Re: [Qemu-devel] [PATCH v2 06/20] include/fpu/softfloat: add some float16 constants

[Alex Bennée]

Philippe Mathieu-Daudé <address@hidden> writes:

> Hi Alex,
>
> On 01/09/2018 09:22 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. I've also removed constants that are not used by anybody.
>>
>> Signed-off-by: Alex Bennée <address@hidden>
>>
>> ---
>> v2
>>   - fixup constants, remove unused onces
>> ---
>>  include/fpu/softfloat.h | 8 +++++---
>>  1 file changed, 5 insertions(+), 3 deletions(-)
>>
>> diff --git a/include/fpu/softfloat.h b/include/fpu/softfloat.h
>> index 8ab5d0df47..e64bf62f3d 100644
>> --- a/include/fpu/softfloat.h
>> +++ b/include/fpu/softfloat.h
>> @@ -368,6 +368,11 @@ 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)
>> +#define float16_one make_float16(0x3a00)
>
> I still disagree with this one, it seems your bits 9/10 are inverted
> (mantissa msb with biased exponent lsb).
>
>          S EEEEE TTTTTTTTTT
> 0x3a00 = 0 01110 1000000000
>
> having:
> S=0
> E=0b01110=14
> T=0b1000000000=512
>
> I get:
> (-1)^0 * 2^(14-15) * (1 + (2^-10) * 512) = 1 * 0.5 * (1 + 0.5) = 0.75
>
> With 0x3c00 I get:
>
>          S EEEEE TTTTTTTTTT
> 0x3c00 = 0 01111 0000000000
> S=0
> E=0b01111=15
> T=0b0000000000=0
>
> (-1)^0 * 2^(15-15) * (1 + (2^-10) * 0) = 1 * 2^0 * (1 + 0) = 1
>
> The rest is OK.
>
> Changing by "#define float16_one make_float16(0x3c00)":
> Reviewed-by: Philippe Mathieu-Daudé <address@hidden>

Dam it, I thought I'd fixed them all.

Thanks for the catch.

>
>> +#define float16_half make_float16(0x3800)
>> +#define float16_infinity make_float16(0x7c00)
>> +
>>  
>> /*----------------------------------------------------------------------------
>>  | The pattern for a default generated half-precision NaN.
>>  
>> *----------------------------------------------------------------------------*/
>> @@ -474,8 +479,6 @@ static inline float32 float32_set_sign(float32 a, int 
>> sign)
>>
>>  #define float32_zero make_float32(0)
>>  #define float32_one make_float32(0x3f800000)
>> -#define float32_ln2 make_float32(0x3f317218)
>> -#define float32_pi make_float32(0x40490fdb)
>>  #define float32_half make_float32(0x3f000000)
>>  #define float32_infinity make_float32(0x7f800000)
>>
>> @@ -588,7 +591,6 @@ static inline float64 float64_set_sign(float64 a, int 
>> sign)
>>  #define float64_zero make_float64(0)
>>  #define float64_one make_float64(0x3ff0000000000000LL)
>>  #define float64_ln2 make_float64(0x3fe62e42fefa39efLL)
>> -#define float64_pi make_float64(0x400921fb54442d18LL)
>>  #define float64_half make_float64(0x3fe0000000000000LL)
>>  #define float64_infinity make_float64(0x7ff0000000000000LL)
>>
>>


--
Alex Bennée