Re: C provenance semantics proposal

[Jens Gustedt <jens dot gustedt at inria dot fr>]

Hello,

On Thu, 18 Apr 2019 14:42:22 +0200 Richard Biener
<richard.guenther@gmail.com> wrote:

> On Thu, Apr 18, 2019 at 2:20 PM Uecker, Martin
> <Martin.Uecker@med.uni-goettingen.de> wrote:

> > 1.) Compilers do not use conditional equivalences for
> > optimizations of pointers (or only when additional
> > conditions apply which make it safe)
> >
> > 2.) We make pointer comparison between a pointer
> > and a one-after pointer of a different object
> > undefined behaviour.  
> 
> Yes please!

No please don't, not UB.

If any of this, make the result unspecified but not UB, please.

> 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.

Jens

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