Port NaN, infinity handling better to VAX

Nowadays .elc files routinely contain tokens like 1.0e+INF and
0.0e+NaN that do not work on antiques like the VAX that lack IEEE fp.
Port Emacs to these platforms, by treating infinities as extreme
values and NaNs as strings that trap if used numerically.
* src/lread.c (INFINITY): Default to HUGE_VAL if non-IEEE.
(not_a_number) [!IEEE_FLOATING_POINT]: New static array.
(syms_of_lread) [!IEEE_FLOATING_POINT]: Initialize it.
(read0): Report invalid syntax for +0.0e+NaN on platforms
that lack NaNs.
(string_to_number): On non-IEEE platforms, return HUGE_VAL
for infinity and a string for NaN.  All callers changed.

3 files changed, 30 insertions, 5 deletions

diff --git a/src/data.c b/src/data.c
index 6de8e0cf1a1..5a31462d8ca 100644
--- a/src/data.c
+++ b/src/data.c
@@ -3033,7 +3033,8 @@ If the base used is not 10, STRING is always parsed as an integer.  */)
     p++;
 
   Lisp_Object val = string_to_number (p, b, 0);
-  return NILP (val) ? make_fixnum (0) : val;
+  return ((IEEE_FLOATING_POINT ? NILP (val) : !NUMBERP (val))
+          ? make_fixnum (0) : val);
 }
 
 enum arithop
diff --git a/src/lread.c b/src/lread.c
index 51d0d2a3c24..6792ef27206 100644
--- a/src/lread.c
+++ b/src/lread.c
@@ -75,6 +75,10 @@ along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.  */
 # ifndef INFINITY
 #  define INFINITY ((union ieee754_double) {.ieee = {.exponent = -1}}.d)
 # endif
+#else
+# ifndef INFINITY
+#  define INFINITY HUGE_VAL
+# endif
 #endif
 
 /* The objects or placeholders read with the #n=object form.
@@ -4477,10 +4481,17 @@ substitute_in_interval (INTERVAL interval, void *arg)
 }
 
 
+#if !IEEE_FLOATING_POINT
+/* Strings that stand in for +NaN, -NaN, respectively.  */
+static Lisp_Object not_a_number[2];
+#endif
+
 /* Convert the initial prefix of STRING to a number, assuming base BASE.
    If the prefix has floating point syntax and BASE is 10, return a
    nearest float; otherwise, if the prefix has integer syntax, return
-   the integer; otherwise, return nil.  If PLEN, set *PLEN to the
+   the integer; otherwise, return nil.  (On antique platforms that lack
+   support for NaNs, if the prefix has NaN syntax return a Lisp object that
+   will provoke an error if used as a number.)  If PLEN, set *PLEN to the
    length of the numeric prefix if there is one, otherwise *PLEN is
    unspecified.  */
 
@@ -4545,7 +4556,6 @@ string_to_number (char const *string, int base, ptrdiff_t *plen)
                 cp++;
               while ('0' <= *cp && *cp <= '9');
             }
-#if IEEE_FLOATING_POINT
           else if (cp[-1] == '+'
                    && cp[0] == 'I' && cp[1] == 'N' && cp[2] == 'F')
             {
@@ -4558,12 +4568,17 @@ string_to_number (char const *string, int base, ptrdiff_t *plen)
             {
               state |= E_EXP;
               cp += 3;
+#if IEEE_FLOATING_POINT
               union ieee754_double u
                 = { .ieee_nan = { .exponent = 0x7ff, .quiet_nan = 1,
                                   .mantissa0 = n >> 31 >> 1, .mantissa1 = n }};
               value = u.d;
-            }
+#else
+              if (plen)
+                *plen = cp - string;
+              return not_a_number[negative];
 #endif
+            }
           else
             cp = ecp;
         }
@@ -5707,6 +5722,14 @@ that are loaded before your customizations are read!  */);
   DEFSYM (Qcomma, ",");
   DEFSYM (Qcomma_at, ",@");
 
+#if !IEEE_FLOATING_POINT
+  for (int negative = 0; negative < 2; negative++)
+    {
+      not_a_number[negative] = build_pure_c_string (&"-0.0e+NaN"[!negative]);
+      staticpro (&not_a_number[negative]);
+    }
+#endif
+
   DEFSYM (Qinhibit_file_name_operation, "inhibit-file-name-operation");
   DEFSYM (Qascii_character, "ascii-character");
   DEFSYM (Qfunction, "function");
diff --git a/src/process.c b/src/process.c
index 67d1d3e425f..2d6e08f16b5 100644
--- a/src/process.c
+++ b/src/process.c
@@ -7130,7 +7130,8 @@ See function `signal-process' for more details on usage.  */)
         {
           ptrdiff_t len;
           tem = string_to_number (SSDATA (process), 10, &len);
-          if (NILP (tem) || len != SBYTES (process))
+          if ((IEEE_FLOATING_POINT ? NILP (tem) : !NUMBERP (tem))
+              || len != SBYTES (process))
             return Qnil;
         }
       process = tem;