detect fma error

test2.c

include <stdio.h>                                                                                       
#include <stdlib.h>
#include <math.h>

static double fct (double a)
{
  double x = fma(a,a,-a);
  return 1.0 - x;
}

static void test (double x)
{
  printf ("test(%.20a) = %.20a\n", x, fct (x));
}

int main (int argc, char **argv)
{
  int i;

  if (argc <= 1)
    {
      fprintf (stderr, "Usage: test2 <double> ...\n");
      return 1;
    }

  for (i = 1; i < argc; i++)
    test (atof (argv[i]));

  return 0;
}

Results:

GCC

$ gcc test2.c -o test2 -lm
$ ./test2 0x1.0000000000001p+0
test(0x1.00000000000010000000p+0) = 0x1.ffffffffffffe0000000p-1

(the correctly rounded answer)

clang

$ clang test2.c -o test2 -lm                                      
$ ./test2 0x1.0000000000001p+0
test(0x1.00000000000010000000p+0) = 0x1.ffffffffffffd0000000p-1

(the incorrectly rounded answer).