On 10/30/2017 05:29 PM, Martin Sebor wrote: > On 10/30/2017 03:48 PM, Jeff Law wrote: >> On 10/30/2017 09:19 AM, Martin Sebor wrote: >>> On 10/30/2017 05:45 AM, Richard Biener wrote: >>>> On Sun, 29 Oct 2017, Martin Sebor wrote: >>>> >>>>> In my work on -Wrestrict, to issue meaningful warnings, I found >>>>> it important to detect both out of bounds array indices as well >>>>> as offsets in calls to restrict-qualified functions like strcpy. >>>>> GCC already detects some of these cases but my tests for >>>>> the enhanced warning exposed a few gaps. >>>>> >>>>> The attached patch enhances -Warray-bounds to detect more instances >>>>> out-of-bounds indices and offsets to member arrays and non-array >>>>> members. For example, it detects the out-of-bounds offset in the >>>>> call to strcpy below. >>>>> >>>>> The patch is meant to be applied on top posted here but not yet >>>>> committed: >>>>> https://gcc.gnu.org/ml/gcc-patches/2017-10/msg01304.html >>>>> >>>>> Richard, since this also touches tree-vrp.c I look for your comments. >>>> >>>> You fail to tell what you are changing and why - I have to reverse >>>> engineer this from the patch which a) isn't easy in this case, b) feels >>>> like a waste of time. Esp. since the patch does many things. >>>> >>>> My first question is why do you add a warning from forwprop? It >>>> _feels_ like you're trying to warn about arbitrary out-of-bound >>>> addresses at the point they are folded to MEM_REFs. And it looks >>>> like you're warning about pointer arithmetic like &p->a + 6. >>>> That doesn't look correct to me. Pointer arithmetic in GIMPLE >>>> is not restricted to operate within fields that are appearantly >>>> accessed here - the only restriction is with respect to the >>>> whole underlying pointed-to-object. >>>> >>>> By doing the warning from forwprop you'll run into all such cases >>>> introduced by GCC itself during quite late optimization passes. >>> >>> I haven't run into any such cases. What would a more appropriate >>> place to detect out-of-bounds offsets? I'm having a hard time >>> distinguishing what is appropriate and what isn't. For instance, >>> if it's okay to detect some out of bounds offsets/indices in vrp >>> why is it wrong to do a better job of it in forwpropI think part of >>> the problem is there isn't a well defined place to do >> this kind of warning. I suspect it's currently in VRP simply because >> that is where we had range information in the past. It's still the >> location with the most accurate range information. >> >> In a world where we have an embedded context sensitive range analysis >> engine, we should *really* look at pulling the out of bounds array >> warnings out of any optimization pass an have a distinct pass to deal >> with them. >> >> I guess in the immediate term the question I would ask Martin is what is >> it about forwprop that makes it interesting? Is it because of the >> lowering issues we touched on last week? If so I wonder if we could >> recreate an array form from a MEM_REF for the purposes of optimization. >> Or if we could just handle MEM_REFs better within the existing warning. > > I put it in forwprop only because that was the last stage where > there's still enough context before the POINTER_PLUS_EXPR is > folded into MEM_REF to tell an offset from the beginning of > a subobject from the one from the beginning of the bigger object > of which the subobject is a member. I certainly don't mind moving > it somewhere else more appropriate if this isn't ideal, just as > long it doesn't cripple the detection (e.g., as long as we still > have range information). Understood.
[ ... ] > > I of course don't want to break anything. I didn't see any fallout > in my testing and I normally test all the front ends, including Ada, > but let me check to make sure I tested it this time (I had made some > temporary changes to my build script and may have disabled it.) Let > me double check it after I get back from my trip. No worries. Hopefully by the time you're back I'll have something publishable on the ripping apart tree-vrp front and we can prototype the effectiveness of doing this kind of stuff outside tree-vrp.c We should also revisit Aldy's work from last year which started the whole effort around fixing how we deal with out out of bounds index testing. He had a version which ran outside tree-vrp.c but used the same basic structure and queried range data for the index. I've got a copy here we can poke at. jeff
