Re: [Qemu-devel] [PATCH v1 05/19] include/fpu/softfloat: add some float16 contants

[Philippe Mathieu-Daudé]

Hi Alex,

On 12/11/2017 09:56 AM, Alex Bennée wrote:
> This defines the same set of common constants for float 16 as defined
> for 32 and 64 bit floats. These are often used by target helper
> functions.
> 
> Signed-off-by: Alex Bennée <address@hidden>
> ---
>  include/fpu/softfloat.h | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/include/fpu/softfloat.h b/include/fpu/softfloat.h
> index 17dfe60dbd..5a9258c57c 100644
> --- a/include/fpu/softfloat.h
> +++ b/include/fpu/softfloat.h
> @@ -395,6 +395,13 @@ static inline float16 float16_set_sign(float16 a, int 
> sign)
>      return make_float16((float16_val(a) & 0x7fff) | (sign << 15));
>  }
>  
> +#define float16_zero make_float16(0)

ok

> +#define float16_one make_float16(0x3a00)

I'm a bit confused...

>>> [np.fromstring(struct.pack("<H", x), dtype=np.float16)[0] for x in
[0, 0x3a00, 0x34d1, 0x4448, 0x3800, 0x7a00]]
[0.0, 0.75, 0.30103, 4.2812, 0.5, 49152.0]

However:

>>> ['0x' + binascii.hexlify(np.array([x], '>f2').tostring()) for x in
[0, 1, math.log(2), np.pi, 0.5, np.inf]]
['0x0000', '0x3c00', '0x398c', '0x4248', '0x3800', '0x7c00']

It seems the MSB bit of the mantissa got shifted as the LSB of the
biased exponent...

> +#define float16_ln2 make_float16(0x34d1)

incorrect? 0x398c

> +#define float16_pi make_float16(0x4448)

incorrect? 0x4248

> +#define float16_half make_float16(0x3800)

ok

> +#define float16_infinity make_float16(0x7a00)

incorrect? 0x7c00

> +
>  
> /*----------------------------------------------------------------------------
>  | The pattern for a default generated half-precision NaN.
>  
> *----------------------------------------------------------------------------*/
>

signature.asc
Description: OpenPGP digital signature