[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <CABqyc3yHSTm7jBy0SLBxrwtmxH=Q+4t_V051xM6yneev5ptdLw@mail.gmail.com>
Date: Tue, 11 Mar 2025 17:19:26 +0100
From: Victor Stinner <vstinner@...hon.org>
To: Rich Felker <dalias@...c.org>
Cc: musl@...ts.openwall.com
Subject: Re: Bug in fma(): should return negative zero
Hi,
On Tue, Mar 11, 2025 at 2:25 PM Rich Felker <dalias@...c.org> wrote:
> What archs have you tested it on?
I tested on x86-64.
> It doesn't happen for me and the general C code looks like it handles
> this case correctly,
Oh. I can reproduce the issue with my C example on:
* Fedora 41: musl 1.2.5-1, musl-gcc version "gcc (GCC) 14.2.1"
* Alpine Linux v3.21: musl-1.2.5-r9, gcc version 14.2.0.
Example on Fedora:
---
$ musl-gcc x.c -o x && ./x
fma(+1e-300, -1e-300, +0) = +0
---
The result is wrong (+0), it must be "-0": fma(x, y, z) = (+1e-300) *
(-1e-300) + (+0) = -0.
Using glibc, I get the correct result (-0):
---
$ gcc x.c -o x -lm && ./x
fma(+1e-300, -1e-300, +0) = -0
---
Note: when I wrote a reproducer for FreeBSD, I add to trick the clang
compiler to prevent it to call fma() at build time. But I didn't need
to do it for this musl reproducer (using gcc). I checked using gdb
that main() actually calls fma() at runtime:
--------
$ gdb ./x
(gdb) disassemble main
...
0x0000000000401192 <+76>: call 0x401030 <fma@plt>
...
--------
Victor
--
Night gathers, and now my watch begins. It shall not end until my death.
Powered by blists - more mailing lists
Confused about mailing lists and their use?
Read about mailing lists on Wikipedia
and check out these
guidelines on proper formatting of your messages.
