C bitfields guarantees

[Timon Gehr]

On 7/7/24 06:47, Walter Bright wrote:
> On 7/6/2024 8:50 PM, Steven Schveighoffer wrote:
>> Let's take another example:
>>
>> ```c
>> struct U {
>>    unsigned int x;
>>    unsigned long long y: 30;
>>    unsigned long long z: 34;
>> }
>>
>> struct U2 {
>>    unsigned int x;
>>    unsigned long long y: 34;
>>    unsigned long long z: 30;
>> }
>> ```
> 
> Simple solution:
> ...

You said: Same type, same alignment. It's clearly not true in Steven's 
example. It seems alignment depends on bit width.

Also consider this:

```d
struct S{
     uint x;
     ulong y:30;
     ulong z:34;
}
pragma(msg, S.y.offsetof, " ", S.y.alignof); // 4LU 8LU

The offset of `y` does not even respect its alignment! This is insanity.

It also happens with `uint`:

```d
struct S{
     ushort x;
     uint y:16;
}
pragma(msg, S.y.offsetof, " ", S.y.alignof); // 2LU 4LU
```

I.e., "stick to `int`/`uint` bitfields, things will be predictable" is 
not even true. They may be laid out differently based on what's before them.

> ```
> struct U {
>      unsigned int x;
>      unsigned int y:30;
>      unsigned long long z:34;
> }
> ```
> 
> or:
> 
> ```
> struct U2 {
>      unsigned int x;
>      unsigned int pad;
>      unsigned long long y:30;
>      unsigned long long z:34;
> }
> ```
> 
> depending on which layout is desired. This is simple,

If it is simple, you should have no trouble stating how it works 
completely in a couple sentences.

> predictable, and 
> portable. It's not going to be a mystery to anyone reading the code - 
> it's eminently readable.
> ...

Walter, this is frustrating. It is only obvious to you because having 
reverse-engineered and implemented it, you already know how it works. 
Note that the things you were saying earlier suggested it would actually 
work differently in Steven's example. I hope you understand that this is 
confusing. I am as a result now not sure whether what you stated is the 
full truth, or it is still some inadmissible simplification that glosses 
over some further dragons.

Also, I hope `.offsetof % .alignof != 0` is just a bug in your bitfield 
implementation.