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Re: avoid alignment of static variables affecting stack's
- From: Jakub Jelinek <jakub at redhat dot com>
- To: Richard Biener <richard dot guenther at gmail dot com>
- Cc: Jan Beulich <JBeulich at suse dot com>, Jeff Law <law at redhat dot com>, GCC Patches <gcc-patches at gcc dot gnu dot org>, "H.J. Lu" <hjl dot tools at gmail dot com>
- Date: Fri, 24 Oct 2014 11:18:49 +0200
- Subject: Re: avoid alignment of static variables affecting stack's
- Authentication-results: sourceware.org; auth=none
- References: <5448BCA30200007800041508 at mail dot emea dot novell dot com> <20141023065015 dot GX10376 at tucnak dot redhat dot com> <5448C5CB0200007800041532 at mail dot emea dot novell dot com> <5449454A dot 9050502 at redhat dot com> <544A31900200007800041C83 at mail dot emea dot novell dot com> <CAFiYyc3KEC1X29vcTO3OVTV+_XO=gymWdadRROnFs3OWuL3KwA at mail dot gmail dot com>
- Reply-to: Jakub Jelinek <jakub at redhat dot com>
On Fri, Oct 24, 2014 at 11:10:08AM +0200, Richard Biener wrote:
> >> 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.
>From what I remember, at RA time you already have to know conservatively
that you'll want to do dynamic stack realignment and what the highest needed
alignment will be, so various parts of expansion etc. conservatively compute
what will be needed. I think that is because you e.g. need to reserve some
registers (vDRAP, etc.) if doing dynamic realignment.
If you conservatively assume you'll need dynamic stack realignment and after
RA you find you really don't need it, there are some optimizations in
prologue threading where it attempts to at least decrease amount of
unnecessary code, but the harm has already been done.
Might be that with LRA perhaps this could be changed and not conservatively
assume more alignment than proven to be needed, but such code isn't there I