Re: [PATCH v6 1/4] xen: introduce SYMBOL

[Stefano Stabellini <sstabellini@kernel.org>]

On Mon, 21 Jan 2019, Jan Beulich wrote:
> >>> On 21.01.19 at 11:22, <julien.grall@arm.com> wrote:
> > Hi Jan,
> > 
> > On 21/01/2019 09:34, Jan Beulich wrote:
> >>>>> On 18.01.19 at 11:48, <julien.grall@arm.com> wrote:
> >>> On 18/01/2019 09:54, Jan Beulich wrote:
> >>>>>>> On 18.01.19 at 02:24, <sstabellini@kernel.org> wrote:
> >>>>> On Thu, 17 Jan 2019, Jan Beulich wrote:
> >>>>>>>>> On 17.01.19 at 01:37, <sstabellini@kernel.org> wrote:
> >>>>>>> On Wed, 16 Jan 2019, Jan Beulich wrote:
> >>>> Stop. No. We very much can prove they are - _end points at
> >>>> one past the last element of _start[]. It is the compiler which
> >>>> can't prove the opposite, and hence it can't leverage
> >>>> undefined behavior for optimization purposes.
> >>>
> >>> You keep saying the compiler can't leverage it for optimization purpose,
> >>> however
> >>> there are confirmations that GCC may actually leverage it (e.g [1]). You
> >>> actually need to trick the compiler to avoid the optimization (e.g
> >>> RELOC_HIDE).
> >>>
> >>> So obviously, this is not only a MISRA "problem" as you state here and
> >>> below.
> >>>
> >>> I believe Stefano, Stewart and I provided plenty of documentation/thread to
> >>> support our positions. Can you provide us documentation/thread showing the
> >>> compiler will not try to leverage that case?
> >>>
> >>> Cheers,
> >>>
> >>> [1]
> >>> 
> > https://kristerw.blogspot.com/2016/12/pointer-comparison-invalid-optimization.html?m=1
> >> 
> >> Btw., the __start[] / __end[] example given there does not match
> >> up with what I see.
> > What you see in a specific version of GCC. This does not mean this behavior is 
> > valid across all the released versions and future one.
> 
> Are you suggesting that for the purpose of certification we need to
> deal with compiler bugs? Imo such a compiler should simply be
> excluded for use to build Xen.
> 
> >> Only symbols defined in the same CU as where
> >> the comparison sits get "optimized" this way. Externs as well as
> >> weak symbols defined locally don't get dealt with like this. And how
> >> could they? Nothing tells the compiler that two distinct symbols
> >> refer to two distinct objects. It is easy to create objects with
> >> multiple names, not only in assembly but also in C (using the "alias"
> >> attribute).
> > 
> > Similarly, nothing tells the compiler that they are not two distinct symbols. 
> > You haven't yet provided evidence a compiler cannot use that for optimization.
> 
> The compiler can leverage for optimization only what it can prove
> (to be undefined behavior or symbols referring to distinct objects
> or ...). A compiler may never use guesses for optimization. That
> is in the case here it is not us who need to tell the compiler that
> two different symbols may refer to the same object, but it is the
> compiler which needs to prove that two symbols cannot possibly
> refer to the same object. This is possible for automatic and static
> objects. This is also possible for some non-static objects defined
> in the CU under compilation. But this is not possible in the general
> case.

Clearly from the GCC thread not everybody agrees with you:

 Just because two pointers print the same and have the same bit-pattern 
 doesn't mean they need to compare equal

 So the only way within the C standard you could deduce that two objects 
 follow each other in memory is that the address of one compares equal to 
 one past the address of the other - but that does not mean they follow 
 each other in memory for any other comparison.

  > Are you saying it's possible that y immediately follows x in the
  > address space when that line of output is printed, and that y *doesn't*
  > immediately follow x in the address space when "inconsistent behavior:"
  > is printed?
  
  Yes.

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