[ros-dev] [ros-diffs] [dgorbachev] 53907: [NTOSKRNL] Use inline asm in KiIsNpxErrataPresent(). C code can be optimized in a wrong way.

[Alex Ionescu]

Care to explain?

If it can be optimized the wrong way -- use a pragma or fix the compiler.

I hate these kinds of patches. I think Timo would agree.

Best regards,
Alex Ionescu


On Fri, Sep 30, 2011 at 5:03 PM, <dgorbachev at svn.reactos.org> wrote:

> Author: dgorbachev
> Date: Fri Sep 30 21:03:29 2011
> New Revision: 53907
>
> URL: http://svn.reactos.org/svn/reactos?rev=53907&view=rev
> Log:
> [NTOSKRNL]
> Use inline asm in KiIsNpxErrataPresent(). C code can be optimized in a
> wrong way.
>
> Modified:
>    trunk/reactos/ntoskrnl/ke/i386/cpu.c
>
> Modified: trunk/reactos/ntoskrnl/ke/i386/cpu.c
> URL:
> http://svn.reactos.org/svn/reactos/trunk/reactos/ntoskrnl/ke/i386/cpu.c?rev=53907&r1=53906&r2=53907&view=diff
>
> ==============================================================================
> --- trunk/reactos/ntoskrnl/ke/i386/cpu.c [iso-8859-1] (original)
> +++ trunk/reactos/ntoskrnl/ke/i386/cpu.c [iso-8859-1] Fri Sep 30 21:03:29
> 2011
> @@ -1254,9 +1254,9 @@
>  INIT_FUNCTION
>  KiIsNpxErrataPresent(VOID)
>  {
> -    BOOLEAN ErrataPresent;
> +    static double Value1 = 4195835.0, Value2 = 3145727.0;
> +    INT ErrataPresent;
>     ULONG Cr0;
> -    volatile double Value1, Value2;
>
>     /* Disable interrupts */
>     _disable();
> @@ -1269,9 +1269,30 @@
>     Ke386FnInit();
>
>     /* Multiply the magic values and divide, we should get the result back
> */
> -    Value1 = 4195835.0;
> -    Value2 = 3145727.0;
> -    ErrataPresent = (Value1 * Value2 / 3145727.0) != 4195835.0;
> +#ifdef __GNUC__
> +    __asm__ __volatile__
> +    (
> +        "fldl %1\n\t"
> +        "fdivl %2\n\t"
> +        "fmull %2\n\t"
> +        "fldl %1\n\t"
> +        "fsubp\n\t"
> +        "fistpl %0\n\t"
> +        : "=m" (ErrataPresent)
> +        : "m" (Value1),
> +          "m" (Value2)
> +    );
> +#else
> +    __asm
> +    {
> +        fld Value1
> +        fdiv Value2
> +        fmul Value2
> +        fld Value1
> +        fsubp
> +        fistp ErrataPresent
> +    };
> +#endif
>
>     /* Restore CR0 */
>     __writecr0(Cr0);
> @@ -1280,7 +1301,7 @@
>     _enable();
>
>     /* Return if there's an errata */
> -    return ErrataPresent;
> +    return ErrataPresent != 0;
>  }
>
>  VOID
>
>
>
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