MIPS: IEEE 754-2008 NaN encoding support

It has been a long practice for software using IEEE 754 floating-point
arithmetic run on MIPS processors to use an encoding of Not-a-Number
(NaN) data different to one used by software run on other processors.
And as of IEEE 754-2008 revision [1] this encoding does not follow one
recommended in the standard, as specified in section 6.2.1, where it
is stated that quiet NaNs should have the first bit (d1) of their
significand set to 1 while signalling NaNs should have that bit set to
0, but MIPS software interprets the two bits in the opposite manner.

As from revision 3.50 [2][3] the MIPS Architecture provides for
processors that support the IEEE 754-2008 preferred NaN encoding format.
As the two formats (further referred to as "legacy NaN" and "2008 NaN")
are incompatible to each other, tools have to provide support for the
two formats to help people avoid using incompatible binary modules.

The change is comprised of two functional groups of features, both of
which are required for correct support.

1. Dynamic linker support.

   To enforce the NaN encoding requirement in dynamic linking a new ELF
   file header flag has been defined.  This flag is set for 2008-NaN
   shared modules and executables and clear for legacy-NaN ones.  The
   dynamic linker silently ignores any incompatible modules it
   encounters in dependency processing.

   To avoid unnecessary processing of incompatible modules in the
   presence of a shared module cache, a set of new cache flags has been
   defined to mark 2008-NaN modules for the three ABIs supported.
   Changes to sysdeps/unix/sysv/linux/mips/readelflib.c have been made
   following an earlier code quality suggestion made here:

   http://sourceware.org/ml/libc-ports/2009-03/msg00036.html

   and are therefore a little bit more extensive than the minimum
   required.

   Finally a new name has been defined for the dynamic linker so that
   2008-NaN and legacy-NaN binaries can coexist on a single system that
   supports dual-mode operation and that a legacy dynamic linker that
   does not support verifying the 2008-NaN ELF file header flag is not
   chosen to interpret a 2008-NaN binary by accident.

2. Floating environment support.

   IEEE 754-2008 features are controlled in the Floating-Point Control
   and Status (FCSR) register and updates are needed to floating
   environment support so that the 2008-NaN flag is set correctly and
   the kernel default, inferred from the 2008-NaN ELF file header flag
   at the time an executable is loaded, respected.

As the NaN encoding format is a property of GCC code generation that is
both a user-selected GCC configuration default and can be overridden
with GCC options, code that needs to know what NaN encoding standard it
has been configured for checks for the __mips_nan2008 macro that is
defined internally by GCC whenever the 2008-NaN mode has been selected.
This mode is determined at the glibc configuration time and therefore a
few consistency checks have been added to catch cases where compilation
flags have been overridden by the user.

The 2008 NaN set of features relies on kernel support as the in-kernel
floating-point emulator needs to be aware of the NaN encoding used even
on hard-float processors and configure the FPU context according to the
value of the 2008 NaN ELF file header flag of the executable being
started.  As at this time work on kernel support is still in progress
and the relevant changes have not made their way yet to linux.org master
repository.

Therefore the minimum version supported has been artificially set to
10.0.0 so that 2008-NaN code is not accidentally run on a Linux kernel
that does not suppport it.  It is anticipated that the version is
adjusted later on to the actual initial linux.org kernel version to
support this feature.  Legacy NaN encoding support is unaffected, older
kernel versions remain supported.

[1] "IEEE Standard for Floating-Point Arithmetic", IEEE Computer
    Society, IEEE Std 754-2008, 29 August 2008

[2] "MIPS Architecture For Programmers, Volume I-A: Introduction to the
    MIPS32 Architecture", MIPS Technologies, Inc., Document Number:
    MD00082, Revision 3.50, September 20, 2012

[3] "MIPS Architecture For Programmers, Volume I-A: Introduction to the
    MIPS64 Architecture", MIPS Technologies, Inc., Document Number:
    MD00083, Revision 3.50, September 20, 2012

9 files changed, 256 insertions, 21 deletions

diff --git a/ports/sysdeps/mips/bits/nan.h b/ports/sysdeps/mips/bits/nan.h
index 80f7866a97..c322523275 100644
--- a/ports/sysdeps/mips/bits/nan.h
+++ b/ports/sysdeps/mips/bits/nan.h
@@ -22,10 +22,11 @@
 
