format: Check for NaN when rendering doubles

- The result of 0.0/0.0 was being rendered as a lot of
  zeroes in the integer portion, as in this example:

  DBGvpp# show physmem
  0: 16 objects, 576k of 582k used, 3k free, 0 reclaimed, 2k overhead,
  16380k capacity
       alloc. from small object cache: 0 hits 0 attempts (0.00%) replacements 0
       alloc. from free-list: 0 attempts, 0 hits (0.00%), 0 considered (per-attempt 0.00)
       alloc. from vector-expand: 16
       allocs: 16 73643.06 clocks/call
       frees: 0 000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00 clocks/call

- Add two macros to vppinfra/math.h that use compiler builtins to check
  for NaN and Infinity and then use them in format_float().

Change-Id: Iccc03997e6e33d6b888d1e7e20cd78df0cfd02e8
Signed-off-by: Chris Luke <chrisy@flirble.org>
(cherry picked from commit bb18ee6f1c7c172d30cb0c98153499af571777ee)

src/vppinfra/format.c

@@ -62,6 +62,7 @@
 #include <vppinfra/error.h>
 #include <vppinfra/string.h>
 #include <vppinfra/os.h>	/* os_puts */
+#include <vppinfra/math.h>
 
 typedef struct
 {
@@ -708,8 +709,12 @@ format_float (u8 * s, f64 x, uword n_fraction_digits, uword output_style)
       sign = 1;
     }
 
+  /* Check for not-a-number. */
+  if (isnan (x))
+    return format (s, "%cNaN", sign ? '-' : '+');
+
   /* Check for infinity. */
-  if (x == x / 2)
+  if (isinf (x))
     return format (s, "%cinfinity", sign ? '-' : '+');
 
   x = normalize (x, &expon, &prec);

src/vppinfra/math.h

@@ -52,6 +52,14 @@ fabs (f64 x)
   return __builtin_fabs (x);
 }
 
+#ifndef isnan
+#define isnan(x) __builtin_isnan(x)
+#endif
+
+#ifndef isinf
+#define isinf(x) __builtin_isinf(x)
+#endif
+
 #endif /* included_math_h */
 
 /*