> GCC supports an "aligned" attribute to specify a minimum alignment for
> types/objects. However, if an object is allocated on the stack and
> its alignment exceeds the "preferred stack boundary", then GCC 4.2
> silently ignores the alignment.
[...]
> Since newer GCC properly support overly aligned stack allocations, it
> seems prudent to at least make base GCC emit a warning if its going to
> ignore an alignment request.
[...]
> I verified this doesn't break an amd64 kernel build, but I haven't had
> time to look beyond that. Sharing in case anyone's interested and/or
> wants to test further themselves.
I love this. I have verified that kernels for all our gcc4 platforms
still build with this diff.
Miod
> Index: gcc/cfgexpand.c
> ===================================================================
> RCS file: /home/matthew/cvs-mirror/cvs/src/gnu/gcc/gcc/cfgexpand.c,v
> retrieving revision 1.4
> diff -u -p -r1.4 cfgexpand.c
> --- gcc/cfgexpand.c 6 May 2014 23:32:34 -0000 1.4
> +++ gcc/cfgexpand.c 20 Jun 2014 22:55:53 -0000
> @@ -159,8 +159,10 @@ get_decl_align_unit (tree decl)
>
> align = DECL_ALIGN (decl);
> align = LOCAL_ALIGNMENT (TREE_TYPE (decl), align);
> - if (align > PREFERRED_STACK_BOUNDARY)
> + if (align > PREFERRED_STACK_BOUNDARY) {
> + warning (0, "ignoring alignment for stack allocated %q+D", decl);
> align = PREFERRED_STACK_BOUNDARY;
> + }
> if (cfun->stack_alignment_needed < align)
> cfun->stack_alignment_needed = align;
>
>