 
 /* IEEE Not A Number.  */
-/* Note that MIPS has the qNaN and sNaN patterns reversed compared to most
-   other architectures.  IEEE 754-1985 left the definition of this open to
-   implementations, and for MIPS the top bit of the mantissa must be SET to
-   indicate a sNaN.  */
+/* In legacy-NaN mode MIPS has the qNaN and sNaN patterns reversed
+   compared to most other architectures.  IEEE 754-1985 left the
+   definition of this open to implementations, and for MIPS the top bit
+   of the mantissa must be SET to indicate a sNaN.  In 2008-NaN mode
+   MIPS aligned to IEEE 754-2008.  */
 
 #if __GNUC_PREREQ(3,3)
 
@@ -33,6 +34,8 @@
 
 #elif defined __GNUC__
 
+/* No 2008-NaN mode support in any GCC version before 4.9.  */
+
 # define NAN \
   (__extension__							      \
    ((union { unsigned __l __attribute__ ((__mode__ (__SI__))); float __d; })  \
@@ -43,10 +46,18 @@
 # include <endian.h>
 
 # if __BYTE_ORDER == __BIG_ENDIAN
-#  define __qnan_bytes		{ 0x7f, 0xbf, 0xff, 0xff }
+#  ifdef __mips_nan2008
+#   define __qnan_bytes		{ 0x7f, 0xc0, 0, 0 }
+#  else
+#   define __qnan_bytes		{ 0x7f, 0xbf, 0xff, 0xff }
+#  endif
 # endif
 # if __BYTE_ORDER == __LITTLE_ENDIAN
-#  define __qnan_bytes		{ 0xff, 0xff, 0xbf, 0x7f }
+#  ifdef __mips_nan2008
+#   define __qnan_bytes		{ 0, 0, 0xc0, 0x7f }
+#  else
+#   define __qnan_bytes		{ 0xff, 0xff, 0xbf, 0x7f }
+#  endif
 # endif
 
 static union { unsigned char __c[4]; float __d; } __qnan_union
diff --git a/ports/sysdeps/mips/configure b/ports/sysdeps/mips/configure
index 898e4c3f20..de8092c970 100644
--- a/ports/sysdeps/mips/configure
+++ b/ports/sysdeps/mips/configure
@@ -1,3 +1,163 @@
 # This file is generated from configure.in by Autoconf.  DO NOT EDIT!
  # Local configure fragment for sysdeps/mips.
 
