Re: [Xen-devel] [PATCH v4 2/2] xen: use SYMBOL when required

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Mon, 7 Jan 2019, Jan Beulich wrote:
> >>> On 04.01.19 at 18:05, <sstabellini@xxxxxxxxxx> wrote:
> > I realize that you are not convinced by these arguments, but let's find
> > a way forward. My preference would be to have SYMBOL returning unsigned
> > long and do unsigned long comparisons when pointers pointing to
> > different objects are involved.
> 
> I continue to fail to see how suitable hiding of the connection to the
> original symbol from the compiler makes code less standard compliant
> when comparing pointers: The compiler simply can't know whether
> the underlying object is (in the extreme case) an array spanning the
> entire address space.

That is because the requirement I am trying to address is MISRA-C
compliance, which in turns requires C language compliance for C language
(I think it allows mixing C with assembly code). I don't particularly
care whether the compiler can or cannot find the connection to the
original symbol. 

The important thing for me is to avoid comparisons (and subtractions)
between pointers pointing to different objects. If we compare unsigned
longs, it is easier to prove that the comparison is not between pointers
pointing to different objects, even if somehow the numeric values
indirectly come from pointers. If we compare pointers, even if they went
through some sort of assembly conversions, we are still comparing
pointers pointing to different objects. The compiler might not be able
to figure it out, but a MISRA-C compliance scanning tool, or a human,
might.

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