* gcc 11.1.0: printf("%.43f\n", 0x1.52f8a8e32e982p-140): printed value is incorrectly rounded
@ 2021-06-12 20:45 Pavel M
2021-06-13 9:42 ` Achim Gratz
` (3 more replies)
0 siblings, 4 replies; 6+ messages in thread
From: Pavel M @ 2021-06-12 20:45 UTC (permalink / raw)
To: cygwin
Sample code (t903.c):
#include <stdio.h>
int main(void)
{
printf("%.43f\n", 0x1.52f8a8e32e982p-140);
return 0;
}
Invocations:
# gcc on Windows 10 (Cygwin)
$ gcc t903.c -Wall -Wextra -std=c11 -pedantic -Wfatal-errors && ./a.exe
0.0000000000000000000000000000000000000000010
$ gcc --version
gcc (GCC) 11.1.0
# gcc on Linux
$ gcc t903.c -Wall -Wextra -std=c11 -pedantic -Wfatal-errors && ./a.exe
0.0000000000000000000000000000000000000000009
# clang on Windows
$ clang t903.c -Wall -Wextra -std=c11 -ffp-model=strict -pedantic
-Wfatal-errors && ./a.exe
0.0000000000000000000000000000000000000000009
# cl on Windows
$ cl t903.c /std:c11 /Za /fp:strict && ./t903.exe
0.0000000000000000000000000000000000000000009
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: gcc 11.1.0: printf("%.43f\n", 0x1.52f8a8e32e982p-140): printed value is incorrectly rounded
2021-06-12 20:45 gcc 11.1.0: printf("%.43f\n", 0x1.52f8a8e32e982p-140): printed value is incorrectly rounded Pavel M
@ 2021-06-13 9:42 ` Achim Gratz
2021-06-13 19:19 ` Brian Inglis
` (2 subsequent siblings)
3 siblings, 0 replies; 6+ messages in thread
From: Achim Gratz @ 2021-06-13 9:42 UTC (permalink / raw)
To: cygwin
Pavel M via Cygwin writes:
> Sample code (t903.c):
> #include <stdio.h>
> int main(void)
> {
> printf("%.43f\n", 0x1.52f8a8e32e982p-140);
> return 0;
> }
>
> Invocations:
> # gcc on Windows 10 (Cygwin)
> $ gcc t903.c -Wall -Wextra -std=c11 -pedantic -Wfatal-errors && ./a.exe
> 0.0000000000000000000000000000000000000000010
You will want to report this against newlib, I'd think. If I remember
correctly, newlib uses a single guard digit for the conversion and for
such an algorithm the result you see is expected and correct. Whether
any later C standard would mandate to use an algorithm that uses higher
precision (in terms of guard digits you'd end up with 15 for your
example before you get the expected result I think) I have no idea.
Regards,
Achim.
--
+<[Q+ Matrix-12 WAVE#46+305 Neuron microQkb Andromeda XTk Blofeld]>+
SD adaptation for Waldorf rackAttack V1.04R1:
http://Synth.Stromeko.net/Downloads.html#WaldorfSDada
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: gcc 11.1.0: printf("%.43f\n", 0x1.52f8a8e32e982p-140): printed value is incorrectly rounded
2021-06-12 20:45 gcc 11.1.0: printf("%.43f\n", 0x1.52f8a8e32e982p-140): printed value is incorrectly rounded Pavel M
2021-06-13 9:42 ` Achim Gratz
@ 2021-06-13 19:19 ` Brian Inglis
2021-11-03 21:07 ` Pavel M
[not found] ` <CAL9Mx1utCxXFPy3vOK0ZOkX2qzt8ntQ0LEyD4WvLpCRRbTLYeQ@mail.gmail.com>
3 siblings, 0 replies; 6+ messages in thread
From: Brian Inglis @ 2021-06-13 19:19 UTC (permalink / raw)
To: cygwin
[-- Attachment #1: Type: text/plain, Size: 1851 bytes --]
On 2021-06-12 14:45, Pavel M via Cygwin wrote:
> Sample code (t903.c):
> #include <stdio.h>
> int main(void)
> {
> printf("%.43f\n", 0x1.52f8a8e32e982p-140);
> return 0;
> }
>
> Invocations:
> # gcc on Windows 10 (Cygwin)
> $ gcc t903.c -Wall -Wextra -std=c11 -pedantic -Wfatal-errors && ./a.exe
> 0.0000000000000000000000000000000000000000010
>
> $ gcc --version
> gcc (GCC) 11.1.0
>
> # gcc on Linux
> $ gcc t903.c -Wall -Wextra -std=c11 -pedantic -Wfatal-errors && ./a.exe
> 0.0000000000000000000000000000000000000000009
>
> # clang on Windows
> $ clang t903.c -Wall -Wextra -std=c11 -ffp-model=strict -pedantic
> -Wfatal-errors && ./a.exe
> 0.0000000000000000000000000000000000000000009
>
> # cl on Windows
> $ cl t903.c /std:c11 /Za /fp:strict && ./t903.exe
> 0.0000000000000000000000000000000000000000009
See newlib mailing list incorrectly rounded square root thread starting:
https://sourceware.org/pipermail/newlib/2021/018369.html
As suggested lately, I tried adding -frounding-math
$ info gcc frounding-math
to see if it makes any difference to your test on gcc 10 and got the
same result.
