Re: [RFA 4.7/4.6] Re: [PATCH v2] Target-specific limits on vector alignment

[Richard Earnshaw <rearnsha at arm dot com>]

On 07/08/12 16:04, Richard Guenther wrote:
> On Tue, Aug 7, 2012 at 4:56 PM, Ulrich Weigand <uweigand@de.ibm.com> wrote:
>> Richard Guenther wrote:
>>> On Fri, Jul 27, 2012 at 5:24 PM, Ulrich Weigand <uweigand@de.ibm.com> wrote:
>>>> ChangeLog:
>>>>
>>>>         * target.def (vector_alignment): New target hook.
>>>>         * doc/tm.texi.in (TARGET_VECTOR_ALIGNMENT): Document new hook.
>>>>         * doc/tm.texi: Regenerate.
>>>>         * targhooks.c (default_vector_alignment): New function.
>>>>         * targhooks.h (default_vector_alignment): Add prototype.
>>>>         * stor-layout.c (layout_type): Use targetm.vector_alignment.
>>>>         * config/arm/arm.c (arm_vector_alignment): New function.
>>>>         (TARGET_VECTOR_ALIGNMENT): Define.
>>>>
>>>>         * tree-vect-data-refs.c (vect_update_misalignment_for_peel): Use
>>>>         vector type alignment instead of size.
>>>>         * tree-vect-loop-manip.c (vect_do_peeling_for_loop_bound): Use
>>>>         element type size directly instead of computing it from alignment.
>>>>         Fix variable naming and comment.
>>>>
>>>> testsuite/ChangeLog:
>>>>
>>>>         * lib/target-supports.exp
>>>>         (check_effective_target_vect_natural_alignment): New function.
>>>>         * gcc.dg/align-2.c: Only run on targets with natural alignment
>>>>         of vector types.
>>>>         * gcc.dg/vect/slp-25.c: Adjust tests for targets without natural
>>>>         alignment of vector types.
>>
>>
>>>> OK for mainline?
>>>
>>> Ok.
>>
>> Would it be OK to backport this to 4.7 and possibly 4.6?
>>
>> This patch represents a change in the ABI on ARM (the alignment could
>> potentially affect struct member offsets, for example), which could
>> conceivably cause incompatibilities with code compiled with older
>> versions of GCC.  We had originally decided we nevertheless want to
>> implement this change on ARM, because:
>>
>> - GCC is now in compliance with the platform ABI document
>> - the old code could actually be buggy since GCC *assumed* 16-byte
>>   alignment that wasn't actually guaranteed
>> - code actually affected by this change ought to be rare (code using
>>   NEON vector types is rare in the first place, code using *structure
>>   members* of such types is even rarer, and code using such structures
>>   in cross-module APIs seems to be extremely rare)
>>
>> Nevertheless, given we do want to make this change, it would be best
>> to roll it out as quickly as possible, to minimize the time period
>> where people might use old (not yet fixed) compilers to generate
>> non-ABI-compliant binaries.  Thus, we think it would be good for
>> the change to be applied to all still open release branches as well.
>>
>> (Note that while the patch contains changes to common code, those
>> should be no-ops for all targets that do not implement the new hook.)
> 
> I'll defer the decision to the target maintainers.  But please double-check
> for any changes in the vectorizer parts when backporting to 4.6.
> 

My inclination is to back-port this to all maintained branches (for
consistency).

However, it does need to be release-noted.

R.