+
+
+
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for grep that handles long lines and -e" >&5
+$as_echo_n "checking for grep that handles long lines and -e... " >&6; }
+if ${ac_cv_path_GREP+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  if test -z "$GREP"; then
+  ac_path_GREP_found=false
+  # Loop through the user's path and test for each of PROGNAME-LIST
+  as_save_IFS=$IFS; IFS=$PATH_SEPARATOR
+for as_dir in $PATH$PATH_SEPARATOR/usr/xpg4/bin
+do
+  IFS=$as_save_IFS
+  test -z "$as_dir" && as_dir=.
+    for ac_prog in grep ggrep; do
+    for ac_exec_ext in '' $ac_executable_extensions; do
+      ac_path_GREP="$as_dir/$ac_prog$ac_exec_ext"
+      { test -f "$ac_path_GREP" && $as_test_x "$ac_path_GREP"; } || continue
+# Check for GNU ac_path_GREP and select it if it is found.
+  # Check for GNU $ac_path_GREP
+case `"$ac_path_GREP" --version 2>&1` in
+*GNU*)
+  ac_cv_path_GREP="$ac_path_GREP" ac_path_GREP_found=:;;
+*)
+  ac_count=0
+  $as_echo_n 0123456789 >"conftest.in"
+  while :
+  do
+    cat "conftest.in" "conftest.in" >"conftest.tmp"
+    mv "conftest.tmp" "conftest.in"
+    cp "conftest.in" "conftest.nl"
+    $as_echo 'GREP' >> "conftest.nl"
+    "$ac_path_GREP" -e 'GREP$' -e '-(cannot match)-' < "conftest.nl" >"conftest.out" 2>/dev/null || break
+    diff "conftest.out" "conftest.nl" >/dev/null 2>&1 || break
+    as_fn_arith $ac_count + 1 && ac_count=$as_val
+    if test $ac_count -gt ${ac_path_GREP_max-0}; then
+      # Best one so far, save it but keep looking for a better one
+      ac_cv_path_GREP="$ac_path_GREP"
+      ac_path_GREP_max=$ac_count
+    fi
+    # 10*(2^10) chars as input seems more than enough
+    test $ac_count -gt 10 && break
+  done
+  rm -f conftest.in conftest.tmp conftest.nl conftest.out;;
+esac
+
+      $ac_path_GREP_found && break 3
+    done
+  done
+  done
+IFS=$as_save_IFS
+  if test -z "$ac_cv_path_GREP"; then
+    as_fn_error $? "no acceptable grep could be found in $PATH$PATH_SEPARATOR/usr/xpg4/bin" "$LINENO" 5
+  fi
+else
+  ac_cv_path_GREP=$GREP
+fi
+
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $ac_cv_path_GREP" >&5
+$as_echo "$ac_cv_path_GREP" >&6; }
+ GREP="$ac_cv_path_GREP"
+
+
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for egrep" >&5
+$as_echo_n "checking for egrep... " >&6; }
+if ${ac_cv_path_EGREP+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  if echo a | $GREP -E '(a|b)' >/dev/null 2>&1
+   then ac_cv_path_EGREP="$GREP -E"
+   else
+     if test -z "$EGREP"; then
+  ac_path_EGREP_found=false
+  # Loop through the user's path and test for each of PROGNAME-LIST
+  as_save_IFS=$IFS; IFS=$PATH_SEPARATOR
+for as_dir in $PATH$PATH_SEPARATOR/usr/xpg4/bin
+do
+  IFS=$as_save_IFS
+  test -z "$as_dir" && as_dir=.
+    for ac_prog in egrep; do
+    for ac_exec_ext in '' $ac_executable_extensions; do
+      ac_path_EGREP="$as_dir/$ac_prog$ac_exec_ext"
+      { test -f "$ac_path_EGREP" && $as_test_x "$ac_path_EGREP"; } || continue
+# Check for GNU ac_path_EGREP and select it if it is found.
+  # Check for GNU $ac_path_EGREP
+case `"$ac_path_EGREP" --version 2>&1` in
+*GNU*)
+  ac_cv_path_EGREP="$ac_path_EGREP" ac_path_EGREP_found=:;;
+*)
+  ac_count=0
+  $as_echo_n 0123456789 >"conftest.in"
+  while :
+  do
+    cat "conftest.in" "conftest.in" >"conftest.tmp"
+    mv "conftest.tmp" "conftest.in"
+    cp "conftest.in" "conftest.nl"
+    $as_echo 'EGREP' >> "conftest.nl"
+    "$ac_path_EGREP" 'EGREP$' < "conftest.nl" >"conftest.out" 2>/dev/null || break
+    diff "conftest.out" "conftest.nl" >/dev/null 2>&1 || break
+    as_fn_arith $ac_count + 1 && ac_count=$as_val
+    if test $ac_count -gt ${ac_path_EGREP_max-0}; then
+      # Best one so far, save it but keep looking for a better one
+      ac_cv_path_EGREP="$ac_path_EGREP"
+      ac_path_EGREP_max=$ac_count
+    fi
+    # 10*(2^10) chars as input seems more than enough
+    test $ac_count -gt 10 && break
+  done
+  rm -f conftest.in conftest.tmp conftest.nl conftest.out;;
+esac
+
+      $ac_path_EGREP_found && break 3
+    done
+  done
+  done
+IFS=$as_save_IFS
+  if test -z "$ac_cv_path_EGREP"; then
+    as_fn_error $? "no acceptable egrep could be found in $PATH$PATH_SEPARATOR/usr/xpg4/bin" "$LINENO" 5
+  fi
+else
+  ac_cv_path_EGREP=$EGREP
+fi
+
+   fi
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $ac_cv_path_EGREP" >&5
+$as_echo "$ac_cv_path_EGREP" >&6; }
+ EGREP="$ac_cv_path_EGREP"
+
+
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking whether the compiler is using the 2008 NaN encoding" >&5
+$as_echo_n "checking whether the compiler is using the 2008 NaN encoding... " >&6; }
+if ${libc_cv_mips_nan2008+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+dnl
+#ifdef __mips_nan2008
+yes
+#endif
+_ACEOF
+if (eval "$ac_cpp conftest.$ac_ext") 2>&5 |
+  $EGREP "yes" >/dev/null 2>&1; then :
+  libc_cv_mips_nan2008=yes
+else
+  libc_cv_mips_nan2008=no
+fi
+rm -f conftest*
+
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $libc_cv_mips_nan2008" >&5
+$as_echo "$libc_cv_mips_nan2008" >&6; }
+if test x$libc_cv_mips_nan2008 = xyes; then
+  $as_echo "#define HAVE_MIPS_NAN2008 1" >>confdefs.h
+
+fi
diff --git a/ports/sysdeps/mips/configure.in b/ports/sysdeps/mips/configure.in
index be9672d823..bcbdaffd9f 100644
--- a/ports/sysdeps/mips/configure.in
+++ b/ports/sysdeps/mips/configure.in
@@ -4,3 +4,12 @@ GLIBC_PROVIDES dnl See aclocal.m4 in the top level source directory.
 dnl No MIPS GCC supports accessing static and hidden symbols in an
 dnl position independent way.
 dnl AC_DEFINE(PI_STATIC_AND_HIDDEN)
+
+AC_CACHE_CHECK([whether the compiler is using the 2008 NaN encoding],
+  libc_cv_mips_nan2008, [AC_EGREP_CPP(yes, [dnl
+#ifdef __mips_nan2008
+yes
+#endif], libc_cv_mips_nan2008=yes, libc_cv_mips_nan2008=no)])
+if test x$libc_cv_mips_nan2008 = xyes; then
+  AC_DEFINE(HAVE_MIPS_NAN2008)
+fi
diff --git a/ports/sysdeps/mips/dl-machine.h b/ports/sysdeps/mips/dl-machine.h
index dae938f039..722c8a0ba8 100644
--- a/ports/sysdeps/mips/dl-machine.h
+++ b/ports/sysdeps/mips/dl-machine.h
@@ -73,6 +73,16 @@ do { if ((l)->l_info[DT_MIPS (RLD_MAP)]) \
        (ElfW(Addr)) (r); \
    } while (0)
 
