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-bugzi...@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 Ã 22:21, jakub at gcc dot gnu.org
>>> <gcc-bugzi...@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
