* [musl] issue with exp10l
@ 2021-01-07 11:17 Paul Zimmermann
2021-01-07 19:49 ` Rich Felker
0 siblings, 1 reply; 4+ messages in thread
From: Paul Zimmermann @ 2021-01-07 11:17 UTC (permalink / raw)
To: musl
Hi,
I am extending my comparison of the accuracy of several mathematical libraries
to the "double extended precision" (long double on x86_64).
First I notice that Musl does not provide j0, j1, y0, and y1 for the long
double format. Do you confirm?
Then I got a segmentation fault using exp10l with NaN input with a non-zero
payload.
$ cat test_exp10.c
#define _GNU_SOURCE
typedef union { __uint128_t n; long double x; } union_t;
#include <stdio.h>
#include <math.h>
int main()
{
union_t u;
u.n = 16383UL;
u.n = u.n << 64;
u.n = u.n | 629329181547216221UL;
/* u.n = 302213637488765131341149 */
long double x = u.x;
printf ("x=%La\n", x);
fflush (stdout);
long double y;
y = exp10l (x);
printf ("y=%La\n", y);
fflush (stdout);
return 0;
}
With glibc this works fine:
$ gcc -fno-builtin test_exp10.c -lm
$ ./a.out
x=nan
y=-nan
With Musl 1.2.1 I get:
$ ./a.out
x=nan
Segmentation fault
According to gdb, the issue is in pow10l:
Program received signal SIGSEGV, Segmentation fault.
0x000055555555d10e in pow10l ()
(gdb) where
#0 0x000055555555d10e in pow10l ()
#1 0x0000000080000000 in ?? ()
#2 0x0000000000003fff in ?? ()
#3 0x0000000000000000 in ?? ()
Best regards,
Paul
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [musl] issue with exp10l
2021-01-07 11:17 [musl] issue with exp10l Paul Zimmermann
@ 2021-01-07 19:49 ` Rich Felker
2021-01-07 20:02 ` Rich Felker
0 siblings, 1 reply; 4+ messages in thread
From: Rich Felker @ 2021-01-07 19:49 UTC (permalink / raw)
To: Paul Zimmermann; +Cc: musl
On Thu, Jan 07, 2021 at 12:17:33PM +0100, Paul Zimmermann wrote:
> Hi,
>
> I am extending my comparison of the accuracy of several mathematical libraries
> to the "double extended precision" (long double on x86_64).
>
> First I notice that Musl does not provide j0, j1, y0, and y1 for the long
> double format. Do you confirm?
I believe that's correct; they're not part of the standard and don't
seem to be an extension we implement at this time.
> Then I got a segmentation fault using exp10l with NaN input with a non-zero
> payload.
>
> $ cat test_exp10.c
> #define _GNU_SOURCE
>
> typedef union { __uint128_t n; long double x; } union_t;
>
> #include <stdio.h>
> #include <math.h>
> int main()
> {
> union_t u;
> u.n = 16383UL;
> u.n = u.n << 64;
> u.n = u.n | 629329181547216221UL;
> /* u.n = 302213637488765131341149 */
> long double x = u.x;
> printf ("x=%La\n", x);
> fflush (stdout);
> long double y;
> y = exp10l (x);
> printf ("y=%La\n", y);
> fflush (stdout);
> return 0;
> }
>
> With glibc this works fine:
>
> $ gcc -fno-builtin test_exp10.c -lm
> $ ./a.out
> x=nan
> y=-nan
>
> With Musl 1.2.1 I get:
>
> $ ./a.out
> x=nan
> Segmentation fault
>
> According to gdb, the issue is in pow10l:
>
> Program received signal SIGSEGV, Segmentation fault.
> 0x000055555555d10e in pow10l ()
> (gdb) where
> #0 0x000055555555d10e in pow10l ()
> #1 0x0000000080000000 in ?? ()
> #2 0x0000000000003fff in ?? ()
> #3 0x0000000000000000 in ?? ()
I can't reproduce this; I get x=nan y=nan. Can you provide a
disassembly and register dump of the point of crash? Did you do
anything weird building musl, or are you using a stock build from a
distro or musl-cross-make?
