2 files changed, 10 insertions, 22 deletions

diff --git a/src/ChangeLog b/src/ChangeLog
index 3b2c9a6777f..381ae6b0989 100644
--- a/src/ChangeLog
+++ b/src/ChangeLog
@@ -1,5 +1,11 @@
 2015-02-09  Paul Eggert  <eggert@cs.ucla.edu>
 
+        Use C99's INFINITY and NAN macros
+        * lread.c: Include <math.h>.
+        (string_to_number): Use INFINITY and NAN rather than rolling our own.
+        This avoids some runtime diagnostics when building with
+        gcc -fsanitize=undefined.
+
         Fix bidi_explicit_dir_char undefined behavior
         * bidi.c (bidi_explicit_dir_char): Avoid subscript error when
         argument is BIDI_EOB.  This can happen in bidi_level_of_next_char.
diff --git a/src/lread.c b/src/lread.c
index 69ec05964be..b42849fc414 100644
--- a/src/lread.c
+++ b/src/lread.c
@@ -28,6 +28,7 @@ along with GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.  */
 #include <sys/file.h>
 #include <errno.h>
 #include <limits.h>     /* For CHAR_BIT.  */
+#include <math.h>
 #include <stat-time.h>
 #include "lisp.h"
 #include "intervals.h"
@@ -3369,10 +3370,6 @@ string_to_number (char const *string, int base, bool ignore_trailing)
   bool float_syntax = 0;
   double value = 0;
 
-  /* Compute NaN and infinities using a variable, to cope with compilers that
-     think they are smarter than we are.  */
-  double zero = 0;
-
   /* Negate the value ourselves.  This treats 0, NaNs, and infinity properly on
      IEEE floating point hosts, and works around a formerly-common bug where
      atof ("-0.0") drops the sign.  */
@@ -3424,30 +3421,15 @@ string_to_number (char const *string, int base, bool ignore_trailing)
             {
               state |= E_EXP;
               cp += 3;
-              value = 1.0 / zero;
+              value = INFINITY;
             }
           else if (cp[-1] == '+'
                    && cp[0] == 'N' && cp[1] == 'a' && cp[2] == 'N')
             {
               state |= E_EXP;
               cp += 3;
-              value = zero / zero;
-
-              /* If that made a "negative" NaN, negate it.  */
-              {
-                int i;
-                union { double d; char c[sizeof (double)]; }
-                  u_data, u_minus_zero;
-                u_data.d = value;
-                u_minus_zero.d = -0.0;
-                for (i = 0; i < sizeof (double); i++)
-                  if (u_data.c[i] & u_minus_zero.c[i])
-                    {
-                      value = -value;
-                      break;
-                    }
-              }
-              /* Now VALUE is a positive NaN.  */
+              /* NAN is a "positive" NaN on all known Emacs hosts.  */
+              value = NAN;
             }
           else
             cp = ecp;