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

[Richard Henderson <rth@twiddle.net>]

On 12/21/2013 01:08 PM, 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(-)

Reviewed-by: Richard Henderson <rth@twiddle.net>


r~