Fix sign of remquo zero remainder in round-downward mode (bug 17987).

Various remquo implementations produce a zero remainder with the wrong
sign (a zero remainder should always have the sign of the first
argument, as specified in IEEE 754) in round-downward mode, resulting
from the sign of 0 - 0.  This patch checks for zero results and fixes
their sign accordingly.

Tested for x86_64, x86, mips64 and powerpc.

	[BZ #17987]
	* sysdeps/ieee754/dbl-64/s_remquo.c (__remquo): Ensure sign of
	zero result does not depend on the sign resulting from
	subtraction.
	* sysdeps/ieee754/dbl-64/wordsize-64/s_remquo.c (__remquo):
	Likewise.
	* sysdeps/ieee754/flt-32/s_remquof.c (__remquof): Likewise.
	* sysdeps/ieee754/ldbl-128/s_remquol.c (__remquol): Likewise.
	* sysdeps/ieee754/ldbl-128ibm/s_remquol.c (__remquol): Likewise.
	* sysdeps/ieee754/ldbl-96/s_remquol.c (__remquol): Likewise.
	* math/libm-test.inc (remquo_test_data): Add more tests.

1 files changed, 3 insertions, 0 deletions

diff --git a/sysdeps/ieee754/dbl-64/s_remquo.c b/sysdeps/ieee754/dbl-64/s_remquo.c
index e07efa89f8..b081d26b0d 100644
--- a/sysdeps/ieee754/dbl-64/s_remquo.c
+++ b/sysdeps/ieee754/dbl-64/s_remquo.c
@@ -101,6 +101,9 @@ __remquo (double x, double y, int *quo)
 
   *quo = qs ? -cquo : cquo;
 
+  /* Ensure correct sign of zero result in round-downward mode.  */
+  if (x == 0.0)
+    x = 0.0;
   if (sx)
     x = -x;
   return x;