Rich
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [musl] issue with exp10l
2021-01-07 19:49 ` Rich Felker
@ 2021-01-07 20:02 ` Rich Felker
2021-01-08 9:06 ` Paul Zimmermann
0 siblings, 1 reply; 4+ messages in thread
From: Rich Felker @ 2021-01-07 20:02 UTC (permalink / raw)
To: Paul Zimmermann; +Cc: musl
On Thu, Jan 07, 2021 at 02:49:03PM -0500, Rich Felker wrote:
> On Thu, Jan 07, 2021 at 12:17:33PM +0100, Paul Zimmermann wrote:
> > Hi,
> >
> > I am extending my comparison of the accuracy of several mathematical libraries
> > to the "double extended precision" (long double on x86_64).
> >
> > First I notice that Musl does not provide j0, j1, y0, and y1 for the long
> > double format. Do you confirm?
>
> I believe that's correct; they're not part of the standard and don't
> seem to be an extension we implement at this time.
>
> > Then I got a segmentation fault using exp10l with NaN input with a non-zero
> > payload.
> >
> > $ cat test_exp10.c
> > #define _GNU_SOURCE
> >
> > typedef union { __uint128_t n; long double x; } union_t;
> >
> > #include <stdio.h>
> > #include <math.h>
> > int main()
> > {
> > union_t u;
> > u.n = 16383UL;
> > u.n = u.n << 64;
> > u.n = u.n | 629329181547216221UL;
> > /* u.n = 302213637488765131341149 */
> > long double x = u.x;
> > printf ("x=%La\n", x);
> > fflush (stdout);
> > long double y;
> > y = exp10l (x);
> > printf ("y=%La\n", y);
> > fflush (stdout);
> > return 0;
> > }
> >
> > With glibc this works fine:
> >
> > $ gcc -fno-builtin test_exp10.c -lm
> > $ ./a.out
> > x=nan
> > y=-nan
> >
> > With Musl 1.2.1 I get:
> >
> > $ ./a.out
> > x=nan
> > Segmentation fault
> >
> > According to gdb, the issue is in pow10l:
> >
> > Program received signal SIGSEGV, Segmentation fault.
> > 0x000055555555d10e in pow10l ()
> > (gdb) where
> > #0 0x000055555555d10e in pow10l ()
> > #1 0x0000000080000000 in ?? ()
> > #2 0x0000000000003fff in ?? ()
> > #3 0x0000000000000000 in ?? ()
>
> I can't reproduce this; I get x=nan y=nan. Can you provide a
> disassembly and register dump of the point of crash? Did you do
> anything weird building musl, or are you using a stock build from a
> distro or musl-cross-make?
OK, on further investigation it looks like your problem is that you're
not passing a NAN but a trap representation. The representation is
only a nan if the exponent value is 0x7fff. For exponent not 0x7fff or
0, the high bit of the significand must be set; otherwise it's a trap
representation.
Rich
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [musl] issue with exp10l
2021-01-07 20:02 ` Rich Felker
@ 2021-01-08 9:06 ` Paul Zimmermann
0 siblings, 0 replies; 4+ messages in thread
From: Paul Zimmermann @ 2021-01-08 9:06 UTC (permalink / raw)
To: Rich Felker; +Cc: musl
Dear Rich,
> OK, on further investigation it looks like your problem is that you're
> not passing a NAN but a trap representation. The representation is
> only a nan if the exponent value is 0x7fff. For exponent not 0x7fff or
> 0, the high bit of the significand must be set; otherwise it's a trap
> representation.
thank you. I have now forced the high bit of the significand to be set,
and it works better indeed. I have found errors up to 40 ulps for exp10l:
exp10l 0xd.41cfe8659e3d7eep+8l 40.1
Best regards,
Paul
^ permalink raw reply [flat|nested] 4+ messages in thread
end of thread, other threads:[~2021-01-08 9:07 UTC | newest]
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2021-01-07 11:17 [musl] issue with exp10l Paul Zimmermann
2021-01-07 19:49 ` Rich Felker
2021-01-07 20:02 ` Rich Felker
2021-01-08 9:06 ` Paul Zimmermann
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