Re: Cache reals for 1/4, 1/6 and 1/9

[Richard Biener <richard dot guenther at gmail dot com>]

On Thu, Oct 1, 2015 at 3:59 PM, Bernd Schmidt <bschmidt@redhat.com> wrote:
> On 10/01/2015 03:51 PM, Richard Sandiford wrote:
>>
>> We have a global 1/2 and a cached 1/3, but recalculate 1/4, 1/6 and 1/9
>> each time we need them.  That seems a bit arbitrary and makes the folding
>> code more noisy (especially once it's moved to match.pd).
>>
>> This patch caches the other three constants too.  Bootstrapped &
>> regression-tested on x86_64-linux-gnu.  OK to install?
>
>
> Looks reasonable enough.

Given

/* Returns the special REAL_VALUE_TYPE corresponding to 1/3.  */

const REAL_VALUE_TYPE *
dconst_third_ptr (void)
{
  static REAL_VALUE_TYPE value;

  /* Initialize mathematical constants for constant folding builtins.
     These constants need to be given to at least 160 bits precision.  */
  if (value.cl == rvc_zero)
    {
      real_arithmetic (&value, RDIV_EXPR, &dconst1, real_digit (3));
    }
  return &value;
}

I wonder if it makes sense to have

template<int a, int b>
const REAL_VALUE_TYPE &
dconst (void)
{
  static REAL_VALUE_TYPE value;
  if (value.cl == rvc_zero)
    real_arithmetic (&value, RDIV_EXPR, real_digit (a), real_digit (b));
  return value;
}

instead which allows us to use

  dconst<1,2>()

in place of dconst_half () and allows arbitrary extra cached constants to be
added (well, double-check that, but I think the function static should be
a .comdat).

Richard.


>
> Bernd