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Message-ID: <20170421213749.GH17319@brightrain.aerifal.cx>
Date: Fri, 21 Apr 2017 17:37:49 -0400
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: [PATCH] fix scalbn when result is in the subnormal range
On Mon, Apr 03, 2017 at 02:38:13AM +0200, Szabolcs Nagy wrote:
> in nearest rounding mode scalbn could introduce double rounding error
> when an intermediate value and the final result were both in the
> subnormal range e.g.
>
> scalbn(0x1.7ffffffffffffp-1, -1073)
>
> returned 0x1p-1073 instead of 0x1p-1074, because the intermediate
> computation got rounded to 0x1.8p-1023.
>
> with the fix an intermediate value can only be in the subnormal range
> if the final result is 0 which is correct even after double rounding.
> (there still can be two roundings so signals may be raised twice, but
> that's only observable with trapping exceptions which is not supported.)
OK, applying.
Rich
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