From @ntyni

#define DOUBLESIZE 8
#define HAS_LONG_DOUBLE
#ifdef HAS_LONG_DOUBLE
#define LONG_DOUBLESIZE 16
#define LONG_DOUBLEKIND 3
#endif
#define I_MATH
#ifdef I_MATH
#include <math.h>
#endif
#include <stdio.h>
#include <string.h>
/* Note that whether the sign bit is on or off
 * for NaN depends on the CPU/FPU, and possibly
 * can be affected by the build toolchain.
 *
 * For example for older MIPS and HP-PA 2.0 the quiet NaN is:
 * 0x7f, 0xf7, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff
 * 0x7f, 0xf4, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00
 * (respectively) as opposed to the more usual
 * 0x7f, 0xf8, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00
 */
static void bytes(unsigned char *p, unsigned int n) {
  int i;
  for (i = 0; i < n; i++) {
    printf("0x%02x%s", p[i], i < n - 1 ? ", " : "\n");
  }
}
int main(int argc, char *argv[]) {
   /* We cannot use 1.0/0.0 and 0.0/0.0 (with L suffixes for long double)
    * because some compilers are 'smart' and not only warn but refuse to
    * compile such 'illegal' values. */
   double dinf = exp(1e9);
   double dnan = sqrt(-1.0);
#ifdef HAS_LONG_DOUBLE
   long double ldinf = (long double)exp(1e9);
   long double ldnan = (long double)sqrt(-1.0);
#endif
  if (argc == 2) {
    switch (argv[1][0]) {
    case '1': bytes(&dinf, sizeof(dinf)); break;
    case '2': bytes(&dnan, sizeof(dnan)); break;
#ifdef HAS_LONG_DOUBLE
# if LONG_DOUBLEKIND == 3 || LONG_DOUBLEKIND == 4
/* the 80-bit long doubles might have garbage in their excess bytes */
    memset((char *)&ldinf + 10, '\0', LONG_DOUBLESIZE - 10);
# endif
    case '3': bytes(&ldinf, sizeof(ldinf)); break;
    case '4': bytes(&ldnan, sizeof(ldnan)); break;
#endif
    }
  }
  return 0;
}

[PATCH] Configure leaves garbage in $Config{longdblinfbytes} #15725

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