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

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

[Joseph Myers]

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.

Signed-off-by: Joseph Myers <joseph@codesourcery.com>
---
 fpu/softfloat.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/fpu/softfloat.c b/fpu/softfloat.c
index ac116c70b8..6094d267b5 100644
--- a/fpu/softfloat.c
+++ b/fpu/softfloat.c
@@ -5866,6 +5866,12 @@ static floatx80 addFloatx80Sigs(floatx80 a, floatx80 b, flag zSign,
         zSig1 = 0;
         zSig0 = aSig + bSig;
         if ( aExp == 0 ) {
+            if ((aSig | bSig) & UINT64_C(0x8000000000000000) && zSig0 < aSig) {
+                /* At least one of the values is a pseudo-denormal,
+                 * and there is a carry out of the result.  */
+                zExp = 1;
+                goto shiftRight1;
+            }
             if (zSig0 == 0) {
                 return packFloatx80(zSign, 0, 0);
             }
-- 
2.17.1


-- 
Joseph S. Myers
joseph@codesourcery.com

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

[Alex Bennée]

Joseph Myers <joseph@codesourcery.com> 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.

>
> Signed-off-by: Joseph Myers <joseph@codesourcery.com>
> ---
>  fpu/softfloat.c | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/fpu/softfloat.c b/fpu/softfloat.c
> index ac116c70b8..6094d267b5 100644
> --- a/fpu/softfloat.c
> +++ b/fpu/softfloat.c
> @@ -5866,6 +5866,12 @@ static floatx80 addFloatx80Sigs(floatx80 a, floatx80 b, flag zSign,
>          zSig1 = 0;
>          zSig0 = aSig + bSig;
>          if ( aExp == 0 ) {
> +            if ((aSig | bSig) & UINT64_C(0x8000000000000000) && zSig0 < aSig) {
> +                /* At least one of the values is a pseudo-denormal,
> +                 * and there is a carry out of the result.  */
> +                zExp = 1;
> +                goto shiftRight1;
> +            }
>              if (zSig0 == 0) {
>                  return packFloatx80(zSign, 0, 0);
>              }
> -- 
> 2.17.1


-- 
Alex Bennée

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

[Joseph Myers]

[-- Attachment #1: Type: text/plain, Size: 1203 bytes --]

On Fri, 1 May 2020, Alex Bennée wrote:

> 
> Joseph Myers <joseph@codesourcery.com> 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
joseph@codesourcery.com