Re: [PATCH 07/11] x86-64: Simplify constraints for fxsave/fxtstor

[Pekka Enberg]

On Sat, Aug 28, 2010 at 7:04 PM, Brian Gerst <brgerst@xxxxxxxxx> wrote:
> Use the "R" constraint (legacy register) instead of listing all the
> possible registers.  Clean up the comments as well.
>
> Signed-off-by: Brian Gerst <brgerst@xxxxxxxxx>
> ---
>  arch/x86/include/asm/i387.h |   44 ++++++++++++++++--------------------------
>  1 files changed, 17 insertions(+), 27 deletions(-)
>
> diff --git a/arch/x86/include/asm/i387.h b/arch/x86/include/asm/i387.h
> index 8b40a83..768fcb2 100644
> --- a/arch/x86/include/asm/i387.h
> +++ b/arch/x86/include/asm/i387.h
> @@ -81,6 +81,7 @@ static inline int fxrstor_checking(struct i387_fxsave_struct *fx)
>  {
>        int err;
>
> +       /* See comment in fxsave() below. */
>        asm volatile("1:  rex64/fxrstor (%[fx])\n\t"
>                     "2:\n"
>                     ".section .fixup,\"ax\"\n"
> @@ -89,11 +90,7 @@ static inline int fxrstor_checking(struct i387_fxsave_struct *fx)
>                     ".previous\n"
>                     _ASM_EXTABLE(1b, 3b)
>                     : [err] "=r" (err)
> -#if 0 /* See comment in fxsave() below. */
> -                    : [fx] "r" (fx), "m" (*fx), "0" (0));
> -#else
> -                    : [fx] "cdaSDb" (fx), "m" (*fx), "0" (0));
> -#endif
> +                    : [fx] "R" (fx), "m" (*fx), "0" (0));

Please correct me if I'm wrong but "legacy registers" also include bp
and sp that are not part of the original constraints:

http://gcc.gnu.org/onlinedocs/gcc/Machine-Constraints.html

So why is it OK to use "R" here?
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