Re: [musl] issue with exp10l

[Rich Felker <dalias@libc.org>]

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