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Message-ID: <CABqyc3weC5TM9iptA1_wdjcnt1kO6QH_c3HE=L9v3kQfbSyfhQ@mail.gmail.com>
Date: Tue, 11 Mar 2025 12:30:58 +0100
From: Victor Stinner <vstinner@...hon.org>
To: musl@...ts.openwall.com
Subject: Bug in fma(): should return negative zero
Hi,
test_math.test_fma_zero_result() of Python 3.13 fails when Python is
built with musl:
https://github.com/python/cpython/issues/131032
The problem is that fma() returns +0.0 instead of -0.0 on some tests.
Example:
---
#include <math.h>
#include <stdio.h>
int main(int argc, char **argv)
{
double tiny = 1e-300;
double zero = 0.0;
double result = fma(tiny, -tiny, zero);
printf("fma(%+g, %+g, %+g) = %+g\n", tiny, -tiny, zero, result);
return 0;
}
---
Output on Alpine with musl-1.2.5-r9:
---
$ gcc x.c -Og -o x -lm && ./x
fma(+1e-300, -1e-300, +0) = +0
---
Expected output:
---
fma(+1e-300, -1e-300, +0) = -0
---
Last year, I reported a similar bug to FreeBSD which was fixed:
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=277783
Link to Python test_fma_zero_result():
https://github.com/python/cpython/blob/425e0af74fb882b95f81923123bd4afad0cda157/Lib/test/test_math.py#L2762-L2816
Tell me if you need more details.
Victor
--
Night gathers, and now my watch begins. It shall not end until my death.
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