[PATCH] Improve hypot performance

[Wilco Dijkstra via Libc-alpha <libc-alpha@sourceware.org>]

Improve hypot performance significantly by using fma when available. The fma
version has twice the throughput of the previous version and 70% of the latency.
The non-fma version has 30% higher throughput and 10% higher latency.

Max ULP error is 0.949 with fma and 0.792 without fma.

Passes GLIBC testsuite.

---

diff --git a/sysdeps/ieee754/dbl-64/e_hypot.c b/sysdeps/ieee754/dbl-64/e_hypot.c
index 9ec4c1ced096160d7360afe2e002b8da81e0cca2..820da4ca6f87f7718510731318a8060705b61ea3 100644
--- a/sysdeps/ieee754/dbl-64/e_hypot.c
+++ b/sysdeps/ieee754/dbl-64/e_hypot.c
@@ -1,164 +1,147 @@
-/* @(#)e_hypot.c 5.1 93/09/24 */
-/*
- * ====================================================
- * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
- *
- * Developed at SunPro, a Sun Microsystems, Inc. business.
- * Permission to use, copy, modify, and distribute this
- * software is freely granted, provided that this notice
- * is preserved.
- * ====================================================
- */
-
-/* __ieee754_hypot(x,y)
- *
- * Method :
- *	If (assume round-to-nearest) z=x*x+y*y
- *	has error less than sqrt(2)/2 ulp, than
- *	sqrt(z) has error less than 1 ulp (exercise).
- *
- *	So, compute sqrt(x*x+y*y) with some care as
- *	follows to get the error below 1 ulp:
- *
- *	Assume x>y>0;
- *	(if possible, set rounding to round-to-nearest)
- *	1. if x > 2y  use
- *		x1*x1+(y*y+(x2*(x+x1))) for x*x+y*y
- *	where x1 = x with lower 32 bits cleared, x2 = x-x1; else
- *	2. if x <= 2y use
- *		t1*y1+((x-y)*(x-y)+(t1*y2+t2*y))
- *	where t1 = 2x with lower 32 bits cleared, t2 = 2x-t1,
- *	y1= y with lower 32 bits chopped, y2 = y-y1.
- *
- *	NOTE: scaling may be necessary if some argument is too
- *	      large or too tiny
- *
- * Special cases:
- *	hypot(x,y) is INF if x or y is +INF or -INF; else
- *	hypot(x,y) is NAN if x or y is NAN.
- *
- * Accuracy:
- *	hypot(x,y) returns sqrt(x^2+y^2) with error less
- *	than 1 ulps (units in the last place)
- */
+/* Euclidean distance function.  Double/Binary64 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/>.  */
+
+/* Fast ~0.949 ULP hypot implementation using fma and fmin/fmax.
+
+   The error of sqrt (x^2 + y^2) is below 1 ULP if x^2 + y^2 is computed
+   with less than 0.707 ULP error.  If |x| >= |2y|, fma (x, x, y^2) has
+   ~0.625 ULP.  If |x| < |2y|, fma (|2x|, |y|, (x - y)^2) has ~0.625 ULP.
+
+   If fma is not available, the implementation uses a correction 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 <math-narrow-eval.h>
 #include <libm-alias-finite.h>
+#include <libm-alias-double.h>
+#include <math_config.h>
+#include <math-svid-compat.h>
+#include <errno.h>
 
