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From: Peter Maydell <peter.maydell@linaro.org>
Date: Thu, 18 Jan 2018 14:31:25 +0000
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Subject: Re: [Qemu-devel] [PATCH v2 10/20] fpu/softfloat: define decompose
 structures
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To: =?UTF-8?Q?Philippe_Mathieu=2DDaud=C3=A9?= <f4bug@amsat.org>
Cc: =?UTF-8?B?QWxleCBCZW5uw6ll?= <alex.bennee@linaro.org>, Francisco Iglesias <frasse.iglesias@gmail.com>, Richard Henderson <richard.henderson@linaro.org>, Laurent Vivier <laurent@vivier.eu>, QEMU Developers <qemu-devel@nongnu.org>, Andrew Dutcher <andrew@andrewdutcher.com>, bharata@linux.vnet.ibm.com, Aurelien Jarno <aurelien@aurel32.net>

On 18 January 2018 at 14:26, Philippe Mathieu-Daud=C3=A9 <f4bug@amsat.org> =
wrote:
>
>
> Le 18 janv. 2018 10:09 AM, "Alex Benn=C3=A9e" <alex.bennee@linaro.org> a =
=C3=A9crit :
>
>
> Philippe Mathieu-Daud=C3=A9 <f4bug@amsat.org> writes:
>>> +typedef struct {
>>> +    uint64_t frac   : 64;
>>
>> I think this does not work on LLP64/IL32P64 model.
>>
>> Should we add a check in ./configure and refuse to build on IL32P64
>> model? This would be safer IMHO.
>>
>>> +    int exp         : 32;
>>> +    float_class cls : 8;
>>> +    int             : 23;
>>> +    bool sign       : 1;
>>
>> checking on "ISO/IEC 14882:1998" 9.6 Bit-fields:
>>
>> Alignment of bit-fields is implementation-defined. Bit-fields are packed
>> into some addressable allocation unit. [Note: bit-fields straddle
>> allocation units on some machines and not on others. Bit-fields are
>> assigned right-to-left on some machines, left-to-right on others. ]
>>
>> I'd still write it:
>>
>>       int             :23, sign :1;
>>
>>> +} decomposed_parts;
>
> I think rather than stuff it into bit fields we can just leave it up to
> the compiler?
>
>
> Yep, my only worry here is the IL32P64 model, if we care.

I don't think we care much about IL32P64, but the code should
still work there, right? It doesn't actually make any assumptions
about bitfield layout.

I think I agree that we shouldn't use bitfields here if we don't
need to, though.

thanks
-- PMM