It may need to be applied to the library functions to have any useful
impact.
I also tried Cygwin clang and got the same result.
It may be an accuracy issue or a bug in the underlying newlib printf
conversion.
I tried fcvtbuf in the attached test program and found it does the
rounding as expected on Cygwin:
$ gcc -o test-printf-round{,.c} && ./test-printf-round
sgn 0 decpt -42 +0.0000000000000000000000000000000000000000009
0.0000000000000000000000000000000000000000010
--
Take care. Thanks, Brian Inglis, Calgary, Alberta, Canada
This email may be disturbing to some readers as it contains
too much technical detail. Reader discretion is advised.
[Data in binary units and prefixes, physical quantities in SI.]
[-- Attachment #2: test-printf-round.c --]
[-- Type: text/plain, Size: 971 bytes --]
/* test-printf-round.c */
#define _POSIX_SOURCE
#define _XOPEN_SOURCE 600
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#if 0
#define V 0X1.52f8a8e32e982P+140
#define V 0X0.0000000000000P+0
#endif
#define V 0X1.52f8a8e32e982P-140
#define DECIMALS 43
int
main(void)
{
int decpt = 0;
int sgn = -1;
char *b;
static char zeros[128] =
"0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000";
static char buf[128] = { 0 };
b = fcvtbuf(V, DECIMALS, &decpt, &sgn, buf);
printf("sgn %d decpt %d %s%.*s%s%s %.*f\n",
sgn, decpt,
!sgn ? "+" : 1 == sgn ? "-" : "",
/* 0 < decpt -> decpt digits | . | buf + decpt digits */
/* decpt < 0 -> 0. | decpt zeros | buf digits */
decpt < 0 ? 2 : decpt,
decpt < 0 ? "0." : b,
decpt < 0 ? zeros + strlen(zeros) + decpt : ".",
decpt < 0 ? b : b + decpt,
DECIMALS, V);
}
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: gcc 11.1.0: printf("%.43f\n", 0x1.52f8a8e32e982p-140): printed value is incorrectly rounded
2021-06-12 20:45 gcc 11.1.0: printf("%.43f\n", 0x1.52f8a8e32e982p-140): printed value is incorrectly rounded Pavel M
2021-06-13 9:42 ` Achim Gratz
2021-06-13 19:19 ` Brian Inglis
@ 2021-11-03 21:07 ` Pavel M
2021-11-03 21:09 ` Pavel M
[not found] ` <CAL9Mx1utCxXFPy3vOK0ZOkX2qzt8ntQ0LEyD4WvLpCRRbTLYeQ@mail.gmail.com>
3 siblings, 1 reply; 6+ messages in thread
From: Pavel M @ 2021-11-03 21:07 UTC (permalink / raw)
To: cygwin
Hello. Is there any update on this? Thanks.
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: gcc 11.1.0: printf("%.43f\n", 0x1.52f8a8e32e982p-140): printed value is incorrectly rounded
2021-11-03 21:07 ` Pavel M
@ 2021-11-03 21:09 ` Pavel M
0 siblings, 0 replies; 6+ messages in thread
From: Pavel M @ 2021-11-03 21:09 UTC (permalink / raw)
To: cygwin
Ah, sorry! Now I see:
https://www.mail-archive.com/cygwin@cygwin.com/msg168488.html.
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: gcc 11.1.0: printf("%.43f\n", 0x1.52f8a8e32e982p-140): printed value is incorrectly rounded
[not found] ` <CAL9Mx1utCxXFPy3vOK0ZOkX2qzt8ntQ0LEyD4WvLpCRRbTLYeQ@mail.gmail.com>
@ 2021-11-03 21:32 ` Pavel M
0 siblings, 0 replies; 6+ messages in thread
From: Pavel M @ 2021-11-03 21:32 UTC (permalink / raw)
To: cygwin
To Brian Inglis: thanks! Forwarded to newlib mailing list.
^ permalink raw reply [flat|nested] 6+ messages in thread
end of thread, other threads:[~2021-11-03 21:33 UTC | newest]
Thread overview: 6+ messages (download: mbox.gz / follow: Atom feed)
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2021-06-12 20:45 gcc 11.1.0: printf("%.43f\n", 0x1.52f8a8e32e982p-140): printed value is incorrectly rounded Pavel M
2021-06-13 9:42 ` Achim Gratz
2021-06-13 19:19 ` Brian Inglis
2021-11-03 21:07 ` Pavel M
2021-11-03 21:09 ` Pavel M
[not found] ` <CAL9Mx1utCxXFPy3vOK0ZOkX2qzt8ntQ0LEyD4WvLpCRRbTLYeQ@mail.gmail.com>
2021-11-03 21:32 ` Pavel M
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