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References: <20170207005930.28327-1-laurent@vivier.eu>
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From: Richard Henderson <rth@twiddle.net>
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Date: Thu, 16 Feb 2017 12:46:35 +1100
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Subject: Re: [Qemu-devel] [PATCH v3 15/16] target-m68k: add more FPU
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To: Laurent Vivier <laurent@vivier.eu>, qemu-devel@nongnu.org
Cc: Aurelien Jarno <aurelien@aurel32.net>

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.

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.


r~