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From: Szabolcs Nagy <nsz@port70.net>
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Subject: Re: correctly rounded sqrt
Date: Thu, 15 Mar 2012 09:02:44 +0100
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* Szabolcs Nagy <nsz@port70.net> [2012-03-15 06:30:11 +0100]:
> * Rich Felker <dalias@aerifal.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