Date: Wed, 19 Oct 2016 11:33:41 +0200 (CEST)
From: Richard Biener <rguenther@suse.de>
To: Peter Zijlstra <peterz@infradead.org>
cc: "Luis R. Rodriguez" <mcgrof@kernel.org>,
        Vegard Nossum <vegard.nossum@oracle.com>, Jiri Slaby <jslaby@suse.cz>,
        linux-kernel@vger.kernel.org,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>, stable@vger.kernel.org,
        Ming Lei <ming.lei@canonical.com>,
        Steven Rostedt <srostedt@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Cesar Eduardo Barros <cesarb@cesarb.eti.br>,
        Michael Matz <matz@suse.de>, David Miller <davem@davemloft.net>,
        Guenter Roeck <linux@roeck-us.net>,
        Fengguang Wu <fengguang.wu@intel.com>, Borislav Petkov <bp@alien8.de>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Juergen Gross <jgross@suse.com>, Kees Cook <keescook@chromium.org>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Ingo Molnar <mingo@kernel.org>, Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH 01/12] extarray: define helpers for arrays defined in
 linker scripts
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On Wed, 19 Oct 2016, Peter Zijlstra wrote:

> On Wed, Oct 19, 2016 at 10:18:43AM +0200, Richard Biener wrote:
> 
> > The commit implements a long-standing failure to optimize trivial pointer
> > comparisons that arise for example from libstdc++.  PR65686 contains
> > a simple C example:
> > 
> > mytype f(struct S *e)
> > {
> >   mytype x;
> >   if(&x != e->pu)
> >     __builtin_memcpy(&x, e->pu, sizeof(unsigned));
> >   return x;
> > }
> > 
> > where GCC before the commit could not optimize the &x != e->pu test
> > as trivial false.
> 
> Which is fine; x is stack based and could not possibly have been handed
> as the argument to this same function.

Sure, it was just one example.

> This is also an entirely different class of optimizations than the whole
> pointer arithmetic is only valid inside an object thing.

Yes, it is not related to that.  I've opened 
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78035 to track an
inconsistency in that new optimization.

> The kernel very much relies on unbounded pointer arithmetic, including
> overflow. Sure, C language says its UB, but we know our memory layout,
> and it would be very helpful if we could define it.

It's well-defined and correctly handled if you do the arithmetic
in uintptr_t.  No need for knobs.

> Can't we get a knob extending -fno-strict-aliasing to define pointer
> arithmetic outside of objects and overflow? I mean, we already use that,
> we also use -fno-strict-overflow and a whole bunch of others.
> 
> At the very least, it would be nice to get a -W flag for when this alias
> analysis stuff kills something so we can at least know when GCC goes and
> defeats us.

What kind of warning do you envision?

"warning: optimized address comparison to always true/false"

?  That would trigger all over the place.

Richard.