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From: Peter Maydell <peter.maydell@linaro.org>
Date: Thu, 18 Jan 2018 15:17:44 +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: Richard Henderson <richard.henderson@linaro.org>, =?UTF-8?B?QWxleCBCZW5uw6ll?= <alex.bennee@linaro.org>, Francisco Iglesias <frasse.iglesias@gmail.com>, 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 14:59, Philippe Mathieu-Daud=C3=A9 <f4bug@amsat.org> =
wrote:
> My comment was for a previous line:
>
>   uint64_t frac   : 64;
>
> I don't have enough compiler knowledge to be sure how this bitfield is
> interpreted by the compiler. I understood the standard as bitfields are
> for 'unsigned', and for IL32 we have sizeof(unsigned) =3D 32, so I wonder
> how a :64 bitfield ends (bits >=3D 32 silently truncated?).

Defining a 64-bit bitfield is a bit pointless (why not just use
uint64_t?) but there's nothing particularly different for IL32P64 here.
The spec says the underlying type is _Bool, signed int, unsigned
into, or an implementation defined type. For QEMU's hosts 'int'
is always 32 bits, so if gcc and clang allow bitfields on a
64-bit type like uint64_t (as an impdef extension) then they
should work on all hosts. (In any case it needs to either work
or give a compiler error, silent truncation isn't an option.)

thanks
-- PMM