Re: [PATCH] DATA_ALIGNMENT vs. DATA_ABI_ALIGNMENT (PR target/56564)

[Jakub Jelinek <jakub@redhat.com>]

On Fri, Jun 14, 2013 at 08:12:02PM +0930, Alan Modra wrote:
> > As for the 
> > typedef int vec_align __attribute__ ((vector_size(16), aligned(32)));                                                                            
> > vec_align x = { 0, 0, 0, 0 };                                                                                                                    
> > changes, that is ABI changing bugfix, so the question is, are you fine with
> > breaking the ABI (between 4.8 and 4.9, or if you wanted to backport it to
> > 4.8 too (I certainly plan to backport the non-ppc DATA_ABI_ALIGNMENT changes
> > to 4.8.2, already am using it in our compilers))?  The other option is
> > to fix the ABI, but keep things backwards ABI compatible.  That would be
> > done by decreasing the alignment as it used to do before in DATA_ABI_ALIGNMENT,
> > and increasing it to the desirable level only in DATA_ALIGNMENT.  That has
> > the effect that when emitting the decls into assembly e.g. the above will
> > now be correctly 32 byte aligned, but accesses to such decl in compiler
> > generated code will only assume that alignment if
> > decl_binds_to_current_def_p, otherwise they will keep assuming the old
> > (broken) lowered alignment.  At least for 4.8 backport IMHO that would be a
> > better idea (but of course would need big comment explaning it).
> 
> I see your point, but for there to be a real problem we'd need
> a) A library exporting such a type with (supposed) increased
>    alignment, and,
> b) gcc would need to make use of the increased alignment.
> 
> (a) must be rare or non-existent or you'd think we would have had a
> bug report about lack of user alignment in vector typedefs.  The code
> has been like this since 2001-11-07, so users have had a long time to
> discover it.  (Of course, this is an argument for just ignoring the
> bug too.)

It doesn't have to be an exported symbol from a library, it is enough to
compile some objects using one compiler and other objects using another
compiler, then link into the same library.

> (b) doesn't happen in the rs6000 backend as far as I'm aware.  Do you
> know whether there is some optimisation based on alignment in generic
> parts of gcc?  A quick test like

Tons of them, the DECL_ALIGN value is used say by get_pointer_alignment,
vectorizer assumptions, is added to MEM_ATTRS, so anything looking at
alignment in RTL can optimize too.

> typedef int vec_align __attribute__ ((vector_size(16), aligned(32)));
> vec_align x = { 0, 0, 0, 0 };
> 
> long f1 (void)
> {
>   return (long) &x & -32;
> }

Try (long) &x & 31; ?  That &x & -32 not being optimized into &x
is guess a missed optimization.

Consider if you put:
typedef int vec_align __attribute__ ((vector_size(16), aligned(32)));
vec_align x = { 0, 0, 0, 0 };
into one TU and compile with gcc 4.8.1, then
typedef int vec_align __attribute__ ((vector_size(16), aligned(32)));
extern vec_align x;

long f1 (void)
{
  return (long) &x & 31;
}
in another TU and compile with gcc trunk after your patch.  I bet
it will be optimized into return 0; by the trunk + your patch compiler,
while the alignment will be actually just 16 byte.

	Jakub