second draft: add Bitfields to D

[Timon Gehr]

On 5/6/24 09:14, Walter Bright wrote:
> On 5/5/2024 2:08 AM, Timon Gehr wrote:
>> GCC 11.4.0, clang 14.0.0:
>>
>> ```c
>> #include <stdio.h>
>> struct __attribute__((packed)) S{
>>      long long x:8;
>> };
>> int main(){
>>      printf("%ld\n",sizeof(long long)); // 8
>>      printf("%ld\n",sizeof(struct S)); // 1
>> }
>> ```
> 
> The sizeof there, in both cases, is giving the size in bytes of the 
> memory object the field is a subset of.
> ...

This is C and neither sizeof is on a memory object, they are both on 
types. sizeof on x gives a compile error. However, with the DIP, given 
that you implement packed bitfields in DMD, when importing an example 
like this one, `x.sizeof` would be eight times as big as the size of the 
`struct` it is a part of.

> 
>> Maybe at least redefine `.sizeof` to give the size of the underlying 
>> storage unit for a bitfield.
> 
> That's what it's doing in the example.
> ...

Well, the DIP now says `bitfield.sizeof` is `typeof(bitfield).sizeof`. 
Unless I misunderstand and `typeof(x)` is not `long long`, this should 
not be the case in this example, because a `long long` is longer than 
the memory location it is packed into in this case. (I think this is 
another broken invariant.)

> BTW, I didn't implement packed bitfields in ImportC. It never occurred 
> to me :-/ I suppose it should get a bugzilla issue.
> 
> https://issues.dlang.org/show_bug.cgi?id=24538

Well, that will help, but the point was the C standard does not give the 
guarantees you assumed to hold earlier, and in practice it in fact does 
not hold, as in this example.