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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 12:51:04 -0700
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mitchalsup@aol.com (MitchAlsup1) writes:

> On Sun, 15 Sep 2024 17:07:58 +0000, Scott Lurndal wrote:
>
>> 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;
>>         uint64_t reserved_9_31               : 23;
>>         uint64_t sbe                         :  9;
>> #else
>>         uint64_t sbe                         :  9;
>>         uint64_t reserved_9_31               : 23;
>>         uint64_t dbe                         :  9;
>>         uint64_t reserved_41_63              : 23;
>> #endif
>>     } s;
>
> Which brings to mind a slight different but related bit-field issue.
>
> If one has an architecture that allows a bit-field to span a register
> sized container, how does one specify that bit-field in C ??
>
> So, assume a register contains 64-bits and we have a 17-bit field
> starting at bit 53 and continuing to bit 69 of a 128-bit struct.
> How would one "properly" specify this in C.

The 17-bit bitfied can be specified in the usual way.  Example:

   struct bitfield_example {
      unsigned  one : 32;
      unsigned  two : 20;
      unsigned  hmm : 17;
   };

An implementation is allowed to use up the last 12 bits of the
first 64-bit unit and the first 5 bits of the next 64-bit unit.
But, whether that happens or not is up to the implementation.
The bitfield for member 'hmm' could instead be put entirely in
the second 64-bit unit, with the last 12 bits of the first 64-bit
unit simply left as padding.  There is no standard way to force
it.