+#if ((defined __mips_nan2008 && !defined HAVE_MIPS_NAN2008) \
+     || (!defined __mips_nan2008 && defined HAVE_MIPS_NAN2008))
+# error "Configuration inconsistency: __mips_nan2008 != HAVE_MIPS_NAN2008, overridden CFLAGS?"
+#endif
+#ifdef __mips_nan2008
+# define ELF_MACHINE_NAN2008 EF_MIPS_NAN2008
+#else
+# define ELF_MACHINE_NAN2008 0
+#endif
+
 /* Return nonzero iff ELF header is compatible with the running host.  */
 static inline int __attribute_used__
 elf_machine_matches_host (const ElfW(Ehdr) *ehdr)
@@ -83,6 +93,10 @@ elf_machine_matches_host (const ElfW(Ehdr) *ehdr)
     return 0;
 #endif
 
+  /* Don't link 2008-NaN and legacy-NaN objects together.  */
+  if ((ehdr->e_flags & EF_MIPS_NAN2008) != ELF_MACHINE_NAN2008)
+    return 0;
+
   switch (ehdr->e_machine)
     {
     case EM_MIPS:
diff --git a/ports/sysdeps/mips/fpu_control.h b/ports/sysdeps/mips/fpu_control.h
index 770cbb31d8..40469627a7 100644
--- a/ports/sysdeps/mips/fpu_control.h
+++ b/ports/sysdeps/mips/fpu_control.h
@@ -29,7 +29,9 @@
  *           available for MIPS III and newer.
  * 23     -> Condition bit
  * 22-21  -> reserved for architecture implementers
- * 20-18  -> reserved (read as 0, write with 0)
+ * 20     -> reserved (read as 0, write with 0)
+ * 19     -> IEEE 754-2008 non-arithmetic ABS.fmt and NEG.fmt enable
+ * 18     -> IEEE 754-2008 recommended NaN encoding enable
  * 17     -> cause bit for unimplemented operation
  * 16     -> cause bit for invalid exception
  * 15     -> cause bit for division by zero exception
@@ -79,22 +81,33 @@ extern fpu_control_t __fpu_control;
 /* flush denormalized numbers to zero */
 #define _FPU_FLUSH_TZ   0x1000000
 
