This is the mail archive of the libc-alpha@sourceware.org mailing list for the glibc project.


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]
Other format: [Raw text]

Fix ldbl-128ibm roundl for non-default rounding modes (bug 19594) [committed]


The ldbl-128ibm implementation of roundl is only correct in
round-to-nearest mode (in other modes, there are incorrect results and
overflow exceptions in some cases).  This patch reimplements it along
the lines used for floorl, ceill and truncl, using __round on the high
part, and on the low part if the high part is an integer, and then
adjusting in the cases where this is incorrect.

Tested for powerpc.  Committed.

2016-02-18  Joseph Myers  <joseph@codesourcery.com>

	[BZ #19594]
	* sysdeps/ieee754/ldbl-128ibm/s_roundl.c (__roundl): Use __round
	on high and low parts then adjust result and use
	ldbl_canonicalize_int if needed.

diff --git a/sysdeps/ieee754/ldbl-128ibm/s_roundl.c b/sysdeps/ieee754/ldbl-128ibm/s_roundl.c
index 20813ed..b01510f 100644
--- a/sysdeps/ieee754/ldbl-128ibm/s_roundl.c
+++ b/sysdeps/ieee754/ldbl-128ibm/s_roundl.c
@@ -38,46 +38,44 @@ __roundl (long double x)
 			&& __builtin_isless (__builtin_fabs (xh),
 					     __builtin_inf ()), 1))
     {
-      double orig_xh;
-
-      /* Long double arithmetic, including the canonicalisation below,
-	 only works in round-to-nearest mode.  */
-
-      /* Convert the high double to integer.  */
-      orig_xh = xh;
-      hi = ldbl_nearbyint (xh);
-
-      /* Subtract integral high part from the value.  */
-      xh -= hi;
-      ldbl_canonicalize (&xh, &xl);
-
-      /* Now convert the low double, adjusted for any remainder from the
-	 high double.  */
-      lo = ldbl_nearbyint (xh);
-
-      /* Adjust the result when the remainder is exactly 0.5.  nearbyint
-	 rounds values halfway between integers to the nearest even
-	 integer.  roundl must round away from zero.
-	 Also correct cases where nearbyint returns an incorrect value
-	 for LO.  */
-      xh -= lo;
-      ldbl_canonicalize (&xh, &xl);
-      if (xh == 0.5)
+      hi = __round (xh);
+      if (hi != xh)
 	{
-	  if (xl > 0.0 || (xl == 0.0 && orig_xh > 0.0))
-	    lo += 1.0;
+	  /* The high part is not an integer; the low part only
+	     affects the result if the high part is exactly half way
+	     between two integers and the low part is nonzero with the
+	     opposite sign.  */
+	  if (fabs (hi - xh) == 0.5)
+	    {
+	      if (xh > 0 && xl < 0)
+		xh = hi - 1;
+	      else if (xh < 0 && xl > 0)
+		xh = hi + 1;
+	      else
+		xh = hi;
+	    }
+	  else
+	    xh = hi;
+	  xl = 0;
 	}
-      else if (-xh == 0.5)
+      else
 	{
-	  if (xl < 0.0 || (xl == 0.0 && orig_xh < 0.0))
-	    lo -= 1.0;
+	  /* The high part is a nonzero integer.  */
+	  lo = __round (xl);
+	  if (fabs (lo - xl) == 0.5)
+	    {
+	      if (xh > 0 && xl < 0)
+		xl = lo + 1;
+	      else if (xh < 0 && lo > 0)
+		xl = lo - 1;
+	      else
+		xl = lo;
+	    }
+	  else
+	    xl = lo;
+	  xh = hi;
+	  ldbl_canonicalize_int (&xh, &xl);
 	}
-
-      /* Ensure the final value is canonical.  In certain cases,
-	 rounding causes hi,lo calculated so far to be non-canonical.  */
-      xh = hi;
-      xl = lo;
-      ldbl_canonicalize (&xh, &xl);
     }
 
   return ldbl_pack (xh, xl);

-- 
Joseph S. Myers
joseph@codesourcery.com


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]