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

[Julien Grall <julien.grall@xxxxxxx>]

Hi Jan,

On 21/01/2019 09:34, Jan Beulich wrote:
> > > > On 18.01.19 at 11:48, <julien.grall@xxxxxxx> wrote:
> > On 18/01/2019 09:54, Jan Beulich wrote:
> > > > > > On 18.01.19 at 02:24, <sstabellini@xxxxxxxxxx> wrote:
> > > > On Thu, 17 Jan 2019, Jan Beulich wrote:
> > > > > > > > On 17.01.19 at 01:37, <sstabellini@xxxxxxxxxx> 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.
> 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.
Cheers,

--
Julien Grall

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