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.

2 files changed, 22 insertions, 0 deletions

diff --git a/src/lread.c b/src/lread.c
index 50fc6ef8f3a..290b0f6bbe9 100644
--- a/src/lread.c
+++ b/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;
diff --git a/src/print.c b/src/print.c
index da6ec1aaedf..add21609cc5 100644
--- a/src/print.c
+++ b/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)