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Re: avoid alignment of static variables affecting stack's


On Fri, Oct 24, 2014 at 11:01 AM, Jan Beulich <JBeulich@suse.com> wrote:
>>>> On 23.10.14 at 20:13, <law@redhat.com> wrote:
>> On 10/23/14 01:09, Jan Beulich wrote:
>>>>>> On 23.10.14 at 08:50, <jakub@redhat.com> wrote:
>>>> On Thu, Oct 23, 2014 at 07:30:27AM +0100, Jan Beulich wrote:
>>>>> Function (or more narrow) scope static variables (as well as others not
>>>>> placed on the stack) should also not have any effect on the stack
>>>>> alignment. I noticed the issue first with Linux'es dynamic_pr_debug()
>>>>> construct using an 8-byte aligned sub-file-scope local variable.
>>>>>
>>>>> According to my checking bad behavior started with 4.6.x (4.5.3 was
>>>>> still okay), but generated code got quite a bit worse as of 4.9.0.
>>>>
>>>> If the static/external var has BLKmode, then perhaps it is safe, but I
>>>> wonder about other vars, say vectors etc.  Such vars are most likely
>>>> loaded from their memory location, and if for some reason that needs to be
>>>> spilled again, stack realignment would not be able to do that.
>>>> Or do we inspect the IL and for any pseudos with modes needing larger
>>>> alignment we adjust the dynamic stack realignment fields?
>>>
>>> I don't know, but it would seem to me that this ought to happen
>>> anyway: If the pseudo holds the result of some computation
>>> other than a simple load from memory and needs spilling, the same
>>> would apply afaict.
>>
>> For something in static storage, this seems OK.  However, I think a hard
>> register variable ought to be left alone -- even if we can't spill it to
>> a stack slot today, there's a reasonable chance we might add that
>> capability in the future.
>
> Hmm, but then wouldn't it need to be the code generating the spill
> that's responsible for enforcing suitable alignment? I can certainly
> re-submit without the hard register special cased (as it would still
> fix the original issue I'm seeing), but it feels wrong to do so.

Yes, ISTR the spilling code is supposed to update the required
stack alignment.  After all the RA decision might affect required
alignment of spills.

Richard.

> Jan
>


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