[Bug middle-end/107370] long double sqrtl constant folding is wrong

["jacob at jacob dot remcomp.fr" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107370

--- Comment #17 from jacob navia <jacob at jacob dot remcomp.fr> ---
Sorry but the program I used was compiled with gcc. It suffers from lack of
precision too.

In ANY case, in decimal now, the square root of 2 is
1.414 213 562 373 095 048 801 688 724 209 698 078 GP PARI calculator
1.414 213 562 373 095 145 547 462 185 874 gcc: only the first 15 digits are OK
                      ^^^^^ 
In ANOTHER example:
#include <stdio.h>
#include <math.h>

int main(void) { printf("%.30Lg \n",sqrtl(powl(2,225))); }

This produces:
7.34301663720716943338259962711e+33
But bc produces 
~/lcc/book/test % bc
bc 1.06
Copyright 1991-1994, 1997, 1998, 2000 Free Software Foundation, Inc.
This is free software with ABSOLUTELY NO WARRANTY.
For details type `warranty'. 
scale=32
sqrt(2^225)
7343016637207168931428032607349397.48923080385562326502615381523522

To make things clearer I put now the results side by side
7.34301663720716943338259962711e+33 gcc 
734301663720716 8931428032607349397.48923080385562326502615381523522 bc

As you can see, after the 15th digits all others are wrong!!!

Jacob


> Le 24 oct. 2022 à 22:50, gcc-bugzilla@gcc.gnu.org a écrit :
> 
> Attachments with a MIME type of "text/html" are not allowed on this installation. 
> 
> jacob navia wrote:
>> 
>> 
>>> Le 24 oct. 2022 à 22:21, jakub at gcc dot gnu.org <gcc-bugzilla@gcc.gnu.org> a écrit :
>>> 
>>> 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; places.
>> 
>>> 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.
>>> 
>>> -- 
>>> You are receiving this mail because:
>>> You reported the bug.
>> 
>> Sorry but the program I used was compiled with gcc. It suffers from lack of precision too.
>> 
>> In ANY case, in decimal now, the square root of 2 is
>> 1.414 213 562 373 095 048 801 688 724 209 698 078 GP PARI calculator
>> 1.414 213 562 373 095 145 547 462 185 874 gcc: only the first 15 digits are OK
>>                      ^^^^^ 
>> In ANOTHER example:
>> #include <stdio.h>
>> #include <math.h>
>> 
>> int main(void) { printf("%.30Lg \n",sqrtl(powl(2,225))); }
>> 
>> This produces:
>> 7.34301663720716943338259962711e+33
>> But bc produces 
>> ~/lcc/book/test % bc
>> bc 1.06
>> Copyright 1991-1994, 1997, 1998, 2000 Free Software Foundation, Inc.
>> This is free software with ABSOLUTELY NO WARRANTY.
>> For details type `warranty'. 
>> scale=32
>> sqrt(2^225)
>> 7343016637207168931428032607349397.48923080385562326502615381523522
>> 
>> To make things clearer I put now the results side by side
>> 7.34301663720716943338259962711e+33 gcc
>> 734301663720716 8931428032607349397.48923080385562326502615381523522 bc
>> 
>> As you can see, after the 15th digits all others are wrong!!!
>> 
>> Jacob