(read1): Don't assume that atof ("-0.0") yields -0.0.

Handle leading '-' uniformly for zeros, infinities, and NaNs.

1 files changed, 25 insertions, 12 deletions

diff --git a/src/lread.c b/src/lread.c
index 8ff3bd4206d..04f34e88690 100644
--- a/src/lread.c
+++ b/src/lread.c
@@ -2106,22 +2106,35 @@ read1 (readcharfun, pch, first_in_list)
 #ifdef LISP_FLOAT_TYPE
             if (isfloat_string (read_buffer))
               {
+                /* Compute NaN and infinities using 0.0 in a variable,
+                   to cope with compilers that think they are smarter
+                   than us.  */
                 double zero = 0.0;
-                double value = atof (read_buffer);
-                if (read_buffer[0] == '-' && value == 0.0)
-                  value *= -1.0;
-                /* The only way this can be true, after isfloat_string
+
+                double value;
+
+                /* Negate the value ourselves.  This treats 0, NaNs,
+                   and infinity properly on IEEE floating point hosts,
+                   and works around a common bug where atof ("-0.0")
+                   drops the sign.  */
+                int negative = read_buffer[0] == '-';
+
+                /* The only way p[-1] can be 'F' or 'N', after isfloat_string
                    returns 1, is if the input ends in e+INF or e+NaN.  */
-                if (p[-1] == 'F' || p[-1] == 'N')
+                switch (p[-1])
                   {
-                    if (p[-1] == 'N')
-                      value = zero / zero;
-                    else if (read_buffer[0] == '-')
-                      value = - 1.0 / zero;
-                    else
-                      value = 1.0 / zero;
+                  case 'F':
+                    value = 1.0 / zero;
+                    break;
+                  case 'N':
+                    value = zero / zero;
+                    break;
+                  default:
+                    value = atof (read_buffer + negative);
+                    break;
                   }
-                return make_float (value);
+
+                return make_float (negative ? - value : value);
               }
 #endif
           }