Fix inaccuracy of clog, clog10 near |z| = 1 (bug 13629).

1 files changed, 32 insertions, 0 deletions

diff --git a/sysdeps/ieee754/dbl-64/x2y2m1f.c b/sysdeps/ieee754/dbl-64/x2y2m1f.c
new file mode 100644
index 0000000000..fcaa70851c
--- /dev/null
+++ b/sysdeps/ieee754/dbl-64/x2y2m1f.c
@@ -0,0 +1,32 @@
+/* Compute x^2 + y^2 - 1, without large cancellation error.
+   Copyright (C) 2012 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
+   <http://www.gnu.org/licenses/>.  */
+
+#include <math.h>
+#include <math_private.h>
+#include <float.h>
+
+/* Return X^2 + Y^2 - 1, computed without large cancellation error.
+   It is given that 1 > X >= Y >= epsilon / 2, and that either X >=
+   0.75 or Y >= 0.5.  */
+
+float
+__x2y2m1f (float x, float y)
+{
+  double dx = x, dy = y;
+  return (float) ((dx - 1) * (dx + 1) + dy * dy);
+}