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_cacoshf.c | 6 ++++++
 1 file changed, 6 insertions(+)

(limited to 'math/s_cacoshf.c')

diff --git a/math/s_cacoshf.c b/math/s_cacoshf.c
index aa4696f..fe04692 100644
--- a/math/s_cacoshf.c
+++ b/math/s_cacoshf.c
@@ -65,6 +65,12 @@ __cacoshf (__complex__ float x)
       __real__ res = 0.0;
       __imag__ res = __copysignf (M_PI_2, __imag__ x);
     }
+  /* The factor 16 is just a guess.  */
+  else if (16.0 * fabsf (__imag__ x) < fabsf (__real__ x))
+    /* Kahan's formula which avoid cancellation through subtraction in
+       some cases.  */
+    res = 2.0 * __clogf (__csqrtf ((x + 1.0) / 2.0)
+			 + __csqrtf ((x - 1.0) / 2.0));
   else
     {
       __complex__ float y;
-- 
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