Re: [PATCH 1/2] softfloat: m68k: infinity is a valid encoding

["Alex Bennée" <alex.bennee@linaro.org>]

KONRAD Frederic <frederic.konrad@adacore.com> writes:

> The MC68881 say about infinities (3.2.4):
>
> "*For the extended precision format, the most significant bit of the
> mantissa (the integer bit) is a don't care."
>
> https://www.nxp.com/docs/en/reference-manual/MC68881UM.pdf
>
> The m68k extended format is implemented with the floatx80 and
> floatx80_invalid_encoding currently treats 0x7fff00000000000000000000 as
> an invalid encoding.  This patch fixes floatx80_invalid_encoding so it
> accepts that the most significant bit of the mantissa can be 0.
>
> This bug can be revealed with the following code which pushes extended
> infinity on the stack as a double and then reloads it as a double.  It
> should normally be converted and read back as infinity and is currently
> read back as nan:

Do you have any real HW on which you could record some .ref files for
the various multiarch float tests we have (float_convs/float_madds)?
Does this different of invalid encoding show up when you add them?

>
>         .global _start
>         .text
> _start:
>         lea val, %a0
>         lea fp, %fp
>         fmovex (%a0), %fp0
>         fmoved %fp0, %fp@(-8)
>         fmoved %fp@(-8), %fp0
> end:
>         bra end
>
> .align 0x4
> val:
>         .fill 1, 4, 0x7fff0000
>         .fill 1, 4, 0x00000000
>         .fill 1, 4, 0x00000000
> .align 0x4
>         .fill 0x100, 1, 0
> fp:
>
> -------------
>
> (gdb) tar rem :1234
> Remote debugging using :1234
> _start () at main.S:5
> 5              lea val, %a0
> (gdb) display $fp0
> 1: $fp0 = nan(0xffffffffffffffff)
> (gdb) si
> 6             lea fp, %fp
> 1: $fp0 = nan(0xffffffffffffffff)
> (gdb) si
> _start () at main.S:7
> 7              fmovex (%a0), %fp0
> 1: $fp0 = nan(0xffffffffffffffff)
> (gdb) si
> 8             fmoved %fp0, %fp@(-8)
> 1: $fp0 = inf
> (gdb) si
> 9             fmoved %fp@(-8), %fp0
> 1: $fp0 = inf
> (gdb) si
> end () at main.S:12
> 12          bra end
> 1: $fp0 = nan(0xfffffffffffff800)
> (gdb) x/1xg $fp-8
> 0x40000120 <val+260>:   0x7fffffffffffffff
>
> Signed-off-by: KONRAD Frederic <frederic.konrad@adacore.com>
> ---
>  include/fpu/softfloat.h | 5 +++++
>  1 file changed, 5 insertions(+)
>
> diff --git a/include/fpu/softfloat.h b/include/fpu/softfloat.h
> index ecb8ba0..dc80298 100644
> --- a/include/fpu/softfloat.h
> +++ b/include/fpu/softfloat.h
> @@ -688,7 +688,12 @@ static inline int floatx80_is_any_nan(floatx80 a)
>  *----------------------------------------------------------------------------*/
>  static inline bool floatx80_invalid_encoding(floatx80 a)
>  {
> +#if defined(TARGET_M68K)
> +    return (a.low & (1ULL << 63)) == 0 && (((a.high & 0x7FFF) != 0)
> +                                           && (a.high != 0x7FFF));
> +#else
>      return (a.low & (1ULL << 63)) == 0 && (a.high & 0x7FFF) != 0;
> +#endif
>  }
>  
>  #define floatx80_zero make_floatx80(0x0000, 0x0000000000000000LL)


-- 
Alex Bennée