From e3a851a21bb69a4b18065dd859a724cfd984d8b3 Mon Sep 17 00:00:00 2001
From: Ulrich Drepper <drepper@gmail.com>
Date: Wed, 21 Dec 2011 22:08:12 -0500
Subject: Use Kahan's formula in cacosh

---
 math/s_cacoshl.c | 6 ++++++
 1 file changed, 6 insertions(+)

(limited to 'math/s_cacoshl.c')

diff --git a/math/s_cacoshl.c b/math/s_cacoshl.c
index b90b196959..84c2715d22 100644
--- a/math/s_cacoshl.c
+++ b/math/s_cacoshl.c
@@ -65,6 +65,12 @@ __cacoshl (__complex__ long double x)
       __real__ res = 0.0;
       __imag__ res = __copysignl (M_PI_2l, __imag__ x);
     }
+  /* The factor 16 is just a guess.  */
+  else if (16.0L * fabsl (__imag__ x) < fabsl (__real__ x))
+    /* Kahan's formula which avoid cancellation through subtraction in
+       some cases.  */
+    res = 2.0L * __clogl (__csqrtl ((x + 1.0L) / 2.0L)
+			  + __csqrtl ((x - 1.0L) / 2.0L));
   else
     {
       __complex__ long double y;
-- 
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