[LLVMdev] Structure Types and ABI sizes

[David A. Greene]

Jochen Wilhelmy <j.wilhelmy at arcor.de> writes:

> struct I {
>    int a;
>    char b;
> };
>
> struct J : I {
>    char c;
> };
>
> Dave said that this translates to
>
> %I = type { i32, i8, i16 }
> %J = type { %I, i8, i16 }

It translates to that in OUR compiler.  It's not the only answer.

> because the frontend has to communicate the ABI to llvm since llvm is 
> language agnostic.

Correct.

> What I really wonder is why it isn't
>
> %I = type { i32, i8 }
> %J = type { %I, i16, i8 }
>
> because llvm at least knows alignment rules by
>
> target  datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16...
>
> Therefore llvm has no other choice than assigning %I a size of 8
> since an array may consist of %I elements and size of 5 would violate
> the aligment of the i32 member.

I can't quite parse this.  %I doesn't get "assigned" a size by anyone.
Do you meant the size of struct I is eight bytes?  Yes, that's true.

> If the ABI requires that member c has an offset of 8 instead of 5 then
> of course a padding behind %I is necessary in %J.

Yes, the padding is required.  I believe %J = type { %I, i16, i8 } would
work just as well as long as %I = type { i32, i8 } as in your example.

Our frontend is far from "perfect" in the sense of aesthetics.  :)

                          -Dave