Re: [PATCH 2/4] softfloat: fix floatx80 pseudo-denormal addition / subtraction

[Joseph Myers]

On Fri, 1 May 2020, Alex Bennée wrote:

> 
> Joseph Myers <address@hidden> writes:
> 
> > The softfloat function addFloatx80Sigs, used for addition of values
> > with the same sign and subtraction of values with opposite sign, fails
> > to handle the case where the two values both have biased exponent zero
> > and there is a carry resulting from adding the significands, which can
> > occur if one or both values are pseudo-denormals (biased exponent
> > zero, explicit integer bit 1).  Add a check for that case, so making
> > the results match those seen on x86 hardware for pseudo-denormals.
> 
> Hmm running the super detailed test:
> 
>   fp-test -s -l 2 -r all  extF80_add extF80_sub
> 
> I don't see any difference between before and after the patch. This
> makes me wonder if we are (or rather TestFloat) is missing something in
> it's test case.

It could well only be testing kinds of floating-point representations that 
are meaningful in IEEE interchange formats.  Pseudo-denormals don't exist 
in IEEE interchange formats (and nor do pseudo-NaNs, pseudo-infinities and 
un-normals, which are dealt with in floatx80_invalid_encoding).

-- 
Joseph S. Myers
address@hidden