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Message-ID: <20120315080244.GL5728@port70.net>
Date: Thu, 15 Mar 2012 09:02:44 +0100
From: Szabolcs Nagy <nsz@...t70.net>
To: musl@...ts.openwall.com
Subject: Re: correctly rounded sqrt
* Szabolcs Nagy <nsz@...t70.net> [2012-03-15 06:30:11 +0100]:
> * Rich Felker <dalias@...ifal.cx> [2012-03-15 01:07:48 -0400]:
> > .global sqrt
> > .type sqrt,@function
> > sqrt: fldl 4(%esp)
> > fsqrt
> > fstsw %ax
> > sub $12,%esp
> > fld %st(0)
> > fstpt (%esp)
> > mov (%esp),%ecx
> > and $0x7ff,%ecx
> > cmp $0x400,%ecx
> > jnz 1f
> > and $0x200,%eax
> > sub $0x100,%eax
> > sub %eax,(%esp)
> here you modify the return value even if it was nan
> eg c99 F.9 recommends no nan modifications
> if ((y & 0x7fff<<64) == 0x7fff<<64) ...
>
posix requires this as well
http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap04.html#tag_04_20
if (signexp >= 0x7fff) is probably a better check
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