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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 21:02:33 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107370

--- Comment #20 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
All I'm arguing is that everything I see from gcc seems to be correct.
30 digits precision is not enough for IEEE quad, 36 is enough.
If I try your sqrt (2^255) case on x86_64, I see:
#define _GNU_SOURCE
#include <stdlib.h>
#include <stdio.h>
#include <math.h>

int
main ()
{
  volatile _Float128 x =3D __builtin_sqrtf128(powf128(2, 225));
  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);
}

0x1.6a09e667f3bcc908b2fb1366ea95p+112=20=20
7343016637207168931428032607349397.000000000000000000000000000000000000
0x1.ffffffffffffffffffffffffffffp+224=20=20
53919893334301279589334030174039256154977430310253516431710891278336.000000=
000000000000000000000000000000
so again, x * x is 1ulp from 0x1.0p+225.=