local register asm variables are *only* guaranteed to live in that
register as operands to an asm.  See
  https://gcc.gnu.org/onlinedocs/gcc/Local-Register-Variables.html#Local-Register-Variables
("The only supported use" etc.)

You can do something like

static inline struct task_struct *get_current(void)
{
	register struct task_struct *task asm ("r2");

	asm("" : "+r"(task));

	return task;
}

which makes sure that "task" actually is in r2 at the point of that asm.

Segher