Fix generic fmax, fmin sNaN handling (bug 20947).

Various fmax and fmin function implementations mishandle sNaN
arguments:

(a) When both arguments are NaNs, the return value should be a qNaN,
but sometimes it is an sNaN if at least one argument is an sNaN.

(b) Under TS 18661-1 semantics, if either argument is an sNaN then the
result should be a qNaN (whereas if one argument is a qNaN and the
other is not a NaN, the result should be the non-NaN argument).
Various implementations treat sNaNs like qNaNs here.

This patch fixes the generic implementations used in the absence of
architecture-specific versions.

Tested for mips64 and powerpc (together with testcases that I'll add
along with the x86_64 / x86 fixes).

	[BZ #20947]
	* math/s_fmax_template.c (M_DECL_FUNC (__fmax)): Add the arguments
	when either is a signaling NaN.
	* math/s_fmin_template.c (M_DECL_FUNC (__fmin)): Likewise.

ChangeLog

@@ -1,5 +1,10 @@
 2016-12-14  Joseph Myers  <joseph@codesourcery.com>
 
+	[BZ #20947]
+	* math/s_fmax_template.c (M_DECL_FUNC (__fmax)): Add the arguments
+	when either is a signaling NaN.
+	* math/s_fmin_template.c (M_DECL_FUNC (__fmin)): Likewise.
+
 	* bits/long-double.h: New file.
 	* sysdeps/ieee754/ldbl-128/bits/long-double.h: Likewise.
 	* sysdeps/ieee754/ldbl-96/bits/long-double.h: Likewise.

math/s_fmax_template.c

@@ -22,7 +22,14 @@
 FLOAT
 M_DECL_FUNC (__fmax) (FLOAT x, FLOAT y)
 {
-  return (isgreaterequal (x, y) || isnan (y)) ? x : y;
+  if (isgreaterequal (x, y))
+    return x;
+  else if (isless (x, y))
+    return y;
+  else if (issignaling (x) || issignaling (y))
+    return x + y;
+  else
+    return isnan (y) ? x : y;
 }
 
 declare_mgen_alias (__fmax, fmax);

math/s_fmin_template.c

@@ -23,7 +23,14 @@
 FLOAT
 M_DECL_FUNC (__fmin) (FLOAT x, FLOAT y)
 {
-  return (islessequal (x, y) || isnan (y)) ? x : y;
+  if (islessequal (x, y))
+    return x;
+  else if (isgreater (x, y))
+    return y;
+  else if (issignaling (x) || issignaling (y))
+    return x + y;
+  else
+    return isnan (y) ? x : y;
 }
 declare_mgen_alias (__fmin, fmin);