Re: [PATCH v4 05/10] x86: Add functions for 64-bit integer arithmetic

["Zhang, Haozhong" <haozhong.zhang@intel.com>]

On 02/05/16 21:36, Jan Beulich wrote:
> >>> On 17.01.16 at 22:58, <haozhong.zhang@intel.com> wrote:
> > This patch adds several functions to take multiplication, division and
> > shifting involving 64-bit integers.
> > 
> > Signed-off-by: Haozhong Zhang <haozhong.zhang@intel.com>
> > Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
> > ---
> > Changes in v4:
> >  (addressing Jan Beulich's comments)
> >  * Rewrite mul_u64_u64_shr() in assembly.
> 
> Thanks, but it puzzles me that the other one didn't get converted
> as well. Anyway, I'm not going to make this a requirement, since
> at least it appears to match Linux'es variant.
>

I can't remember why I didn't rewrite mul_u64_u32_div(), especially when
it can be easily implemented as

static inline u64 mul_u64_u32_div(u64 a, u32 mul, u32 divisor)
{
    u64 quotient, remainder;
    
    asm volatile ( "mulq %3; divq %4"
                   : "=a" (quotient), "=d" (remainder)
                   : "0" (a), "rm" ((u64) mul), "c" ((u64) divisor) );

    return quotient;
}

I'll modify it in the next version.

> > +static inline u64 mul_u64_u64_shr(u64 a, u64 mul, unsigned int n)
> > +{
> > +    u64 hi, lo;
> > +
> > +    asm volatile ( "mulq %2; shrdq %1,%0"
> > +                   : "=a" (lo), "=d" (hi)
> > +                   : "rm" (mul), "0" (a), "c" (n) );
> 
> SHRD formally is a 3-operand instruction, and the fact that gas'
> AT&T syntax supports a 2-operand "alias" is, well, odd. Please
> let's use the specification mandated 3-operand form properly,
> to avoid surprises with e.g. clang.
>

OK, I'll change it to the 3-operand form.

Haozhong