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From: Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups: comp.arch
Subject: Re: Computer architects leaving Intel...
Date: Sun, 15 Sep 2024 12:37:28 -0700
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scott@slp53.sl.home (Scott Lurndal) writes:

> Robert Finch <robfi680@gmail.com> writes:
>
>> On 2024-09-15 12:09 p.m., David Brown wrote:
>>
>>>> In addition, some padding-related things can be defined by Standard
>>>> itself.  Not in this particular case, but, for example, it could be
>>>> defined that when field of one integer type is immediately followed by
>>>> another field of integer type with the same or narrower width then
>>>> there should be no padding in-between.
>>>>
>>>
>> What about bit-fields in a struct?  I believe they are usually packed.  In
>> case its for something like an I/O device.
>
> That's a bit more complicated as it depends on the target byte-order.
>
> e.g.
>
>     struct GIC_ECC_INT_STATUSR_s {
> #if __BYTE_ORDER == __BIG_ENDIAN
>         uint64_t reserved_41_63              : 23;
>         uint64_t dbe                         :  9; /**< R/W1C/H - RAM ECC DBE detected. */
>         uint64_t reserved_9_31               : 23;
>         uint64_t sbe                         :  9; /**< R/W1C/H - RAM ECC SBE detected. */
> #else
>         uint64_t sbe                         :  9;
>         uint64_t reserved_9_31               : 23;
>         uint64_t dbe                         :  9;
>         uint64_t reserved_41_63              : 23;
> #endif
>     } s;

Probably many people know that this code depends on an
implementation-defined extension (allowing uint64_t as
the type of a bitfield) and is not guaranteed to be
portable.  Using 'unsigned' instead would be portable
(assuming typical 32-bit ints, etc).