Re: [Qemu-devel] [RFC PATCH 23/30] softfloat: add float16_rem and float16_muladd (!CHECK)

[Richard Henderson]

On 10/13/2017 09:24 AM, Alex Bennée wrote:
> +float16 float16_rem(float16 a, float16 b, float_status *status)
> +{
> +    flag aSign, zSign;
> +    int aExp, bExp, expDiff;
> +    uint32_t aSig, bSig;
> +    uint32_t q;
> +    uint64_t aSig64, bSig64, q64;
> +    uint32_t alternateASig;
> +    int32_t sigMean;
> +    a = float16_squash_input_denormal(a, status);
> +    b = float16_squash_input_denormal(b, status);
> +
> +    aSig = extractFloat32Frac( a );
> +    aExp = extractFloat32Exp( a );
> +    aSign = extractFloat32Sign( a );
> +    bSig = extractFloat32Frac( b );
> +    bExp = extractFloat32Exp( b );
> +    if ( aExp == 0xFF ) {
> +        if ( aSig || ( ( bExp == 0xFF ) && bSig ) ) {
> +            return propagateFloat32NaN(a, b, status);
> +        }
> +        float_raise(float_flag_invalid, status);
> +        return float16_default_nan(status);
> +    }
> +    if ( bExp == 0xFF ) {
> +        if (bSig) {
> +            return propagateFloat32NaN(a, b, status);
> +        }
> +        return a;
> +    }

s/0xff/0x1f/.

> +    if ( bExp == 0 ) {
> +        if ( bSig == 0 ) {
> +            float_raise(float_flag_invalid, status);
> +            return float16_default_nan(status);
> +        }
> +        normalizeFloat32Subnormal( bSig, &bExp, &bSig );
> +    }
> +    if ( aExp == 0 ) {
> +        if ( aSig == 0 ) return a;
> +        normalizeFloat32Subnormal( aSig, &aExp, &aSig );
> +    }
> +    expDiff = aExp - bExp;
> +    aSig |= 0x00800000;
> +    bSig |= 0x00800000;

These implicit bits are set for float32.

> +    if ( expDiff < 32 ) {
> +        aSig <<= 8;
> +        bSig <<= 8;

This algorithm isn't going to work unless the fractions are normalized to
0b1xxx_xxxx_xxx0_0000, just like for float32.  Indeed, I think you should
actually share code.

> +    return normalizeRoundAndPackFloat32(aSign ^ zSign, bExp, aSig, status);

... but you really will have to pack into the correct format.

> +    if (((aExp == 0xff) && aSig) ||
> +        ((bExp == 0xff) && bSig) ||
> +        ((cExp == 0xff) && cSig)) {

0x1f, lots more times.

> +    /* Calculate the actual result a * b + c */
> +
> +    /* Multiply first; this is easy. */
> +    /* NB: we subtract 0x7e where float16_mul() subtracts 0x7f
> +     * because we want the true exponent, not the "one-less-than"
> +     * flavour that roundAndPackFloat16() takes.
> +     */
> +    pExp = aExp + bExp - 0x7e;
> +    aSig = (aSig | 0x00800000) << 7;
> +    bSig = (bSig | 0x00800000) << 8;

All of these constants are for float32, not float16.


r~