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Message-ID: <20210107194901.GF22981@brightrain.aerifal.cx>
Date: Thu, 7 Jan 2021 14:49:03 -0500
From: Rich Felker <dalias@...c.org>
To: Paul Zimmermann <Paul.Zimmermann@...ia.fr>
Cc: musl@...ts.openwall.com
Subject: Re: issue with exp10l
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
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