Alignment of unions

[Don]

Stewart Gordon wrote:
> Don wrote:
>> Stewart Gordon wrote:
>>> Don wrote:
> <snip>
>>>> I'm not sure why you think unions are so different to structs. They 
>>>> are identical in most respects -- including requirements for 
>>>> alignment of members.
>>>
>>> I still don't know what you mean.
>>
>> You're acting as if there's a big difference between unions and 
>> structs, and there isn't. Read the spec, the only differences are at 
>> construction, and struct literals.
> 
> No, you have to add to that the differences inherited from C.  Moreover,
> what difference could possibly be bigger than that between the basic 
> essence of structs and the basic essence of unions?

In the compiler, struct and union share almost all of their code.
Basically, a union is just a struct where all the elements are on top of 
each other. Which is a tiny difference compared (say) to the difference 
between structs and classes. For example, this works:

union foo(T) {
public:
    static int bar(int x) { return x*2; }
}

void main()
{
   assert (foo!(int).bar(7)==14);
}


> 
> http://www.digitalmars.com/d/1.0/struct.html
> "They work like they do in C, with the following exceptions:"
> 
> It's true that "alignment can be explicitly specified" is in there.  But 
> I'm not sure that Walter really meant it to apply to unions as well as 
> structs.  Especially given that it makes no comment (that I've found) on 
> what align is meant to do in a union. 

In a struct, align(4) int X; means, pad with zero bytes until the 
address you're up to is a multiple of 4, then put the int at that 
address. The address for the next element is the first byte past the int.
In a union, align(4) int X; means, pad with zero bytes until the address 
you're up to is a multiple of 4, then put the int at that address.  The 
address for the next element is the first byte of the union.

  Moreover,
> 
> http://www.digitalmars.com/d/1.0/attribute.html#align
> "Specifies the alignment of struct members."
> 
>>>>> If you want a union to have a certain alignment relative to a 
>>>>> struct in which it's contained, in what cases is it not sufficient 
>>>>> to put the align attribute on the union's instance in the struct?
>>>>
>>>> In cases where you don't know what the containing struct is. The 
>>>> union may just be a type returned from a template, for example.
>>>
>>> If you don't know what the containing struct is, you probably also 
>>> don't know what member alignment that struct requires. 
>>
>> Yes you do. The union doesn't control the alignment of the surrounding 
>> struct. It only controls the alignment of itself inside that struct.
> 
> I thought that was meant to be controlled by an align applied to the 
> union instance as a member of the struct, rather than by an align 
> applied to the union itself.  Only in the case of anonymous unions are 
> they one and the same.
> 
>>> The person who creates the struct, OTOH, does know.
>>
>> They do NOT know. Not without probing every member in the union.
> 
> ?? ISTM all they really need to know is the overall size of the union, 
> which is equal to the size of the union's largest member (which is easy 
> to find out).
> 
>> So why are you trying to do that person's
>>> job?
>>
>> No, you know what alignment the member requires.
> <snip>
> 
> What are the cases in which a member _requires_ a certain alignment?

Static arrays, eg float[4]. Must be aligned if you want to use the 
movaps instruction (which is 4X faster than the unaligned instruction).

Alignment requirements are rare, even for structs.

> 
> OK, so there's one case: pointers in order to make sure the GC works 
> properly.  But this is absolute alignment.  Maybe it's just me, but 
> given that talk of alignment is nearly always in relation to structs, 
> I'd made it out to be talking about relative alignment.  Maybe the spec 
> just needs to be clearer....
> 
> Stewart.

Probably. Unions tend to get forgotten.