Read and print NaN significand if <ieee754.h>

* configure.ac: Check for ieee754.h.
* doc/lispref/numbers.texi (Float Basics): Document
that NaN string representation digits are machine-dependent.
* etc/NEWS: Mention the change.
* src/lread.c, src/print.c [HAVE_IEEE754_H]: Include ieee754.h.
* src/lread.c (string_to_number) [HAVE_IEEE754_H]:
* src/print.c (float_to_string) [HAVE_IEEE754_H]:
Read and print NaN significand.

configure.ac

@@ -1668,6 +1668,7 @@ fi
 
 dnl checks for header files
 AC_CHECK_HEADERS_ONCE(
+  ieee754.h
   linux/fs.h
   malloc.h
   sys/systeminfo.h

doc/lispref/numbers.texi

@@ -241,7 +241,7 @@ A NaN is never numerically equal to any value, not even to itself.
 NaNs carry a sign and a significand, and non-numeric functions treat
 two NaNs as equal when their
 signs and significands agree.  Significands of NaNs are
-machine-dependent and are not directly visible to Emacs Lisp.
+machine-dependent, as are the digits in their string representation.
 
   When NaNs and signed zeros are involved, non-numeric functions like
 @code{eql}, @code{equal}, @code{sxhash-eql}, @code{sxhash-equal} and

etc/NEWS

@@ -880,6 +880,9 @@ Formerly, some of these functions ignored signs and significands of
 NaNs.  Now, all these functions treat NaN signs and significands as
 significant.  For example, (eql 0.0e+NaN -0.0e+NaN) now returns nil
 because the two NaNs have different signs; formerly it returned t.
+Also, on platforms that have <ieee754.h> Emacs now reads and prints
+NaN significands; e.g., if X is a NaN, (format "%s" X) now returns
+"0.0e+NaN", "1.0e+NaN", etc., depending on X's significand.
 
 +++
 ** The function 'make-string' accepts an additional optional argument.

src/lread.c

@@ -72,6 +72,10 @@ along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.  */
 #define file_tell ftell
 #endif
 
+#if HAVE_IEEE754_H
+# include <ieee754.h>
+#endif
+
 /* The objects or placeholders read with the #n=object form.
 
    A hash table maps a number to either a placeholder (while the
@@ -3757,8 +3761,15 @@ string_to_number (char const *string, int base, int flags)
 	    {
 	      state |= E_EXP;
 	      cp += 3;
+#if HAVE_IEEE754_H
+	      union ieee754_double u
+		= { .ieee_nan = { .exponent = -1, .quiet_nan = 1,
+				  .mantissa0 = n >> 31 >> 1, .mantissa1 = n }};
+	      value = u.d;
+#else
 	      /* NAN is a "positive" NaN on all known Emacs hosts.  */
 	      value = NAN;
+#endif
 	    }
 	  else
 	    cp = ecp;

src/print.c

@@ -40,6 +40,10 @@ along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.  */
 #include <ftoastr.h>
 #include <math.h>
 
+#if HAVE_IEEE754_H
+# include <ieee754.h>
+#endif
+
 #ifdef WINDOWSNT
 # include <sys/socket.h> /* for F_DUPFD_CLOEXEC */
 #endif
@@ -1011,6 +1015,12 @@ float_to_string (char *buf, double data)
     }
   if (isnan (data))
     {
+#if HAVE_IEEE754_H
+      union ieee754_double u = { .d = data };
+      uprintmax_t hi = u.ieee_nan.mantissa0;
+      return sprintf (buf, &"-%"pMu".0e+NaN"[!u.ieee_nan.negative],
+		      (hi << 31 << 1) + u.ieee_nan.mantissa1);
+#else
       /* Prepend "-" if the NaN's sign bit is negative.
 	 The sign bit of a double is the bit that is 1 in -0.0.  */
       static char const NaN_string[] = "0.0e+NaN";
@@ -1029,6 +1039,7 @@ float_to_string (char *buf, double data)
 
       strcpy (buf + negative, NaN_string);
       return negative + sizeof NaN_string - 1;
+#endif
     }
 
   if (NILP (Vfloat_output_format)