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Message-ID: <20260803160024.GP27423@brightrain.aerifal.cx>
Date: Mon, 3 Aug 2026 12:00:25 -0400
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: fix acosh for x<0
On Mon, Aug 03, 2026 at 02:10:37PM +0200, Szabolcs Nagy wrote:
> i somehow missed this earlier.
>
> >From 84bf2a51418196f96192d7fc8f11cf268b02e1b1 Mon Sep 17 00:00:00 2001
> From: Szabolcs Nagy <nsz@...t70.net>
> Date: Mon, 3 Aug 2026 06:41:28 +0000
> Subject: [PATCH 1/2] math: fix acosh for x<0
>
> acosh(-0x1.8p15) returned -3.7534177368329567 instead of nan.
>
> the same issue got fixed for acoshf and acoshl in commits
> c4c38e6364323b6d83ba3428464e19987b981d7a and
> 6d10102709df4bc966d2846c1c45cd667e5048e5 here we follow the latter.
>
> reported by Paul Zimmermann.
> ---
> src/math/acosh.c | 13 +++++++------
> 1 file changed, 7 insertions(+), 6 deletions(-)
>
> diff --git a/src/math/acosh.c b/src/math/acosh.c
> index badbf908..75ce95aa 100644
> --- a/src/math/acosh.c
> +++ b/src/math/acosh.c
> @@ -9,16 +9,17 @@
> double acosh(double x)
> {
> union {double f; uint64_t i;} u = {.f = x};
> - unsigned e = u.i >> 52 & 0x7ff;
> -
> - /* x < 1 domain error is handled in the called functions */
> + unsigned e = u.i >> 52;
>
> if (e < 0x3ff + 1)
> - /* |x| < 2, up to 2ulp error in [1,1.125] */
> + /* 0 <= x < 2, up to 2ulp error in [1,1.125] */
> return log1p(x-1 + sqrt((x-1)*(x-1)+2*(x-1)));
> if (e < 0x3ff + 26)
> - /* |x| < 0x1p26 */
> + /* 2 <= x < 0x1p26 */
> return log(2*x - 1/(x+sqrt(x*x-1)));
> - /* |x| >= 0x1p26 or nan */
> + if (e & 0x800)
> + /* x < 0 or x = -0, invalid */
> + return (x-x)/(x-x);
> + /* x >= 0x1p26 or nan */
> return log(x) + 0.693147180559945309417232121458176568;
> }
I don't think you need the special case for negative. The final case
of passing a negative x to log will already produce a nan.
Rich
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