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From: David Brown <david.brown@hesbynett.no>
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Date: Thu, 11 Jul 2024 12:46:18 +0200
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On 11/07/2024 04:51, Lawrence D'Oliveiro wrote:
> On Wed, 10 Jul 2024 03:16:18 -0400, James Kuyper wrote:
> 
>> On 7/9/24 20:57, Lawrence D'Oliveiro wrote:
>>
>>> On Sat, 6 Jul 2024 21:34:29 -0400, James Kuyper wrote:
>>>
>>>> On many platforms, if _Alignof(type) is less than the word size, then
>>>> a C pointer to that type is implemented as the combination of the
>>>> machine address of the correct word, combined with an offset within
>>>> that word of the first byte of that object.
>>>
>>> Which is a terrific idea, except it cannot be carried to its logical
>>> conclusion (addressing of arbitrarily-aligned dynamically-defined
>>> bitfields) because of the requirement in the C spec that the size of a
>>> “byte” be at least 8 bits.
>>
>> I will grant you that I failed to mention the fact that this is a
>> feasible way of implementing C only on platforms with a word size of 16
>> bits or longer.
> 
> Don’t you think C needs a better way of handling bitfields than shift-and-
> mask? Many architectures have bitfield instructions, but C cannot easily
> make use of them without the ability to have arbitrarily-bit-aligned
> pointers.

There are pros and cons of different ways to support bitfields in a 
language, and there are certainly aspects of C's bitfields that many 
people think are less than ideal.

But (good) C compilers have no problem using bitfield instructions on 
architectures that have these, so that at least is not an issue.