-double
-__ieee754_hypot (double x, double y)
+#define SCALE     0x1p-600
+#define LARGE_VAL 0x1p+511
+#define TINY_VAL  0x1p-459
+#define EPS       0x1p-54
+
+#ifndef FAST_FMINMAX
+# define FAST_FMINMAX 0
+#endif
+
+static inline double
+handle_errno (double r)
 {
-  double a, b, t1, t2, y1, y2, w;
-  int32_t j, k, ha, hb;
-
-  GET_HIGH_WORD (ha, x);
-  ha &= 0x7fffffff;
-  GET_HIGH_WORD (hb, y);
-  hb &= 0x7fffffff;
-  if (hb > ha)
-    {
-      a = y; b = x; j = ha; ha = hb; hb = j;
-    }
+  r = math_narrow_eval (r);
+  if (isinf (r))
+    __set_errno (ERANGE);
+  return r;
+}
+
+/* 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 double
+kernel (double ax, double ay)
+{
+  double t1, t2;
+#ifdef __FP_FAST_FMA
+  t1 = ay + ay;
+  t2 = ax - ay;
+
+  if (t1 >= ax)
+    return sqrt (fma (t1, ax, t2 * t2));
   else
+    return sqrt (fma (ax, ax, ay * ay));
+
+#else
+  double h = sqrt (ax * ax + ay * ay);
+
+  if (h <= 2.0 * ay)
     {
-      a = x; b = y;
-    }
-  SET_HIGH_WORD (a, ha);        /* a <- |a| */
-  SET_HIGH_WORD (b, hb);        /* b <- |b| */
-  if ((ha - hb) > 0x3c00000)
-    {
-      return a + b;
-    }                                       /* x/y > 2**60 */
-  k = 0;
-  if (__glibc_unlikely (ha > 0x5f300000))                  /* a>2**500 */
-    {
-      if (ha >= 0x7ff00000)             /* Inf or NaN */
-	{
-	  uint32_t low;
-	  w = a + b;                    /* for sNaN */
-	  if (issignaling (a) || issignaling (b))
-	    return w;
-	  GET_LOW_WORD (low, a);
-	  if (((ha & 0xfffff) | low) == 0)
-	    w = a;
-	  GET_LOW_WORD (low, b);
-	  if (((hb ^ 0x7ff00000) | low) == 0)
-	    w = b;
-	  return w;
-	}
-      /* scale a and b by 2**-600 */
-      ha -= 0x25800000; hb -= 0x25800000;  k += 600;
-      SET_HIGH_WORD (a, ha);
-      SET_HIGH_WORD (b, hb);
+      double delta = h - ay;
+      t1 = ax * (2.0 * delta - ax);
+      t2 = (delta - 2.0 * (ax - ay)) * delta;
     }
-  if (__builtin_expect (hb < 0x23d00000, 0))            /* b < 2**-450 */
+  else
     {
-      if (hb <= 0x000fffff)             /* subnormal b or 0 */
-	{
-	  uint32_t low;
-	  GET_LOW_WORD (low, b);
-	  if ((hb | low) == 0)
-	    return a;
-	  t1 = 0;
-	  SET_HIGH_WORD (t1, 0x7fd00000);       /* t1=2^1022 */
-	  b *= t1;
-	  a *= t1;
-	  k -= 1022;
-	  GET_HIGH_WORD (ha, a);
-	  GET_HIGH_WORD (hb, b);
-	  if (hb > ha)
-	    {
-	      t1 = a;
-	      a = b;
-	      b = t1;
-	      j = ha;
-	      ha = hb;
-	      hb = j;
-	    }
-	}
-      else                      /* scale a and b by 2^600 */
-	{
-	  ha += 0x25800000;             /* a *= 2^600 */
-	  hb += 0x25800000;             /* b *= 2^600 */
-	  k -= 600;
-	  SET_HIGH_WORD (a, ha);
-	  SET_HIGH_WORD (b, hb);
-	}
+      double delta = h - ax;
+      t1 = 2.0 * delta * (ax - 2.0 * ay);
+      t2 = (4.0 * delta - ay) * ay + delta * delta;
     }
-  /* medium size a and b */
-  w = a - b;
-  if (w > b)
+
+  h -= (t1 + t2) / (2.0 * h);
+  return h;
+#endif
+}
+
+
+double
+__ieee754_hypot (double x, double y)
+{
+  if (!isfinite (x) || !isfinite (y))
     {
-      t1 = 0;
-      SET_HIGH_WORD (t1, ha);
-      t2 = a - t1;
-      w = sqrt (t1 * t1 - (b * (-b) - t2 * (a + t1)));
+      if ((isinf (x) || isinf (y))
+	  && !issignaling_inline (x) && !issignaling_inline (y))
+	return INFINITY;
+      return x + y;
     }
-  else
+
+  x = fabs (x);
+  y = fabs (y);
+
+  /* Ensure ax >= ay >= 0 without branches if possible.  */
+  double ax = FAST_FMINMAX ? fmax (x, y) : (x < y ? y : x);
+  double ay = FAST_FMINMAX ? fmin (x, y) : (x < y ? x : y);
+
+  /* If ax is huge, scale both inputs down.  */
+  if (__glibc_unlikely (ax > LARGE_VAL))
     {
-      a = a + a;
-      y1 = 0;
-      SET_HIGH_WORD (y1, hb);
-      y2 = b - y1;
-      t1 = 0;
-      SET_HIGH_WORD (t1, ha + 0x00100000);
-      t2 = a - t1;
-      w = sqrt (t1 * y1 - (w * (-w) - (t1 * y2 + t2 * b)));
+      if (__glibc_unlikely (ay <= ax * EPS))
+	return handle_errno (ax + ay);
+
+      return handle_errno (kernel (ax * SCALE, ay * SCALE) / SCALE);
     }
-  if (k != 0)
+
+  /* If ay is tiny, scale both inputs up.  */
+  if (__glibc_unlikely (ay < TINY_VAL))
     {
-      uint32_t high;
-      t1 = 1.0;
-      GET_HIGH_WORD (high, t1);
-      SET_HIGH_WORD (t1, high + (k << 20));
-      w *= t1;
-      math_check_force_underflow_nonneg (w);
-      return w;
+      if (__glibc_unlikely (ax >= ay / EPS))
+	return math_narrow_eval (ax + ay);
+
+      ax = math_narrow_eval (kernel (ax / SCALE, ay / SCALE) * SCALE);
+      math_check_force_underflow_nonneg (ax);
+      return ax;
     }
-  else
-    return w;
+
+  /* Common case: ax is not huge and ay is not tiny.  */
+  if (__glibc_unlikely (ay <= ax * EPS))
+    return math_narrow_eval (ax + ay);
+
+  return math_narrow_eval (kernel (ax, ay));
 }
 #ifndef __ieee754_hypot
 libm_alias_finite (__ieee754_hypot, __hypot)