This is the mail archive of the
gcc@gcc.gnu.org
mailing list for the GCC project.
Re: [RFD] Using the 'memory constraint' trick to avoid memory clobber doesn't work
- From: Richard Biener <richard dot guenther at gmail dot com>
- To: Hans-Peter Nilsson <hp at bitrange dot com>
- Cc: David Wohlferd <dw at limegreensocks dot com>, y dot gribov at samsung dot com, "gcc at gcc dot gnu dot org" <gcc at gcc dot gnu dot org>, Gerald Pfeifer <gerald at pfeifer dot com>
- Date: Thu, 13 Nov 2014 15:02:04 +0100
- Subject: Re: [RFD] Using the 'memory constraint' trick to avoid memory clobber doesn't work
- Authentication-results: sourceware.org; auth=none
- References: <5422761C dot 2010507 at LimeGreenSocks dot com> <CAFiYyc17coKQ71HcupK5ciabRzzrcTeN2xPJ3zsTPs5BboqSyg at mail dot gmail dot com> <5423C5F2 dot 80509 at samsung dot com> <542541D9 dot 400 at LimeGreenSocks dot com> <CAFiYyc0gqKpCYGi3eLLuo8uPuvN+mDCC7Qr2X0yinNP3AEgedQ at mail dot gmail dot com> <542E4507 dot 7060200 at LimeGreenSocks dot com> <CAFiYyc0o3t1EkZ9VmRHxDjTiJvem+_qF4=ie72Gpm2yq1LTmvg at mail dot gmail dot com> <54649DFD dot 2030000 at LimeGreenSocks dot com> <alpine dot BSF dot 2 dot 02 dot 1411130846300 dot 16267 at arjuna dot pair dot com>
On Thu, Nov 13, 2014 at 2:53 PM, Hans-Peter Nilsson <hp@bitrange.com> wrote:
> On Thu, 13 Nov 2014, David Wohlferd wrote:
>> Sorry for the (very) delayed response. I'm still looking for feedback here so
>> I can fix the docs.
>
> Thank you for your diligence.
>
>> As I said before, triggering a full memory clobber for anything over 16 bytes
>> (and most sizes under 16 bytes) makes this feature all but useless. So if
>> that's really what's happening, we need to decide what to do next:
>>
>> 1) Can this be "fixed?"
>> 2) Do we want to doc the current behavior?
>> 3) Or do we just remove this section?
>>
>> I think it could be a nice performance win for inline asm if it could be made
>> to work right, but I have no idea what might be involved in that. Failing
>> that, I guess if it doesn't work and isn't going to work, I'd recommend
>> removing the text for this feature.
>>
>> Since all 3 suggestions require a doc change, I'll just say that I'm prepared
>> to start work on the doc patch as soon as someone lets me know what the plan
>> is.
>>
>> Richard? Hans-Peter? Your thoughts?
>
> I've forgot if someone mentioned whether we have a test-case in
> our test-suite for this feature. If we don't, then 3; removal.
> If we do, I guess it's flawed or at least not agreeing with the
> documentation? Then it *might* be worth the effort fixing that
> and additional test-coverage (depending on the person stepping
> up...) but 3 is IMHO still an arguably sane option.
Well, as what is missing is just an optimization I'd say we should
try to fix it. And surely the docs should not promise that optimization
will happen - it should just mention that doing this might allow
optimization to happen.
Richard.
> brgds, H-P