Re: NULL pointer arithmetic issues

[Joerg Sonnenberger <joerg%bec.de@localhost>]

On Mon, Mar 09, 2020 at 09:50:50AM -0400, Aaron Ballman wrote:
> On Sun, Mar 8, 2020 at 2:30 PM Taylor R Campbell
> > I ask because in principle a conformant implementation could compile
> > the NetBSD kernel into a useless blob that does nothing -- we rely on
> > all sorts of behaviour relative to a real physical machine that is not
> > defined by the letter of the standard, like inline asm, or converting
> > integers from the VM system's virtual address allocator into pointers
> > to objects.  But such an implementation would not be useful.
> 
> Whether an optimizer elects to use this form of UB to make
> optimization decisions is a matter of QoI. My personal feeling is that
> I don't trust this sort of optimization -- it takes code the
> programmer wrote and makes it behave in a fundamentally different
> manner. I'm in support of UBSan diagnosing these constructs because it
> is UB and an optimizer is definitely allowed to optimize based on it
> but I wouldn't be in support of an optimizer that aggressively
> optimizes on this.

I consider it as something even worse. Just like the case of passing
NULL pointers to memcpy and friends with zero as size, this
interpretation / restriction in the standard is actively harmful to some
code for the sake of potential optimisation opportunities in other code.
It seems to be a poor choice at that. I.e. it requires adding
conditional branches for something that behaves sanely everywhere but
may the DS9k.

Joerg