Re: [REPOST] Invalid Code when reading from unaligned zero-sized array

[Eric Botcazou <ebotcazou at adacore dot com>]

> Instead I'd suggest to keep a 'last_field_array_p' flag that you can
> check at the end of the loop.

OK, I can do that.

> +      && TREE_CODE (TREE_TYPE (field)) == ARRAY_TYPE
> +      && !integer_zerop (TYPE_SIZE (TREE_TYPE (TREE_TYPE (field)))))
> +    return;
> +
> 
> Why does this exclude zero-sized element types?  That looks odd to me ;)

Because the size cannot change if you add zero to it, even multiple times?

> Btw, the loop already has
> 
>       if (TREE_CODE (TREE_TYPE (field)) == ERROR_MARK
> 
>           || (TYPE_MODE (TREE_TYPE (field)) == BLKmode
> 
>               && ! TYPE_NO_FORCE_BLK (TREE_TYPE (field))
>               && !(TYPE_SIZE (TREE_TYPE (field)) != 0
>                    && integer_zerop (TYPE_SIZE (TREE_TYPE (field)))))
> 
>           || ! tree_fits_uhwi_p (bit_position (field))
>           || DECL_SIZE (field) == 0
>           || ! tree_fits_uhwi_p (DECL_SIZE (field)))
> 
>         return;
> 
> which then is supposed to
> handle struct { struct { char c[8]; } a; } - but it seems to
> special-case zero-sized
> members again, thus struct { struct { char c[0]; } a; } would still be
> broken after your patch?

Probably indeed, this test was added by the same patch that added the 0-sized 
bitfield test in the RTL expander
   http://gcc.gnu.org/ml/gcc-patches/2003-10/msg00823.html
and that I think is obsolete now.  I can remove it.

> The issue Bernd raises is real as well, though we probably should fix this
> in a different way by using a different DECL_MODE based on the types
> size for asm register vars?

Really?  The number of open PRs for register variables not behaving properly 
at run time would make me think that this wouldn't be necessary bad...

> Btw, do you think we can recover from some of the now BLKmodes by
> having DECL_MODE != TYPE_MODE?

Well, if you want to do that, the straightforward solution is to keep the non-
BLKmode for the TYPE, put a flag on it and treat it as BLKmode when needed.

-- 
Eric Botcazou