Re: Aligning stack offsets for spills

[Jeff Law <jeffreyalaw@gmail.com>]

On 6/8/2021 8:55 AM, Jakub Jelinek wrote:
> On Tue, Jun 08, 2021 at 08:47:26AM -0600, Jeff Law wrote:
>>> Why is the machinery involving STACK_SLOT_ALIGNMENT and
>>> spill_slot_alignment() (for spilling) or get_stack_local_alignment() (for
>>> backing stack slots) not working for you?  If everything is setup
>>> correctly the input alignment to try_fit_stack_local ought to be correct
>>> already.
>> We don't need the MEM as a whole aligned, just the offset in the address
>> calculation due to how we encode those instructions.  If I've read that code
>> correctly, it would arrange for a dynamic realignment of the stack  so that
>> it could then align the slot. None of that is necessary for us and we'd like
>> to avoid forcing the dynamic stack realignment.  Or did I misread the code?
> I think dynamic stack realignment is done only on x86, no other backend has
> that support, on all the other arches larger alignments are done
> in expand_stack_vars by effectively performing __builtin_alloca_with_align
> for the block containing all such variables.
> So I'd the the functions Michael mentioned shouldn't be doing dynamic stack
> realignment, though perhaps by pretending the vars have higher alignment
> might be recorded in MEM_ALIGN and perhaps might result in wrong-code if
> something will try to e.g. test if least significant bits of certain MEM
> address are 0.
Hmm, I thought we'd do a dynamic realignment, if that's not the case 
then we've got another approach to consider.

Thanks everyone...

jeff