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authorUlrich Drepper <drepper@gmail.com>2011-12-21 22:08:12 -0500
committerUlrich Drepper <drepper@gmail.com>2011-12-21 22:08:12 -0500
commite3a851a21bb69a4b18065dd859a724cfd984d8b3 (patch)
tree684234898c2ac6b0d426658503315e9e4855a9d5
parentb27e24b8746b20d9ded426f202b79c3b7aeb953e (diff)
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Use Kahan's formula in cacosh
-rw-r--r--ChangeLog7
-rw-r--r--math/s_cacosh.c5
-rw-r--r--math/s_cacoshf.c6
-rw-r--r--math/s_cacoshl.c6
4 files changed, 24 insertions, 0 deletions
diff --git a/ChangeLog b/ChangeLog
index 1ccc3ddab0..4d2b1fcc98 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,10 @@
+2011-12-21 Ulrich Drepper <drepper@gmail.com>
+
+ * math/s_cacosh.c: Use Kahan's formula if the subtraction could lead
+ to large cancellation.
+ * math/s_cacoshf.c: Likewise.
+ * math/s_cacoshl.c: Likewise.
+
2011-11-18 Richard B. Kreckel <kreckel@ginac.de>
[BZ #13305]
diff --git a/math/s_cacosh.c b/math/s_cacosh.c
index df5ce6945d..cb01ca87c5 100644
--- a/math/s_cacosh.c
+++ b/math/s_cacosh.c
@@ -65,6 +65,11 @@ __cacosh (__complex__ double x)
__real__ res = 0.0;
__imag__ res = __copysign (M_PI_2, __imag__ x);
}
+ /* The factor 16 is just a guess. */
+ else if (16.0 * fabs (__imag__ x) < fabs (__real__ x))
+ /* Kahan's formula which avoid cancellation through subtraction in
+ some cases. */
+ res = 2.0 * __clog (__csqrt ((x + 1.0) / 2.0) + __csqrt ((x - 1.0) / 2.0));
else
{
__complex__ double y;
diff --git a/math/s_cacoshf.c b/math/s_cacoshf.c
index aa4696f10b..fe046929e8 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;
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;