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/* Euclidean distance function. Long Double/Binary128 version.
Copyright (C) 2021 Free Software Foundation, Inc.
This file is part of the GNU C Library.
The GNU C Library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
The GNU C Library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with the GNU C Library; if not, see
<https://www.gnu.org/licenses/>. */
/* This implementation is based on 'An Improved Algorithm for hypot(a,b)' by
Carlos F. Borges [1] using the MyHypot3 with the following changes:
- Handle qNaN and sNaN.
- Tune the 'widely varying operands' to avoid spurious underflow
due the multiplication and fix the return value for upwards
rounding mode.
- Handle required underflow exception for subnormal results.
[1] https://arxiv.org/pdf/1904.09481.pdf */
#include <math.h>
#include <math_private.h>
#include <math-underflow.h>
#include <libm-alias-finite.h>
#define SCALE L(0x1p-8303)
#define LARGE_VAL L(0x1.6a09e667f3bcc908b2fb1366ea95p+8191)
#define TINY_VAL L(0x1p-8191)
#define EPS L(0x1p-114)
/* Hypot kernel. The inputs must be adjusted so that ax >= ay >= 0
and squaring ax, ay and (ax - ay) does not overflow or underflow. */
static inline _Float128
kernel (_Float128 ax, _Float128 ay)
{
_Float128 t1, t2;
_Float128 h = sqrtl (ax * ax + ay * ay);
if (h <= L(2.0) * ay)
{
_Float128 delta = h - ay;
t1 = ax * (L(2.0) * delta - ax);
t2 = (delta - L(2.0) * (ax - ay)) * delta;
}
else
{
_Float128 delta = h - ax;
t1 = L(2.0) * delta * (ax - L(2.0) * ay);
t2 = (L(4.0) * delta - ay) * ay + delta * delta;
}
h -= (t1 + t2) / (L(2.0) * h);
return h;
}
_Float128
__ieee754_hypotl(_Float128 x, _Float128 y)
{
if (!isfinite(x) || !isfinite(y))
{
if ((isinf (x) || isinf (y))
&& !issignaling (x) && !issignaling (y))
return INFINITY;
return x + y;
}
x = fabsl (x);
y = fabsl (y);
_Float128 ax = x < y ? y : x;
_Float128 ay = x < y ? x : y;
/* If ax is huge, scale both inputs down. */
if (__glibc_unlikely (ax > LARGE_VAL))
{
if (__glibc_unlikely (ay <= ax * EPS))
return ax + ay;
return kernel (ax * SCALE, ay * SCALE) / SCALE;
}
/* If ay is tiny, scale both inputs up. */
if (__glibc_unlikely (ay < TINY_VAL))
{
if (__glibc_unlikely (ax >= ay / EPS))
return ax + ay;
ax = kernel (ax / SCALE, ay / SCALE) * SCALE;
math_check_force_underflow_nonneg (ax);
return ax;
}
/* Common case: ax is not huge and ay is not tiny. */
if (__glibc_unlikely (ay <= ax * EPS))
return ax + ay;
return kernel (ax, ay);
}
libm_alias_finite (__ieee754_hypotl, __hypotl)
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