https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107370
--- Comment #15 from jacob navia <jacob at jacob dot remcomp.fr> --- > Le 24 oct. 2022 à 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
