Fix sign of inexact zero results for ldbl-128ibm fmal

[Joseph S. Myers]

This patch fixes signs of inexact zero results from fmal (x, y, 0) for
ldbl-128ibm in the same way as fixed for other fma implementations.
(The ldbl-128ibm implementation still needs replacing by a proper
implementation that actually does a fused operation, this patch is simply
about eliminating some of the testsuite failures.)

Tested that this eliminates the fmal testsuite failures for zero
results for ldbl-128ibm.

2012-11-22  Joseph Myers  <joseph@codesourcery.com>

	[BZ #14645]
	* sysdeps/ieee754/ldbl-128ibm/s_fmal.c (__fmal): Compute result as
	x * y if x and y are nonzero and z is zero.

diff --git a/sysdeps/ieee754/ldbl-128ibm/s_fmal.c b/sysdeps/ieee754/ldbl-128ibm/s_fmal.c
index a868b8d..6706eb2 100644
--- a/sysdeps/ieee754/ldbl-128ibm/s_fmal.c
+++ b/sysdeps/ieee754/ldbl-128ibm/s_fmal.c
@@ -1,5 +1,5 @@
 /* Compute x * y + z as ternary operation.
-   Copyright (C) 2011 Free Software Foundation, Inc.
+   Copyright (C) 2011-2012 Free Software Foundation, Inc.
    This file is part of the GNU C Library.
    Contributed by David Flaherty <flaherty@linux.vnet.ibm.com>.
 
@@ -29,6 +29,12 @@ __fmal (long double x, long double y, long double z)
        if ((finite ((double)x) && finite ((double)y)) && isinf ((double)z))
                return (z);
 
+       /* If z is zero and x are y are nonzero, compute the result
+	  as x * y to avoid the wrong sign of a zero result if x * y
+	  underflows to 0.  */
+       if (z == 0 && x != 0 && y != 0)
+	 return x * y;
+
        return (x * y) + z;
 }
 #ifdef IS_IN_libm

-- 
Joseph S. Myers
joseph@codesourcery.com