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

[Peter Maydell <peter.maydell@linaro.org>]

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