Re: [Qemu-devel] [PATCH v3 15/16] target-m68k: add more FPU instructions

[Andreas Schwab <schwab@linux-m68k.org>]

On Feb 16 2017, Richard Henderson <rth@twiddle.net> wrote:

> On 02/07/2017 11:59 AM, Laurent Vivier wrote:
>> +static long double floatx80_to_ldouble(floatx80 val)
>> +{
>> +    if (floatx80_is_infinity(val)) {
>> +            if (floatx80_is_neg(val)) {
>> +                    return -__builtin_infl();
>> +            }
>> +            return __builtin_infl();
>> +    }
>> +    if (floatx80_is_any_nan(val)) {
>> +            char low[20];
>> +            sprintf(low, "0x%016"PRIx64, val.low);
>> +
>> +            return nanl(low);
>> +    }
>> +
>> +    return *(long double *)&val;
>> +}
>
> This doesn't work except for x86 host.

Not even then.

> You ought to extract the mantissa, convert the 64-bit value to
> long-double, and use ldexpl to scale the result for the exponent.
>
> Similarly converting the other way use frexpl and ldexpl.

There is no guarantee that the host long double has the same range and
precision as floatx80.

Andreas.

-- 
Andreas Schwab, schwab@linux-m68k.org
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