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Re: About GSOC.


Hello.
I have implemented roundeven function in real.c as follows: (and
respective changes in real.h)

/* Round X to nearest even integer towards zero. */

void
real_roundeven (REAL_VALUE_TYPE *r, format_helper fmt,
        const REAL_VALUE_TYPE *x)
{
  REAL_VALUE_TYPE t;

  do_fix_trunc (&t, x);
  HOST_WIDE_INT i = real_to_integer (&t);
  if(i % 2)
    do_add (r, &t, &dconstm1, 0);
  else
    *r = t;
}

Although I cant get it to test like

int foo()
{
    double x = __builtin_roundeven (3.5);
    printf("%f",x);
    return (int) x;
}
Because I do not know its dependencies through other files. I tried to
track them down by inspecting real_ceil function, but it also includes
other optimization procedures like folding. How do I know enough
declarations to be made in respective files?

Thanks.
-Tejas

On Mon, 28 Jan 2019 at 22:33, Tejas Joshi <tejasjoshi9673@gmail.com> wrote:
>
> Hello.
> Representations of real numbers in real.c are a little complex to
> understand right now for me. I am still trying to understand them and
> figure them out using gdb and cscope. Though conventions are given in
> comments in real.c, I will still be trying to figure it out. The
> equation and its bitwise representation is not pretty elaborated in
> any documentation I could find.
>
> x = s * b^e * \sum_{k=1}^p f_k * b^{-k}
>
>     where
>         s = sign (+- 1)
>         b = base or radix, here always 2
>         e = exponent
>         p = precision (the number of base-b digits in the significand)
>         f_k = the digits of the significand.
>
> In mean time, I've tried real_round function to work like roundeven. I
> will try to submit a clean patch along with roundeven implemented
> separately with changes like in builtins.def, adding cases, etc.
>
> void
> real_round (REAL_VALUE_TYPE *r, format_helper fmt,
>         const REAL_VALUE_TYPE *x)
> {
> #if 0
>   do_add (r, x, &dconsthalf, x->sign);
>   do_fix_trunc (r, r);
>   if (fmt)
>     real_convert (r, fmt, r);
> #endif
>   fprintf (stderr, "\nhere\n");
>   real_value z;
>   do_fix_trunc (&z, x);
>   HOST_WIDE_INT i = real_to_integer (&z);
>   fprintf (stderr, "\n i = %ld\n", i);
>   if (i % 2)
>     do_add (r, &z, &dconstm1, 0);
>   else
>     *r = z;
> }
>
> Thanks.
> -Tejas
>
> On Sat, 26 Jan 2019 at 03:02, Joseph Myers <joseph@codesourcery.com> wrote:
> >
> > On Sat, 26 Jan 2019, Tejas Joshi wrote:
> >
> > > function with byte-byte comparison which also include mpfr. (Correct
> > > me if I am wrong.) What is the significance of mpfr related to these
> > > internal representations?
> >
> > real.c provides a fixed-size representation of floating-point numbers that
> > allows for various non-IEEE formats supported by GCC, and also allows
> > functions from dfp.c to be used for decimal floating-point formats.
> >
> > MPFR is used in GCC to provide operations that are nontrivial to
> > implement, especially those that are nontrivial to implement in such a
> > fixed-size context.  real.c operations wrap around MPFR ones where
> > appropriate, doing whatever's needed in cases where there are non-IEEE
> > semantics or sets of values.
> >
> > --
> > Joseph S. Myers
> > joseph@codesourcery.com


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