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From: Andreas Schwab <schwab@linux-m68k.org>
References: <20170207005930.28327-1-laurent@vivier.eu>
	<20170207005930.28327-16-laurent@vivier.eu>
	<ca2f9cdf-c18c-bc8a-05a7-05b1d9748a48@twiddle.net>
Date: Thu, 16 Feb 2017 11:18:52 +0100
In-Reply-To: <ca2f9cdf-c18c-bc8a-05a7-05b1d9748a48@twiddle.net> (Richard
	Henderson's message of "Thu, 16 Feb 2017 12:46:35 +1100")
Message-ID: <87poiiv3oz.fsf@linux-m68k.org>
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Subject: Re: [Qemu-devel] [PATCH v3 15/16] target-m68k: add more FPU
 instructions
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To: Richard Henderson <rth@twiddle.net>
Cc: Laurent Vivier <laurent@vivier.eu>, qemu-devel@nongnu.org, Aurelien Jarno <aurelien@aurel32.net>

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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"And now for something completely different."