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

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Date:	Sun, 29 Aug 2010 19:44:32 -0400
From:	Brian Gerst <brgerst@...il.com>
To:	Pekka Enberg <penberg@...nel.org>
Cc:	hpa@...or.com, x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 07/11] x86-64: Simplify constraints for fxsave/fxtstor

On Sun, Aug 29, 2010 at 2:45 PM, Pekka Enberg <penberg@...nel.org> wrote:
> On Sat, Aug 28, 2010 at 7:04 PM, Brian Gerst <brgerst@...il.com> 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@...il.com>
>> ---
>>  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?
>

There is no constraint to explicitly request %bp (or %sp), but there
is no reason it could not be used.  The compiler would never choose
%sp for "R" for the same reason it wouldn't for "r": it's not
available as a general purpose register.

--
Brian Gerst
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