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Date: Fri, 21 Apr 2017 17:37:49 -0400
From: Rich Felker <>
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.


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