Re: [Qemu-devel] [PATCH v2] softfloat: Fix factor 2 error for scalbn on denormal inputs

[Aurelien Jarno <aurelien@aurel32.net>]

On Sat, Dec 21, 2013 at 09:08:19PM +0000, Peter Maydell wrote:
> If the input to float*_scalbn() is denormal then it represents
> a number 0.[mantissabits] * 2^(1-exponentbias) (and the actual
> exponent field is all zeroes). This means that when we convert
> it to our unpacked encoding the unpacked exponent must be one
> greater than for a normal number, which represents
> 1.[mantissabits] * 2^(e-exponentbias) for an exponent field e.
> 
> This meant we were giving answers too small by a factor of 2 for
> all denormal inputs.
> 
> Note that the float-to-int routines also have this behaviour
> of not adjusting the exponent for denormals; however there it is
> harmless because denormals will all convert to integer zero anyway.
> 
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> ---
> Changes v1->v2: propagated fix to 80 bit and 128 bit float functions.
> 
> These function names remind me of Ken Thompson's reply to a question
> about what he'd do differently if he were redesigning UNIX:
> "I'd spell creat with an e."
> 
>  fpu/softfloat.c | 29 +++++++++++++++++++++--------
>  1 file changed, 21 insertions(+), 8 deletions(-)
> 
> diff --git a/fpu/softfloat.c b/fpu/softfloat.c
> index dbda61b..7f5e8c7 100644
> --- a/fpu/softfloat.c
> +++ b/fpu/softfloat.c
> @@ -6795,10 +6795,13 @@ float32 float32_scalbn( float32 a, int n STATUS_PARAM )
>          }
>          return a;
>      }
> -    if ( aExp != 0 )
> +    if (aExp != 0) {
>          aSig |= 0x00800000;
> -    else if ( aSig == 0 )
> +    } else if (aSig == 0) {
>          return a;
> +    } else {
> +        aExp++;
> +    }
>  
>      if (n > 0x200) {
>          n = 0x200;
> @@ -6828,10 +6831,13 @@ float64 float64_scalbn( float64 a, int n STATUS_PARAM )
>          }
>          return a;
>      }
> -    if ( aExp != 0 )
> +    if (aExp != 0) {
>          aSig |= LIT64( 0x0010000000000000 );
> -    else if ( aSig == 0 )
> +    } else if (aSig == 0) {
>          return a;
> +    } else {
> +        aExp++;
> +    }
>  
>      if (n > 0x1000) {
>          n = 0x1000;
> @@ -6861,8 +6867,12 @@ floatx80 floatx80_scalbn( floatx80 a, int n STATUS_PARAM )
>          return a;
>      }
>  
> -    if (aExp == 0 && aSig == 0)
> -        return a;
> +    if (aExp == 0) {
> +        if (aSig == 0) {
> +            return a;
> +        }
> +        aExp++;
> +    }
>  
>      if (n > 0x10000) {
>          n = 0x10000;
> @@ -6891,10 +6901,13 @@ float128 float128_scalbn( float128 a, int n STATUS_PARAM )
>          }
>          return a;
>      }
> -    if ( aExp != 0 )
> +    if (aExp != 0) {
>          aSig0 |= LIT64( 0x0001000000000000 );
> -    else if ( aSig0 == 0 && aSig1 == 0 )
> +    } else if (aSig0 == 0 && aSig1 == 0) {
>          return a;
> +    } else {
> +        aExp++;
> +    }
>  
>      if (n > 0x10000) {
>          n = 0x10000;

Reviewed-by: Aurelien Jarno <aurelien@aurel32.net>

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