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 (gcc does); similarly, if one of the pointers is
&object or &object + sizeof (object).
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.
Jakub
