https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107370
--- Comment #12 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
I have to agree with Joseph (on x86_64-linux I get):
#define _GNU_SOURCE
#include <stdlib.h>
#include <stdio.h>
int
main ()
{
volatile _Float128 x = __builtin_sqrtf128(2.0f128);
char buf[256];
strfromf128 (buf, 128, "%a", x);
strfromf128 (buf + 128, 128, "%.36f", x);
printf ("%s\t%s\n", buf, buf + 128);
x = x * x;
strfromf128 (buf, 128, "%a", x);
strfromf128 (buf + 128, 128, "%.36f", x);
printf ("%s\t%s\n", buf, buf + 128);
// The value you want to see
x = 0x1.6a09e667f3bcc908b2fb1366dc6dp+0f128;
strfromf128 (buf, 128, "%a", x);
strfromf128 (buf + 128, 128, "%.36f", x);
printf ("%s\t%s\n", buf, buf + 128);
x = x * x;
strfromf128 (buf, 128, "%a", x);
strfromf128 (buf + 128, 128, "%.36f", x);
printf ("%s\t%s\n", buf, buf + 128);
}
0x1.6a09e667f3bcc908b2fb1366ea95p+0 1.414213562373095048801688724209697984
0x1.ffffffffffffffffffffffffffffp+0 1.999999999999999999999999999999999807
0x1.6a09e667f3bcc908b2fb1366dc6dp+0 1.414213562373095048801688724209000027
0x1.ffffffffffffffffffffffffd7f4p+0 1.999999999999999999999999999998025537
Clearly the value GCC uses for sqrtf128(2.0f128) multiplied by itself is 1ulp
smaller than 2.0,
while the value you want to see is 10252ulps smaller than 2.0.
