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Re: Creating fastest Mul31


On 17 nov 2005, at 10:27, Rustam Muginov wrote:

I am interested what would be fastest implementation of the following multiplication:

#define Mul31(x,y) ((int32)(((int64)(x) * (y)) >> 31))

obviously, int32 and int64 are signed 32 and 64 integers.

Currently i am using: 
inline int32 Mul31(int32 x, int32 y)
{
  register int hi_prod, lo_prod, result;
  asm
  {
    mullw lo_prod, x, y
    srwi lo_prod, lo_prod, 31

    mulhw hi_prod, x, y
    slwi hi_prod, hi_prod, 1

    or result, hi_prod, lo_prod;
  }
  return result;
}

Is it possible to make it faster?

You can replace the "slwi" and "or" at the end with a rlwimi (if you put the result of the mullhw immediately in result instead of in hi_prod), but I doubt it will make that much of a difference (majority of the time is spent multiplying). Something like (untested)

mullhw result, x, y
rlwimi result, lo_prod, 1, 0, 30


Jonas

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