Re: [Qemu-devel] [V4 PATCH 02/22] softfloat: Add float32_to_uint64()

[Tom Musta <tommusta@gmail.com>]

On 12/19/2013 3:31 PM, Peter Maydell wrote:
> On 18 December 2013 20:19, Tom Musta <tommusta@gmail.com> wrote:
>> This patch adds the float32_to_uint64() routine, which converts a
>> 32-bit floating point number to an unsigned 64 bit number.
>>
>> This contribution can be licensed under either the softfloat-2a or -2b
>> license.
>>
>> V2: Reduced patch to just this single routine per feedback from Peter
>> Maydell.
>>
>> V4: Now passing sign to roundAndPackUint64()
>>
>> Signed-off-by: Tom Musta <tommusta@gmail.com>
>> ---
>>  fpu/softfloat.c         |   45 +++++++++++++++++++++++++++++++++++++++++++++
>>  include/fpu/softfloat.h |    1 +
>>  2 files changed, 46 insertions(+), 0 deletions(-)
>>
>> diff --git a/fpu/softfloat.c b/fpu/softfloat.c
>> index ec23908..1ff59d0 100644
>> --- a/fpu/softfloat.c
>> +++ b/fpu/softfloat.c
>> @@ -1558,6 +1558,51 @@ int64 float32_to_int64( float32 a STATUS_PARAM )
>>
>>  /*----------------------------------------------------------------------------
>>  | Returns the result of converting the single-precision floating-point value
>> +| `a' to the 64-bit unsigned integer format.  The conversion is
>> +| performed according to the IEC/IEEE Standard for Binary Floating-Point
>> +| Arithmetic---which means in particular that the conversion is rounded
>> +| according to the current rounding mode.  If `a' is a NaN, the largest
>> +| unsigned integer is returned.  Otherwise, if the conversion overflows, the
>> +| largest unsigned integer is returned.  If the 'a' is negative, zero is
>> +| returned.
>> +*----------------------------------------------------------------------------*/
>> +
>> +uint64 float32_to_uint64(float32 a STATUS_PARAM)
>> +{
>> +    flag aSign;
>> +    int_fast16_t aExp, shiftCount;
>> +    uint32_t aSig;
>> +    uint64_t aSig64, aSigExtra;
>> +    a = float32_squash_input_denormal(a STATUS_VAR);
>> +
>> +    aSig = extractFloat32Frac(a);
>> +    aExp = extractFloat32Exp(a);
>> +    aSign = extractFloat32Sign(a);
>> +    if (aSign) {
>> +        if (aExp) {
>> +            float_raise(float_flag_invalid STATUS_VAR);
> 
> NaNs with the sign bit set will wind up in this case and return 0
> rather than largest-unsigned-integer.
> 
> Also it seems like this code says "negative inputs return
> zero if they're denormal or signal Invalid and return 0
> if they're not". Are you sure this does the right thing for
> (a) values which are not denormal but are close enough
> to zero to round to it and (b) different rounding modes?
> 
>> +        } else if (aSig) { /* negative denormalized */
>> +            float_raise(float_flag_inexact STATUS_VAR);
>> +        }
>> +        return 0;
>> +    }
>> +    shiftCount = 0xBE - aExp;
>> +    if (aExp) {
>> +        aSig |= 0x00800000;
>> +    }
>> +    if (shiftCount < 0) {
>> +        float_raise(float_flag_invalid STATUS_VAR);
>> +        return (int64_t)LIT64(0xFFFFFFFFFFFFFFFF);
>> +    }
>> +
>> +    aSig64 = aSig;
>> +    aSig64 <<= 40;
>> +    shift64ExtraRightJamming(aSig64, 0, shiftCount, &aSig64, &aSigExtra);
>> +    return roundAndPackUint64(aSign, aSig64, aSigExtra STATUS_VAR);
>> +}
> 
> thanks
> -- PMM
> 

Peter:  I agree ... this still isn't quite right.