Re: [PATCH v3 07/13] x86: Add functions for 64-bit integer arithmetic

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

On 01/08/16 02:34, Jan Beulich wrote:
> >>> On 31.12.15 at 04:03, <haozhong.zhang@intel.com> wrote:
> > --- /dev/null
> > +++ b/xen/include/asm-x86/math64.h
> > @@ -0,0 +1,105 @@
> > +#ifndef __X86_MATH64
> > +#define __X86_MATH64
> > +
> > +/*
> > + * Functions defined in this file are derived from Linux kernel
> > + * (include/linux/math64.h).
> > + */
> 
> This is not true. At least mul64() doesn't exist in Linux (as of 4.4-rc8).
>

My fault. I took them from my KVM patches and didn't notice they were
refactored a little when merging.

> > +static inline void mul64(u64 *lo, u64 *hi, u64 a, u64 b)
> > +{
> > +    typedef union {
> > +        u64 ll;
> > +        struct {
> > +            u32 low, high;
> > +        } l;
> > +    } LL;
> > +    LL rl, rm, rn, rh, a0, b0;
> > +    u64 c;
> > +
> > +    a0.ll = a;
> > +    b0.ll = b;
> > +
> > +    rl.ll = (u64)a0.l.low * b0.l.low;
> > +    rm.ll = (u64)a0.l.low * b0.l.high;
> > +    rn.ll = (u64)a0.l.high * b0.l.low;
> > +    rh.ll = (u64)a0.l.high * b0.l.high;
> > +
> > +    c = (u64)rl.l.high + rm.l.low + rn.l.low;
> > +    rl.l.high = c;
> > +    c >>= 32;
> > +    c = c + rm.l.high + rn.l.high + rh.l.low;
> > +    rh.l.low = c;
> > +    rh.l.high += (u32)(c >> 32);
> > +
> > +    *lo = rl.ll;
> > +    *hi = rh.ll;
> > +}
> 
> For this one in particular, but likely also for the others: If this was
> put in include/xen/math64.h I might buy it. As an x86-specific
> implementation this could (and should) be done with a single asm()
> statement using the MUL instruction (I didn't check whether gcc
> also offers a built-in, which then could be used generically).
>

gcc does provide a 128-bit integer type __int128. If my memory is
correct, it can be used for above mul64, but can not be compiled in
Xen if being used for division. Anyway, as above functions are
x86-specific so far, I'll reimplement them by assembly in the next
version.

Haozhong