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Date: Wed, 20 Apr 2011 11:21:04 +0200
From: Aurelien Jarno <aurelien@aurel32.net>
Message-ID: <20110420092104.GD6388@volta.aurel32.net>
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Subject: Re: [Qemu-devel] [PATCH 07/20] softfloat: fix float*_scalnb()
 corner cases
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To: Peter Maydell <peter.maydell@linaro.org>
Cc: qemu-devel@nongnu.org

On Tue, Apr 19, 2011 at 12:57:23PM +0100, Peter Maydell wrote:
> On 18 April 2011 21:59, Aurelien Jarno <aurelien@aurel32.net> wrote:
> 
> > @@ -6349,6 +6352,12 @@ float32 float32_scalbn( float32 a, int n STATUS_PARAM )
> >     else if ( aSig == 0 )
> >         return a;
> >
> > +    if (n > 0x80) {
> > +        n = 0x80;
> > +    } else if (n < -0x80) {
> > +        n = -0x80;
> > +    }
> > +
> >     aExp += n - 1;
> >     aSig <<= 7;
> >     return normalizeRoundAndPackFloat32( aSign, aExp, aSig STATUS_VAR );
> 
> I don't think your if() condition is right here. Consider the
> float32 00800000 (1.0 * 2 ^ -126 ; the smallest possible normalised
> number); you can multiply this by, say, 2^253, without overflowing
> to infinity. However your if() here means we'll incorrectly
> compute the result of multiplying by 2^128 instead. s/0x80/0x200/
> should work.
> 

Correct, will be fixed in the next version.


-- 
Aurelien Jarno                          GPG: 1024D/F1BCDB73
aurelien@aurel32.net                 http://www.aurel32.net