Re: [Qemu-devel] [RFC PATCH v0] softfloat: Add float128_to_uint64_round_to_zero()

[Bharata B Rao <bharata@linux.vnet.ibm.com>]

On Fri, Feb 03, 2017 at 02:40:09PM +0000, Peter Maydell wrote:
> On 1 February 2017 at 10:49, Bharata B Rao <bharata@linux.vnet.ibm.com> wrote:
> > Implement float128_to_uint64() and use that to implement
> > float128_to_uint64_round_to_zero()
> >
> > This is required by xscvqpudz instruction of PowerPC ISA 3.0.
> >
> > Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
> > ---
> >  fpu/softfloat.c         | 65 +++++++++++++++++++++++++++++++++++++++++++++++++
> >  include/fpu/softfloat.h |  2 ++
> >  2 files changed, 67 insertions(+)
> >
> > diff --git a/fpu/softfloat.c b/fpu/softfloat.c
> > index c295f31..49a06c5 100644
> > --- a/fpu/softfloat.c
> > +++ b/fpu/softfloat.c
> > @@ -6110,6 +6110,71 @@ int64_t float128_to_int64_round_to_zero(float128 a, float_status *status)
> >
> >  /*----------------------------------------------------------------------------
> >  | Returns the result of converting the quadruple-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
> > +| positive integer is returned.  Otherwise, if the conversion overflows, the
> > +| largest unsigned integer is returned. If 'a' is negative, the value is
> > +| rounded and zero is returned; negative values that do not round to zero
> > +| will raise the inexact exception.
> > +*----------------------------------------------------------------------------*/
> > +
> > +uint64_t float128_to_uint64(float128 a, float_status *status)
> > +{
> > +    flag aSign;
> > +    int32_t aExp, shiftCount;
> > +    uint64_t aSig0, aSig1;
> 
> I think we should have a float128_squash_input_denormal() function
> which we call here (compare float64_to_uint64).

I followed float128_to_int64() which doesn't have that _denormal() call.

> 
> > +
> > +    aSig1 = extractFloat128Frac1( a );
> 
> Can you use the QEMU coding style for this rather than following
> the softfloat weird one, please?

Sure, I will henceforth switch to QEMU coding style, I was under the
impression that we should stick to the existing style since almost
entire softfloat.c is in different style.

> 
> > +    aSig0 = extractFloat128Frac0( a );
> > +    aExp = extractFloat128Exp( a );
> > +    aSign = extractFloat128Sign( a );
> > +    if ( aExp ) aSig0 |= LIT64( 0x0001000000000000 );
> > +    shiftCount = 0x402F - aExp;
> > +    if ( shiftCount <= 0 ) {
> > +        if ( 0x403E < aExp ) {
> > +            float_raise(float_flag_invalid, status);
> > +            if (    ! aSign
> > +                 || (    ( aExp == 0x7FFF )
> > +                      && ( aSig1 || ( aSig0 != LIT64( 0x0001000000000000 ) ) )
> > +                    )
> > +               ) {
> > +                return LIT64( 0xFFFFFFFFFFFFFFFF );
> > +            }
> > +            return 0;
> > +        }
> > +        shortShift128Left( aSig0, aSig1, - shiftCount, &aSig0, &aSig1 );
> > +    }
> > +    else {
> > +        shift64ExtraRightJamming( aSig0, aSig1, shiftCount, &aSig0, &aSig1 );
> > +    }
> > +    return roundAndPackUint64(aSign, aSig0, aSig1, status);
> 
> I'm finding this a bit difficult to understand, because it doesn't
> follow the code pattern of (for instance) float64_to_uint64().
> Is it based on some other existing function?

As I said above, it is based on float128_to_int64()

Actually what I really need is float128_to_uint64_round_to_zero().

However it is a bit confusing as to which existing routine to follow here.
I see there are 3 different ways floatXX_to_uintYY_round_to_zero is done:

1. Eg float64_to_uint32_round_to_zero()
   Uses float64_to_uint64_round_to_zero()

2. Eg float128_to_int32_round_to_zero()
   Doesn't use float128_to_int32() but instead is implemented
   fully separately.

3. Eg float64_to_uint64_round_to_zero()
   Sets the rounding mode to round-to-zero
   Uses float64_to_uint64()

I don't know if the above variants came about during different points
in time or they are actually implemented that way due to some
subtlety involved. I am following the 3rd pattern above as
I found it to be easier for this particular case (float128_to_uint128)

In fact I need float128_to_uint32() also next, but haven't yet been
able to figure out which way to do it.

> 
> > +}
> > +
> > +/*----------------------------------------------------------------------------
> > +| Returns the result of converting the quadruple-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, except that the conversion is always rounded toward zero.
> > +| according to the current rounding mode.  If `a' is a NaN, the largest
> > +| positive integer is returned.  Otherwise, if the conversion overflows, the
> > +| largest unsigned integer is returned. If 'a' is negative, the value is
> > +| rounded and zero is returned; negative values that do not round to zero
> > +| will raise the inexact exception.
> > +*----------------------------------------------------------------------------*/
> > +
> > +uint64_t float128_to_uint64_round_to_zero(float128 a, float_status *status)
> > +{
> > +    signed char current_rounding_mode = status->float_rounding_mode;
> > +    set_float_rounding_mode(float_round_to_zero, status);
> > +    uint64_t v = float128_to_uint64(a, status);
> > +    set_float_rounding_mode(current_rounding_mode, status);
> > +    return v;
> > +}
> 
> This function is OK, though our coding style would suggest putting
> the declaration of 'uint64_t v;' at the top of the function.

Sure will change in the next iteration when I switch to QEMU style.

Regards,
Bharata.