On Fri, Apr 19, 2019 at 11:09 AM Jens Gustedt <jens.gust...@inria.fr> wrote: > > Hello Jakub, > > On Fri, 19 Apr 2019 10:49:08 +0200 Jakub Jelinek <ja...@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 > > -- > :: INRIA Nancy Grand Est ::: Camus ::::::: ICube/ICPS ::: > :: ::::::::::::::: office Strasbourg : +33 368854536 :: > :: :::::::::::::::::::::: gsm France : +33 651400183 :: > :: ::::::::::::::: gsm international : +49 15737185122 :: > :: http://icube-icps.unistra.fr/index.php/Jens_Gustedt ::
