Re: [PATCH] softfloat: Handle m68k extended precision denormals properly

[Richard Henderson <richard.henderson@linaro.org>]

On 8/20/23 18:24, Keith Packard wrote:
> 
>> Motorola treats denormals with explicit integer bit set as
>> having unbiased exponent 0, unlike Intel which treats it as
>> having unbiased exponent 1 (like all other IEEE formats).
> 
> Thanks for having a look at this. Your patch fixes a couple of cases,
> but there are further adventures that await if you're interested.
> 
>             x:  0x1p0                      0x3fff 0x80000000 0x00000000
>             y:  0x1p-16383                 0x0000 0x80000000 0x00000000
>     build_mul:  0x1p-16382                 0x0000 0x80000000 0x00000000
>   runtime_mul:  0x1p-16383                 0x0001 0x80000000 0x00000000
> 
> I think the enclosed additional patch fixes this. I've still got 75 fmal
> failures on this target, but the obvious 'multiply is broken' problem
> appears fixed.
> 
>  From b722c92f8329f56f5243496eca3779f1156aff4f Mon Sep 17 00:00:00 2001
> From: Keith Packard <keithp@keithp.com>
> Date: Sun, 20 Aug 2023 18:20:13 -0700
> Subject: [PATCH] softfloat: Handle m68k LDBL_MIN_EXP normal values
> 
> Unlike Intel 80-bit floats, Motorola allows for normal values with a
> zero exponent. Handle that by not setting exponent to 1 when the value
> is normal for this format.
> 
> Signed-off-by: Keith Packard <keithp@keithp.com>
> ---
>   fpu/softfloat-parts.c.inc | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fpu/softfloat-parts.c.inc b/fpu/softfloat-parts.c.inc
> index d0c43c28fb..cea854cdf1 100644
> --- a/fpu/softfloat-parts.c.inc
> +++ b/fpu/softfloat-parts.c.inc
> @@ -288,7 +288,7 @@ static void partsN(uncanon_normal)(FloatPartsN *p, float_status *s,
>               p->frac_lo &= ~round_mask;
>           }
>   
> -        exp = (p->frac_hi & DECOMPOSED_IMPLICIT_BIT) != 0;
> +        exp = (p->frac_hi & DECOMPOSED_IMPLICIT_BIT) != 0 && !fmt->m68k_denormal;

That does look like a correct change.  I'll fold it in.
Please let us know if you encounter anything else.


r~