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Subject: Re: [memcg:auto-latest 238/243]
 include/linux/compiler-gcc.h:243:38: error: impossible constraint in 'asm'
From: Balbir Singh <bsingharora@gmail.com>
Date: Wed, 03 Aug 2016 15:17:01 +1000
In-Reply-To: <CAOSf1CG1OB+tQx=u5C5RSEFydPy4Rsa04L=Cwm4PfENWJa658A@mail.gmail.com>
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To: oliver <oohall@gmail.com>, Martin =?UTF-8?Q?Li=C5=A1ka?= <mliska@suse.cz>
Cc: Michal Hocko <mhocko@suse.cz>, kbuild test robot <fengguang.wu@intel.com>, kbuild-all@01.org, linux-mm@kvack.org, Jason Baron <jbaron@akamai.com>, Andrew Morton <akpm@linux-foundation.org>

On Mon, 2016-08-01 at 22:41 +1000, oliver wrote:
> On Mon, Aug 1, 2016 at 9:27 PM, Martin LiA!ka <mliska@suse.cz> wrote:
> >A 
> > On 08/01/2016 01:09 PM, Michal Hocko wrote:
> > >A 
> > > [CC our gcc guy - I guess he has some theory for this]
> > >A 
> > > On Sun 31-07-16 14:11:25, Michal Hocko wrote:
> > > >A 
> > > > It seems that this has been already reported and Jason has noticed [1] that
> > > > the problem is in the disabled optimizations:
> > > >A 
> > > > $ grep CRYPTO_DEV_UX500_DEBUG .config
> > > > CONFIG_CRYPTO_DEV_UX500_DEBUG=y
> > > >A 
> > > > if I disable this particular option the code compiles just fine. I have
> > > > no idea what is wrong about the code but it seems to depend on
> > > > optimizations enabled which sounds a bit scrary...
> > > >A 
> > > > [1] http://www.spinics.net/lists/linux-mm/msg109590.html
> > Hi.
> >A 
> > The difference is that w/o any optimization level, GCC doesn't make %c0 an
> > intermediate integer operand [1] (see description of "i" constraint).
> We recently hit a similar problem on ppc where the compiler couldn't
> satisfy an "i" when it was wrapped in an function and optimisations
> were disabled. The fix[1] was to change the function signature so that
> it's arguments were explicitly const. I don't know enough about gcc to
> tell if that behaviour is arch specific or not, but it's worth trying.
>A 
> Oliver
>A 
> [1] https://lists.ozlabs.org/pipermail/skiboot/2016-July/004061.html

Yes, the way I solved the issue was to look at the RTL and provide
hints to the compiler that the passed argument was a constant and it
needed to be passed as such to the instruction

I would suggest just looking at the RTL and figuring out why the constraints
break

Balbir Singh.

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