Re: [PATCH RFC 2/4] include/linux/compiler.h: allow memory operands

[Will Deacon]

On Wed, Jan 02, 2019 at 03:57:54PM -0500, Michael S. Tsirkin wrote:
> We don't really care whether the variable is in-register
> or in-memory. Relax the constraint accordingly.
> 
> Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
> ---
>  include/linux/compiler.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> index 1ad367b4cd8d..6601d39e8c48 100644
> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -154,7 +154,7 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
>  #ifndef OPTIMIZER_HIDE_VAR
>  /* Make the optimizer believe the variable can be manipulated arbitrarily. */
>  #define OPTIMIZER_HIDE_VAR(var)						\
> -	__asm__ ("" : "=r" (var) : "0" (var))
> +	__asm__ ("" : "=rm" (var) : "0" (var))
>  #endif

I think this can break for architectures with write-back addressing modes
such as arm, where the "m" constraint is assumed to be evaluated precisely
once in the asm block.

Will