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From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/107370] long double sqrtl constant folding is wrong
Date: Mon, 24 Oct 2022 20:21:43 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107370

--- 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 =3D __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 =3D 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 =3D 0x1.6a09e667f3bcc908b2fb1366dc6dp+0f128;
  strfromf128 (buf, 128, "%a", x);
  strfromf128 (buf + 128, 128, "%.36f", x);
  printf ("%s\t%s\n", buf, buf + 128);
  x =3D x * x;
  strfromf128 (buf, 128, "%a", x);
  strfromf128 (buf + 128, 128, "%.36f", x);
  printf ("%s\t%s\n", buf, buf + 128);
}

0x1.6a09e667f3bcc908b2fb1366ea95p+0     1.414213562373095048801688724209697=
984
0x1.ffffffffffffffffffffffffffffp+0     1.999999999999999999999999999999999=
807
0x1.6a09e667f3bcc908b2fb1366dc6dp+0     1.414213562373095048801688724209000=
027
0x1.ffffffffffffffffffffffffd7f4p+0     1.999999999999999999999999999998025=
537

Clearly the value GCC uses for sqrtf128(2.0f128) multiplied by itself is 1u=
lp
smaller than 2.0,
while the value you want to see is 10252ulps smaller than 2.0.=