From: Paul Zimmermann <Paul.Zimmermann@inria.fr>
To: newlib@sourceware.org
Subject: incorrectly rounded square root
Date: Tue, 04 May 2021 10:08:54 +0200 [thread overview]
Message-ID: <mwfsz29azt.fsf@tomate.loria.fr> (raw)
Hi,
according to IEEE 754, the square root function should be correctly rounded
for all rounding modes. I noticed this is not the case in Newlib:
$ cat test_sqrt.c
#include <stdio.h>
#include <math.h>
#include <fenv.h>
#ifdef NEWLIB
int errno;
int* __errno () { return &errno; }
#endif
int main()
{
int rnd[4] = { FE_TONEAREST, FE_TOWARDZERO, FE_UPWARD, FE_DOWNWARD };
char Rnd[4] = "NZUD";
float x = 0x1.ff07fep+127f;
float y;
for (int i = 0; i < 4; i++)
{
fesetround (rnd[i]);
y = sqrtf (x);
printf ("RND%c: %a\n", Rnd[i], y);
}
}
$ gcc -DNEWLIB -fno-builtin test_sqrt.c /localdisk/zimmerma/newlib-4.1.0/libm.a -lm
$ ./a.out
RNDN: 0x1.ff83fp+63
RNDZ: 0x1.ff83fp+63
RNDU: 0x1.ff83fp+63
RNDD: 0x1.ff83fp+63
The RNDZ and RNDD results are wrong. With glibc I get:
$ gcc -fno-builtin test_sqrt.c -lm
$ ./a.out
RNDN: 0x1.ff83fp+63
RNDZ: 0x1.ff83eep+63
RNDU: 0x1.ff83fp+63
RNDD: 0x1.ff83eep+63
Best regards,
Paul Zimmermann
next reply other threads:[~2021-05-04 8:08 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-05-04 8:08 Paul Zimmermann [this message]
2021-05-31 20:52 ` Jeff Johnston
2021-06-01 7:11 ` Paul Zimmermann
2021-06-01 16:28 ` Jeff Johnston
2021-06-02 7:51 ` Paul Zimmermann
2021-06-02 13:07 ` Joel Sherrill
2021-06-02 18:43 ` Jeff Johnston
2021-06-02 19:07 ` Marco Atzeri
2021-06-02 19:12 ` Joel Sherrill
2021-06-03 3:01 ` Jeff Johnston
2021-06-03 10:21 ` Paul Zimmermann
2021-06-03 15:50 ` Jeff Johnston
2021-06-04 7:14 ` Paul Zimmermann
2021-06-04 18:44 ` Jeff Johnston
2021-06-04 18:59 ` Joel Sherrill
2021-06-05 13:25 ` Brian Inglis
2021-06-07 9:51 ` Paul Zimmermann
2021-06-12 22:31 ` Maciej W. Rozycki
2021-06-03 11:25 ` Paul Zimmermann
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