Re: C provenance semantics proposal

[Richard Biener <richard.guenther@gmail.com>]

On Fri, Apr 19, 2019 at 11:09 AM Jens Gustedt <jens.gustedt@inria.fr> wrote:
>
> Hello Jakub,
>
> On Fri, 19 Apr 2019 10:49:08 +0200 Jakub Jelinek <jakub@redhat.com>
> wrote:
>
> > On Fri, Apr 19, 2019 at 10:19:28AM +0200, Jens Gustedt wrote:
> > > > OTOH GCC transforms
> > > > (uintptr_t)&a != (uintptr_t)(&b+1)
> > > > into &a != &b + 1 (for equality compares) and then
> > > > doesn't follow this C rule anyways.
> > >
> > > Actually our proposal we are discussing here goes exactly the other
> > > way around. It basically reduces
> > >
> > >   &a != &b + 1
> > >
> > > to
> > >
> > >   (uintptr_t)&a != (uintptr_t)(&b+1)
> > >
> > > with only an exception for null pointers, but which probably don't
> > > matter for a platform where null pointers are just all bits 0.
> >
> > That penalizes quite a few optimizations though.
> > If you have
> > ptr != ptr2
> > and points-to analysis finds a set of variables ptr as well as ptr2
> > points to and the sets would be disjoint, it would be nice to be able
> > to optimize that comparison away
>
> yes
>
> > (gcc does);
>
> great
>
> > similarly, if one of the
> > pointers is &object or &object + sizeof (object).
>
> Here I don't follow. Why would one waste brain and ressources to
> optimize code that does such tricks?
>
> > By requiring what you request above, it can be pretty much never
> > optimized, unless the points-to analysis is able to also record if
> > the pointer points to the start, middle or end of object and only if
> > it is known to be in the middle it can safely optimize, for start or
> > end it would need to prove the other pointer is to end or start and
> > only non-zero sized objects are involved.
>
> I have the impression that you just propose an inversion of the
> roles. What you require is the user to keep track of this kind of
> information, and to know when they do (or should not) compare a
> one-passed pointer to something with a different provenance.
>
> I just don't feel that it is adequate to impose such a detailed
> knowledge on users, which is basically about a marginal use
> case. One-off pointers don't occur "naturally" in many places,

They occur in the single important place - loop IV tests in
C++ style iterator != end where end is a "pointer" to one after
the last valid iterator value.

I'd
> guess. Using them for anything else than to test bounds for array
> traversal is insane, and there "usually" the test is with `<`, anyhow,
> which has different rules.

Unfortunately then C++ arrived and compilers were expected to
also optimize that nasty code.

Richard.

>
> Jens
>
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