Fix sign of zero part from ctan / ctanh when argument infinite (bug 17118).

C99/C11 Annex G specifies the sign of the zero part of the result of
ctan (x +/- i * Inf) and ctanh (+/-Inf + i * y).  This patch fixes glibc
to follow that specification, along the lines I described in my review
of Andreas's previous patch for this issue
<https://sourceware.org/ml/libc-alpha/2014-08/msg00142.html>.

Tested for x86_64.

2015-09-17  Joseph Myers  <joseph@codesourcery.com>
	    Andreas Schwab  <schwab@suse.de>

	[BZ #17118]
	* math/s_ctan.c (__ctan): Determine sign of zero real part of
	result when imaginary part of argument is infinite using sine and
	cosine.
	* math/s_ctanf.c (__ctanf): Likewise.
	* math/s_ctanl.c (__ctanl): Likewise.
	* math/s_ctanh.c (__ctanh): Determine sign of zero imaginary part
	of result when real part of argument is infinite using sine and
	cosine.
	* math/s_ctanhf.c (__ctanhf): Likewise.
	* math/s_ctanhl.c (__ctanhl): Likewise.
	* math/libm-test.inc (ctan_test_data): Add more tests of ctan.
	(ctanh_test_data): Add more tests of ctanh.

1 files changed, 8 insertions, 1 deletions

diff --git a/math/s_ctan.c b/math/s_ctan.c
index bcf5cfcec0..7ed6abd814 100644
--- a/math/s_ctan.c
+++ b/math/s_ctan.c
@@ -32,7 +32,14 @@ __ctan (__complex__ double x)
     {
       if (__isinf_ns (__imag__ x))
 	{
-	  __real__ res = __copysign (0.0, __real__ x);
+	  if (isfinite (__real__ x) && fabs (__real__ x) > 1.0)
+	    {
+	      double sinrx, cosrx;
+	      __sincos (__real__ x, &sinrx, &cosrx);
+	      __real__ res = __copysign (0.0, sinrx * cosrx);
+	    }
+	  else
+	    __real__ res = __copysign (0.0, __real__ x);
 	  __imag__ res = __copysign (1.0, __imag__ x);
 	}
       else if (__real__ x == 0.0)