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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.

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[PATCH 00/12] external array access helpers Vegard Nossum <vegard.nossum@oracle.com> - 2016-10-16 17:30 +0200
  [PATCH 07/12] tracing: declare __{start,stop}_ftrace_enum_maps as external array Vegard Nossum <vegard.nossum@oracle.com> - 2016-10-16 17:30 +0200
  [PATCH 01/12] extarray: define helpers for arrays defined in linker scripts Vegard Nossum <vegard.nossum@oracle.com> - 2016-10-16 17:30 +0200
    Re: [PATCH 01/12] extarray: define helpers for arrays defined in  linker scripts Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-10-17 09:10 +0200
    Re: [PATCH 01/12] extarray: define helpers for arrays defined in  linker scripts Peter Zijlstra <peterz@infradead.org> - 2016-10-17 10:40 +0200
      Re: [PATCH 01/12] extarray: define helpers for arrays defined in  linker scripts Jiri Slaby <jslaby@suse.cz> - 2016-10-17 11:10 +0200
        Re: [PATCH 01/12] extarray: define helpers for arrays defined in  linker scripts Peter Zijlstra <peterz@infradead.org> - 2016-10-17 11:10 +0200
          Re: [PATCH 01/12] extarray: define helpers for arrays defined in  linker scripts Vegard Nossum <vegard.nossum@oracle.com> - 2016-10-17 13:30 +0200
            Re: [PATCH 01/12] extarray: define helpers for arrays defined in  linker scripts Peter Zijlstra <peterz@infradead.org> - 2016-10-17 13:50 +0200
              Re: [PATCH 01/12] extarray: define helpers for arrays defined in  linker scripts Vegard Nossum <vegard.nossum@oracle.com> - 2016-10-18 10:10 +0200
                Re: [PATCH 01/12] extarray: define helpers for arrays defined in  linker scripts "Luis R. Rodriguez" <mcgrof@kernel.org> - 2016-10-18 23:20 +0200
                Re: [PATCH 01/12] extarray: define helpers for arrays defined in  linker scripts Richard Biener <rguenther@suse.de> - 2016-10-19 16:30 +0200
                Re: [PATCH 01/12] extarray: define helpers for arrays defined in  linker scripts Peter Zijlstra <peterz@infradead.org> - 2016-10-19 16:40 +0200
                Re: [PATCH 01/12] extarray: define helpers for arrays defined in  linker scripts Richard Biener <rguenther@suse.de> - 2016-10-19 18:00 +0200
                Re: [PATCH 01/12] extarray: define helpers for arrays defined in  linker scripts Peter Zijlstra <peterz@infradead.org> - 2016-10-19 18:10 +0200
                Re: [PATCH 01/12] extarray: define helpers for arrays defined in  linker scripts Richard Biener <rguenther@suse.de> - 2016-10-19 17:10 +0200
                Re: [PATCH 01/12] extarray: define helpers for arrays defined in  linker scripts Peter Zijlstra <peterz@infradead.org> - 2016-10-19 18:20 +0200
              Re: [PATCH 01/12] extarray: define helpers for arrays defined in  linker scripts Jiri Slaby <jslaby@suse.cz> - 2016-10-19 16:50 +0200
  [PATCH 12/12] dynamic debug: declare table as external array Vegard Nossum <vegard.nossum@oracle.com> - 2016-10-16 17:30 +0200
  [PATCH 10/12] serial_core: declare __earlycon_table{,_end} as external array Vegard Nossum <vegard.nossum@oracle.com> - 2016-10-16 18:00 +0200
  [PATCH 08/12] tracing: declare __trace_bprintk_fmt/__tracepoint_str as external arrays Vegard Nossum <vegard.nossum@oracle.com> - 2016-10-16 18:00 +0200
  [PATCH 09/12] tracing: declare __{start,stop}_syscalls_metadata as external array Vegard Nossum <vegard.nossum@oracle.com> - 2016-10-16 18:20 +0200
  Re: [PATCH 00/12] external array access helpers Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-10-16 18:20 +0200
    Re: [PATCH 00/12] external array access helpers Vegard Nossum <vegard.nossum@oracle.com> - 2016-10-16 19:30 +0200
      Re: [PATCH 00/12] external array access helpers Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-10-17 09:10 +0200
    Re: [PATCH 00/12] external array access helpers Jiri Slaby <jslaby@suse.cz> - 2016-10-17 08:30 +0200

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