Re: [Qemu-devel] [PATCH 2/2] fpu: Bound increment for scalbn

[Peter Maydell]

On 17 April 2018 at 14:51, Alex Bennée <address@hidden> wrote:
>
> Richard Henderson <address@hidden> writes:
>
>> Without bounding the increment, we can overflow exp either here
>> in scalbn_decomposed or when adding the bias in round_canonical.
>> This can result in e.g. underflowing to 0 instead of overflowing
>> to infinity.
>>
>> The old softfloat code did bound the increment.
>>
>> Signed-off-by: Richard Henderson <address@hidden>
>> ---
>>  fpu/softfloat.c | 6 ++++++
>>  1 file changed, 6 insertions(+)
>>
>> diff --git a/fpu/softfloat.c b/fpu/softfloat.c
>> index ba6e654050..a589f328c9 100644
>> --- a/fpu/softfloat.c
>> +++ b/fpu/softfloat.c
>> @@ -1883,6 +1883,12 @@ static FloatParts scalbn_decomposed(FloatParts a, int 
>> n, float_status *s)
>>          return return_nan(a, s);
>>      }
>>      if (a.cls == float_class_normal) {
>> +        /* The largest float type (even though not supported by FloatParts)
>> +         * is float128, which has a 15 bit exponent.  Bounding N to 16 bits
>> +         * still allows rounding to infinity, without allowing overflow
>> +         * within the int32_t that backs FloatParts.exp.
>> +         */
>> +        n = MIN(MAX(n, -0x10000), 0x10000);
>>          a.exp += n;
>>      }
>>      return a;
>
> Reviewed-by: Alex Bennée <address@hidden>
> Tested-by: Alex Bennée <address@hidden>
>
> (risu FWIW although it obviously didn't catch this failure ;-)

Thanks; applied this patch to master.

-- PMM