Re: [musl] [PATCH #2] Properly simplified nextafter()

[Damian McGuckin <damianm@esi.com.au>]

Hi Stefan,

On Sun, 15 Aug 2021, Stefan Kanthak wrote:

> __attribute__((noinline))
> double nextafter(double x, double y)
> {
> union {double f; unsigned long long i;} ux={x}, uy={y};
> unsigned long long ax, ay;
> int e;
>
> if (isnan(x) || isnan(y))
>  return x + y;
> if (ux.i == uy.i)
>  return y;
> #ifdef PATCH
> ax = ux.i << 1;
> ay = uy.i << 1;
> #else
> ax = ux.i & -1ULL/2;
> ay = uy.i & -1ULL/2;
> #endif
> if (ax == 0) {
>  if (ay == 0)
>   return y;
>  ux.i = (uy.i & 1ULL<<63) | 1;
> #ifdef PATCH
> } else if (ax < ay == (long long) ux.i < 0)
> #else
> } else if (ax > ay || ((ux.i ^ uy.i) & 1ULL<<63))
> #endif
>  ux.i--;
> else
>  ux.i++;
> e = ux.i >> 52 & 0x7ff;
> /* raise overflow if ux.f is infinite and x is finite */
> if (e == 0x7ff)
>  FORCE_EVAL(x + x);
> /* raise underflow if ux.f is subnormal or zero */
> if (e == 0)
>  FORCE_EVAL(x*x + ux.f*ux.f);
> return ux.f;
> }

Maybe I am missing something and my brain is in weekend-mode ...

I did a quick check and ran the above code for some test cases:

nextafter(-9.7500000000e+01, 3.5000000000e+01) = -9.7500000000e+01
yourpatch(-9.7500000000e+01, 3.5000000000e+01) = -9.7500000000e+01

The error is 2.8421709430e-14

nextafter(-9.7500000000e+01, -3.5000000000e+01) = -9.7500000000e+01
yourpatch(-9.7500000000e+01, -3.5000000000e+01) = -9.7500000000e+01

The error is 2.8421709430e-14

nextafter(-inf, inf) = -1.7976931349e+308 Correct
yourpatch(-inf, inf) = -nan Incorrect

This is against standard GLIB.

Comparing the normal MUSL code against GLIB, there are no errors.

If I change this line:

 	} else if (ax < ay == (long long) ux.i < 0)

to
 	} else if (x < y == (long long) ux.i < 0)

Your code works flawlessly. But it introduces a floating point comparison.

- Damian

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