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    SubjectRe: [PATCH 07/11] x86-64: Simplify constraints for fxsave/fxtstor
    From
    On Sun, Aug 29, 2010 at 2:45 PM, Pekka Enberg <penberg@kernel.org> wrote:
    > On Sat, Aug 28, 2010 at 7:04 PM, Brian Gerst <brgerst@gmail.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@gmail.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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