[Aldor-l] alignof

[Stephen Watt]

On Fri, Mar 21, 2008 at 11:22:36PM +0100, Pippijn van Steenhoven wrote:
> First of all, it is not important whether or not there is an example. The
> compiler is free to add padding to a struct { char a; short b; }; and then the
> code is broken. On x86, it would make sense to align the short to 4 bytes and
> the struct itself, too, leading to an alignof (that struct) of 0. I can
> probably live with it as long as no issues arise. The problem is really just,
> the code is broken and it would be good if a better alternative was found.
> 
> -- 
> Pippijn van Steenhoven


On 3/21/08, Pippijn van Steenhoven wrote:
>
>  I keep wondering what to do with this:
>
>  # define alignof(T)             (sizeof(struct {char a; T b;}) - sizeof(T))
>                
>  as it's incorrect. When the compiler adds padding to the structure,
> this macro may return a negative value, breaking everything.

-----------------------------------------------------

Pippijn,

As Gabriel pointed out, we can have conditional implementation in terms of 
__alignof__.  There will be some platforms without it, however, so this 
discussion remains useful.

The reason I ask for an example where it is wrong is because I don't understand
your statement that the macro can return a negative value.  

Can you please explain?

I do not recall whether the C standard allows the compiler to re-order 
fields of a structure.  

If the compiler does not re-order fields, then I believe 
sizeof(struct {char a; T b; }) is strictly greater than sizeof(T) 
so the value of the macro will always be positive. 

If the compiler can re-order fields, then I believe 
sizeof(struct {char a; T b; }) is greater than or equal to sizeof(T) 
so the value of the macro will always be non-negative, but possibly zero.

So if this is what you are worried about, it could probably be fixed as
something along the lines of


if have __alignof__
  use that
else
  correct alignof = max(old alignof, 1)


-- Stephen