Date: Mon, 18 Nov 2019 21:55:03 -0800 From: Jeff Walden <jwalden@mit.edu> To: sgk@troutmask.apl.washington.edu, Dimitry Andric <dim@freebsd.org> Cc: freebsd-numerics@freebsd.org Subject: Re: UB in various hypot() implementations (left-shifting a negative, number) Message-ID: <11f139aa-708a-4799-dfee-1a5b988a3c0b@mit.edu> In-Reply-To: <20191116211419.GA40056@troutmask.apl.washington.edu> References: <a3fe6fb4-8f33-6d10-64d4-e722bfebcc85@FreeBSD.org> <20191116211419.GA40056@troutmask.apl.washington.edu>
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On 11/16/19 1:14 PM, Steve Kargl wrote:
> Well, clearly, the patch to e_hypotl.c is wrong. It
> clear the significand when t1 = 0 whereas t1 = 1 leaves
> one bit set in the significand. Simply looking at the
> value of t1 under a poor man's debugger shows the
> difference. Adding "printf("%Le %La\n", t1, t1);" after
> the SET_HIGH_WORD gives
>
> 2.962347e-2493 0x1p-8280 <-- t1 = 1
> 0.000000e+00 0x1p-8280 <-- t1 = 0
>
> for hypotl(ldexpl(1.1,-16000), ldexpl(2.1, -16000)).
Hrm. My understanding based on an earlier line in e_hypotl.c was that that change should be identical, but something mildly subtle is going on and my understanding of |long double| is not quite as good as I thought it was. I've removed that file's changes and left just the original two in, then resubmitted the diff.
Jeff
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