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Re: avoid alignment of static variables affecting stack's
- From: "Jan Beulich" <JBeulich at suse dot com>
- To: "Richard Biener" <richard dot guenther at gmail dot com>, "Jakub Jelinek" <jakub at redhat dot com>
- Cc: "GCC Patches" <gcc-patches at gcc dot gnu dot org>, "H.J. Lu" <hjl dot tools at gmail dot com>,"Jeff Law" <law at redhat dot com>
- Date: Fri, 24 Oct 2014 11:12:26 +0100
- 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> <20141024091849 dot GV10376 at tucnak dot redhat dot com> <CAFiYyc0CELJQZ07JvU7NYdBGDExP99cLMEkXECb_vQTA+JCBng at mail dot gmail dot com>
>>> On 24.10.14 at 11:52, <email@example.com> wrote:
> On Fri, Oct 24, 2014 at 11:18 AM, Jakub Jelinek <firstname.lastname@example.org> wrote:
>> 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
> I stand corrected then.
So am I to conclude then that I need to take out the hard register
check in order for this to be accepted?