Re: [RFC] Target-specific limits on vector alignment

[Richard Guenther <richard dot guenther at gmail dot com>]

On Mon, Jun 11, 2012 at 4:38 PM, Richard Earnshaw <rearnsha@arm.com> wrote:
> On 11/06/12 15:17, Richard Guenther wrote:
>> On Mon, Jun 11, 2012 at 3:16 PM, Richard Earnshaw <rearnsha@arm.com> wrote:
>>> The ARM ABI states that vectors larger than 64 bits in size still have
>>> 64-bit alignment; never-the-less, the HW supports alignment hints of up
>>> to 128-bits in some cases and will trap in a vector has an alignment
>>> that less than the hint. ?GCC currently hard-codes larger vectors to be
>>> aligned by the size of the vector, which means that 128-bit vectors are
>>> marked as being 128-bit aligned.
>>>
>>> The ARM ABI unfortunately does not support generating such alignment for
>>> parameters passed by value and this can lead to traps at run time. ?It
>>> seems that the best way to solve this problem is to allow the back-end
>>> to set an upper limit on the alignment permitted for a vector.
>>>
>>> I've implemented this as a separate hook, rather than using the existing
>>> hooks because there's a strong likelihood of breaking some existing ABIs
>>> if I did it another way.
>>>
>>> There are a couple of tests that will need some re-working before this
>>> can be committed to deal with the fall-out of making this change; I'll
>>> prepare those changes if this patch is deemed generally acceptable.
>>
>> Hm. ?Where would you use that target hook?
>>
>
> Doh!
>
> It was supposed to be in the patch set... :-(
>
> in layout_type(), where the alignment of a vector is forced to the size
> of the vector.

Ok.

+	/* Naturally align vectors, but let the target set an upper
+	   limit.  This prevents ABI changes depending on whether or
+	   not native vector modes are supported.  */
+	TYPE_ALIGN (type)
+	  = targetm.vector_alignment (type, tree_low_cst (TYPE_SIZE (type), 0));

The type argument or the size argument looks redundant.  Note that we still
can have such vector "properly" aligned, thus the vectorizer would need to
use build_aligned_type also if it knows the type is aligned, not only when
it thinks it is misaligned.  You basically change the alignment of the "default"
vector type.

Richard.

> R.