Re: [PATCH] New function to generate mp_no from powers of two

[Carlos O'Donell <carlos at systemhalted dot org>]

On 01/17/2013 06:05 AM, Siddhesh Poyarekar wrote:
> Hi,
> 
> The __mpexp_twomm1 array (and a lot of unnecessary computation around
> it) is required in mpexp because there is currently no way to generate
> mp_no numbers that are powers of two.  These are actually very simple
> numbers since they just have the exponent, sign and the first mantissa
> digit set and hence are a good special case to have (although I
> haven't done a code search to see yet if it's useful in any other
> functions).  Attached patch implements an inline function __pow_mp
> (inline since it is not a very big or complicated function) and
> implements it in its first user, which is the __mpexp function.  I
> have verified that this does not break any test cases on x86_64.
> 
> Performance:
> 
> With the patch:
> 
> Total:26237847075, Fastest:255700, Slowest:893136, Avg:262378.470750
> 
> Without the patch:
> 
> Total:26245174617, Fastest:256384, Slowest:895106, Avg:262451.746170
> 
> That's a miniscule improvement in performance (0.2% in the fastest
> case and barely 0.02% in the average case) but the real advantage is
> that the code is cleaner.  The numbers are on top of the fast
> multiplication patch[1], which is why they're so low.
> 
> OK to commit?

I like that you compute these numbers efficiently and just-in-time
with minimal text and data space used.

> [1] http://sourceware.org/ml/libc-alpha/2013-01/msg00614.html
> 
> 
> 	* sysdeps/ieee754/dbl-64/mpa.h (__pow_mp): New function to get an
> 	mp_no from a power of two.
> 	* sysdeps/ieee754/dbl-64/mpexp.c (__mpexp): Remove
> 	__mpexp_twomm1.  Use __pow_mp.
> 
> 
> diff --git a/sysdeps/ieee754/dbl-64/mpa.h b/sysdeps/ieee754/dbl-64/mpa.h
> index debb3b2..70a3614 100644
> --- a/sysdeps/ieee754/dbl-64/mpa.h
> +++ b/sysdeps/ieee754/dbl-64/mpa.h
> @@ -123,3 +123,25 @@ extern void __mpsqrt (mp_no *, mp_no *, int);
>  extern void __mpexp (mp_no *, mp_no *, int);
>  extern void __c32 (mp_no *, mp_no *, mp_no *, int);
>  extern int __mpranred (double, mp_no *, int);
> +
> +/* Given a power POW, build a multiprecision number 2^POW.  */
> +static inline void
> +__pow_mp (int pow, mp_no *y, int p)
> +{
> +  int i, rem;
> +
> +  EY = pow / 24;
> +  rem = pow - EY * 24;
> +  EY++;
> +
> +  if (rem < 0)
> +    {
> +      EY--;
> +      rem += 24;
> +    }
> +  Y[0] = ONE;
> +  Y[1] = 1 << rem;
> +
> +  for (i = 2; i <= p; i++)
> +    Y[i] = ZERO;
> +}

OK to commit if add a few more comments __pow_mp describing what
each step does.

Cheers,
Carlos.