On 4/24/19 4:24 AM, Richard Biener wrote: > 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 don't think this is limited to C++.
Jeff
