Re: avoid alignment of static variables affecting stack's

[Jeff Law <law at redhat dot com>]

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.
>
> Furthermore, shouldn't there be an existing test case for what you
> describe, and hence me not seeing regressions with the patch in
> place should be sufficient proof of there not being an issue?

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.

The testsuite is not exhaustive, and even if it were today, it may not 
be tomorrow as it can't anticipate what might change.



jeff