Re: Aligning stack offsets for spills

[Jakub Jelinek <jakub@redhat.com>]

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.

	Jakub