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Message-ID: <20261029232855.GS3542221@port70.net>
Date: Fri, 30 Oct 2026 00:28:55 +0100
From: Szabolcs Nagy <nsz@...t70.net>
To: Bernie Innocenti <bernie@...ewiz.org>
Cc: musl@...ts.openwall.com
Subject: Re: [PATCH] math: fix fma(x,y,0) double rounding with excess
precision
* Bernie Innocenti <bernie@...ewiz.org> [2026-10-30 06:57:38 +0900]:
> with FLT_EVAL_METHOD==2 (i386 x87, m68k with 68881) x*y is rounded
> to long double first and then to double, e.g.
>
> fma(0x1.27365a6456ba5p+0, 0x1.63fdaf7759a4fp+0, 0.0)
>
> returned 0x1.9a84ea6842dccp+0 instead of 0x1.9a84ea6842dcdp+0.
>
> add the zero z exactly on the integer path instead: then x*y is
> rounded once in the current rounding mode with its own sign, which
> keeps the fix of commit 9683bd62414604d3bd56cf6bd7be8f54aa31e7d3.
>
> found by arczi84 (GitHub) reviewing a port of this code to newlib.
> ---
> src/math/fma.c | 10 +++++++---
> 1 file changed, 7 insertions(+), 3 deletions(-)
>
> diff --git a/src/math/fma.c b/src/math/fma.c
> index adfadca8..072e8e5d 100644
> --- a/src/math/fma.c
> +++ b/src/math/fma.c
> @@ -52,9 +52,13 @@ double fma(double x, double y, double z)
> if (nx.e >= ZEROINFNAN || ny.e >= ZEROINFNAN)
> return x*y + z;
> if (nz.e >= ZEROINFNAN) {
> - if (nz.e > ZEROINFNAN) /* z==0 */
> - return x*y;
> - return z;
> + if (nz.e == ZEROINFNAN)
> + return z;
> + /* z==0: x*y would round twice with FLT_EVAL_METHOD==2,
> + so add z exactly, x*y then rounds once with its own sign */
> + nz.m = 0;
> + nz.e = nx.e + ny.e;
> + nz.sign = nx.sign ^ ny.sign;
this looks good
thanks.
> }
>
> /* mul: r = x*y */
> --
> 2.56.0
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