Re: [PATCH] softfloat,m68k: disable floatx80_invalid_encoding() for m68k

[Alex Bennée]

Laurent Vivier <laurent@vivier.eu> writes:

> According to the comment, this definition of invalid encoding is given
> by intel developer's manual, and doesn't comply with 680x0 FPU.
>
> With m68k, the explicit integer bit can be zero in the case of:
>  - zeros                (exp == 0, mantissa == 0)
>  - denormalized numbers (exp == 0, mantissa != 0)
>  - unnormalized numbers (exp != 0, exp < 0x7FFF)
>  - infinities           (exp == 0x7FFF, mantissa == 0)
>  - not-a-numbers        (exp == 0x7FFF, mantissa != 0)
>
> For infinities and NaNs, the explicit integer bit can be either one or
> zero.
>
> The IEEE 754 standard does not define a zero integer bit. Such a number
> is an unnormalized number. Hardware does not directly support
> denormalized and unnormalized numbers, but implicitly supports them by
> trapping them as unimplemented data types, allowing efficient conversion
> in software.
>
> See "M68000 FAMILY PROGRAMMER’S REFERENCE MANUAL",
>     "1.6 FLOATING-POINT DATA TYPES"
>
> We will implement in the m68k TCG emulator the FP_UNIMP exception to
> trap into the kernel to normalize the number. In case of linux-user,
> the number will be normalized by QEMU.
>
> Signed-off-by: Laurent Vivier <laurent@vivier.eu>

Apologies for the private reply, was using my fallback tooling while
email was down and that doesn't automatically include the group address.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>

By all means take it via the m68k tree. 

> ---
>  include/fpu/softfloat.h | 24 ++++++++++++++++++++++++
>  1 file changed, 24 insertions(+)
>
> diff --git a/include/fpu/softfloat.h b/include/fpu/softfloat.h
> index 16ca697a73b7..f6eda4ca8e6c 100644
> --- a/include/fpu/softfloat.h
> +++ b/include/fpu/softfloat.h
> @@ -791,7 +791,31 @@ static inline bool floatx80_unordered_quiet(floatx80 a, 
> floatx80 b,
>  
> *----------------------------------------------------------------------------*/
>  static inline bool floatx80_invalid_encoding(floatx80 a)
>  {
> +#if defined(TARGET_M68K)
> +    
> /*-------------------------------------------------------------------------
> +    | With m68k, the explicit integer bit can be zero in the case of:
> +    | - zeros                (exp == 0, mantissa == 0)
> +    | - denormalized numbers (exp == 0, mantissa != 0)
> +    | - unnormalized numbers (exp != 0, exp < 0x7FFF)
> +    | - infinities           (exp == 0x7FFF, mantissa == 0)
> +    | - not-a-numbers        (exp == 0x7FFF, mantissa != 0)
> +    |
> +    | For infinities and NaNs, the explicit integer bit can be either one or
> +    | zero.
> +    |
> +    | The IEEE 754 standard does not define a zero integer bit. Such a number
> +    | is an unnormalized number. Hardware does not directly support
> +    | denormalized and unnormalized numbers, but implicitly supports them by
> +    | trapping them as unimplemented data types, allowing efficient 
> conversion
> +    | in software.
> +    |
> +    | See "M68000 FAMILY PROGRAMMER’S REFERENCE MANUAL",
> +    |     "1.6 FLOATING-POINT DATA TYPES"
> +    
> *------------------------------------------------------------------------*/
> +    return false;
> +#else
>      return (a.low & (1ULL << 63)) == 0 && (a.high & 0x7FFF) != 0;
> +#endif
>  }
>  
>  #define floatx80_zero make_floatx80(0x0000, 0x0000000000000000LL)

-- 
Alex Bennée