+/* IEEE 754-2008 compliance control.  */
+#define _FPU_ABS2008    0x80000
+#define _FPU_NAN2008    0x40000
+
 /* rounding control */
 #define _FPU_RC_NEAREST 0x0     /* RECOMMENDED */
 #define _FPU_RC_ZERO    0x1
 #define _FPU_RC_UP      0x2
 #define _FPU_RC_DOWN    0x3
 
-#define _FPU_RESERVED 0xfe9c0000  /* Reserved bits in cw */
+#define _FPU_RESERVED 0xfe840000  /* Reserved bits in cw, incl NAN2008.  */
 
 
 /* The fdlibm code requires strict IEEE double precision arithmetic,
    and no interrupts for exceptions, rounding to nearest.  */
+#ifdef __mips_nan2008
+# define _FPU_DEFAULT 0x00040000
+#else
+# define _FPU_DEFAULT 0x00000000
+#endif
 
-#define _FPU_DEFAULT  0x00000000
-
-/* IEEE:  same as above, but exceptions */
-#define _FPU_IEEE     0x00000F80
+/* IEEE: same as above, but exceptions.  */
+#ifdef __mips_nan2008
+# define _FPU_IEEE    0x00040F80
+#else
+# define _FPU_IEEE    0x00000F80
+#endif
 
 /* Type of the control word.  */
 typedef unsigned int fpu_control_t __attribute__ ((__mode__ (__SI__)));
diff --git a/ports/sysdeps/mips/math_private.h b/ports/sysdeps/mips/math_private.h
index f0ba4ee706..6b99957c7b 100644
--- a/ports/sysdeps/mips/math_private.h
+++ b/ports/sysdeps/mips/math_private.h
@@ -18,9 +18,13 @@
 
 #ifndef _MATH_PRIVATE_H
 
+#ifdef __mips_nan2008
+/* MIPS aligned to IEEE 754-2008.  */
+#else
 /* One of the few architectures where the meaning of the quiet/signaling bit is
    inverse to IEEE 754-2008 (as well as common practice for IEEE 754-1985).  */
-#define HIGH_ORDER_BIT_IS_SET_FOR_SNAN
+# define HIGH_ORDER_BIT_IS_SET_FOR_SNAN
+#endif
 
 #include_next <math_private.h>
 
diff --git a/ports/sysdeps/mips/mips64/soft-fp/sfp-machine.h b/ports/sysdeps/mips/mips64/soft-fp/sfp-machine.h
index 9cfd6fbb7b..5be50927d0 100644
--- a/ports/sysdeps/mips/mips64/soft-fp/sfp-machine.h
+++ b/ports/sysdeps/mips/mips64/soft-fp/sfp-machine.h
@@ -24,15 +24,25 @@
 #define _FP_DIV_MEAT_D(R,X,Y)	_FP_DIV_MEAT_1_udiv_norm(D,R,X,Y)
 #define _FP_DIV_MEAT_Q(R,X,Y)	_FP_DIV_MEAT_2_udiv(Q,R,X,Y)
 
-#define _FP_NANFRAC_S		(_FP_QNANBIT_S - 1)
-#define _FP_NANFRAC_D		(_FP_QNANBIT_D - 1)
-#define _FP_NANFRAC_Q		(_FP_QNANBIT_Q - 1), -1
+#ifdef __mips_nan2008
+# define _FP_NANFRAC_S		((_FP_QNANBIT_S << 1) - 1)
+# define _FP_NANFRAC_D		((_FP_QNANBIT_D << 1) - 1)
+# define _FP_NANFRAC_Q		((_FP_QNANBIT_Q << 1) - 1), -1
+#else
+# define _FP_NANFRAC_S		(_FP_QNANBIT_S - 1)
+# define _FP_NANFRAC_D		(_FP_QNANBIT_D - 1)
+# define _FP_NANFRAC_Q		(_FP_QNANBIT_Q - 1), -1
+#endif
 #define _FP_NANSIGN_S		0
 #define _FP_NANSIGN_D		0
 #define _FP_NANSIGN_Q		0
 
