On Fri, Jan 9, 2009 at 4:59 PM, Ian Lance Taylor <i...@google.com> wrote: > "Richard Guenther" <richard.guent...@gmail.com> writes: > >> On Fri, Jan 9, 2009 at 4:31 PM, Ian Lance Taylor <i...@google.com> wrote: >>> "Richard Guenther" <richard.guent...@gmail.com> writes: >>> >>>>> There are many ways to align data without exposing it in the >>>>> ABI--e.g., the alignment of a global array is not part of the ABI, in >>>>> that nothing breaks if the alignment is increased. Also, there are >>>>> many programs which simply don't care about an external ABI. >>>> >>>> I'd say if programs want to use vectorization they should use >>>> alignof (vector_type) instead of a magic attribute((aligned(max))). >>> >>> It's not that programs explicitly want to use vectorization. It's >>> that auto-vectorization happens on ordinary scalar types, and programs >>> want to permit that to happen without worrying about the details. >> >> Sure. The vectorizer increases alignment of variables for this reason. >> But as people cannot adjust their memory allocator behavior they >> cannot use attribute((aligned(max))) for that either. Or do you suggest >> they should stick that on every decl? (which is what the autovectorizer >> does) > > People can and do adjust their memory allocator. Doing that in > portable code requires something like attribute((aligned)).
Well. As HJ says, an attribute((aligned)) is not the best way to communicate this. You would need sth like char a[] __attribute__((aligned)); and then query alignof (a) for your alignment bit-twiddling inside your allocator. Certainly possible, but also "interesting" use of attribute((aligned)) ;) Richard.