 #define _FP_KEEPNANFRACP 1
-#define _FP_QNANNEGATEDP 1
+#ifdef __mips_nan2008
+# define _FP_QNANNEGATEDP 0
+#else
+# define _FP_QNANNEGATEDP 1
+#endif
 
 /* From my experiments it seems X is chosen unless one of the
    NaNs is sNaN,  in which case the result is NANSIGN/NANFRAC.  */
diff --git a/ports/sysdeps/mips/shlib-versions b/ports/sysdeps/mips/shlib-versions
index 780939369b..b153732c27 100644
--- a/ports/sysdeps/mips/shlib-versions
+++ b/ports/sysdeps/mips/shlib-versions
@@ -3,7 +3,11 @@ mips.*-.*-linux.*	libm=6			GLIBC_2.0 GLIBC_2.2
 # Working mips versions were never released between 2.0 and 2.2.
 mips.*-.*-linux.*	libc=6			GLIBC_2.0 GLIBC_2.2
 
+%ifdef HAVE_MIPS_NAN2008
+mips.*-.*-linux.*	ld=ld-linux-mipsn8.so.1	GLIBC_2.0 GLIBC_2.2
+%else
 mips.*-.*-linux.*	ld=ld.so.1		GLIBC_2.0 GLIBC_2.2
+%endif
 mips.*-.*-linux.*	libdl=2			GLIBC_2.0 GLIBC_2.2
 
 mips.*-.*-linux.*	libresolv=2		GLIBC_2.0 GLIBC_2.2
diff --git a/ports/sysdeps/mips/soft-fp/sfp-machine.h b/ports/sysdeps/mips/soft-fp/sfp-machine.h
index a60bef7665..fff3b3c613 100644
--- a/ports/sysdeps/mips/soft-fp/sfp-machine.h
+++ b/ports/sysdeps/mips/soft-fp/sfp-machine.h
@@ -21,15 +21,25 @@
 #define _FP_DIV_MEAT_D(R,X,Y)	_FP_DIV_MEAT_2_udiv(D,R,X,Y)
 #define _FP_DIV_MEAT_Q(R,X,Y)	_FP_DIV_MEAT_4_udiv(Q,R,X,Y)
 
-#define _FP_NANFRAC_S		(_FP_QNANBIT_S - 1)
-#define _FP_NANFRAC_D		(_FP_QNANBIT_D - 1), -1
-#define _FP_NANFRAC_Q		(_FP_QNANBIT_Q - 1), -1, -1, -1
+#ifdef __mips_nan2008
+# define _FP_NANFRAC_S		((_FP_QNANBIT_S << 1) - 1)
+# define _FP_NANFRAC_D		((_FP_QNANBIT_D << 1) - 1), -1
+# define _FP_NANFRAC_Q		((_FP_QNANBIT_Q << 1) - 1), -1, -1, -1
+#else
+# define _FP_NANFRAC_S		(_FP_QNANBIT_S - 1)
+# define _FP_NANFRAC_D		(_FP_QNANBIT_D - 1), -1
+# define _FP_NANFRAC_Q		(_FP_QNANBIT_Q - 1), -1, -1, -1
+#endif
 #define _FP_NANSIGN_S		0
 #define _FP_NANSIGN_D		0
 #define _FP_NANSIGN_Q		0
 
 #define _FP_KEEPNANFRACP 1
-#define _FP_QNANNEGATEDP 1
+#ifdef __mips_nan2008
+# define _FP_QNANNEGATEDP 0
+#else
+# define _FP_QNANNEGATEDP 1
+#endif
 
 /* From my experiments it seems X is chosen unless one of the
    NaNs is sNaN,  in which case the result is NANSIGN/NANFRAC.  */