I'm still looking at the pointer arithmetic warnings to determine which are false positives, but I realized I screwed up my previous stats a bit as I forgot to account for duplicated/repeated warnings (e.g. the same header included in multiple compilation units and so generating the same warnings multiple times). For unique warnings, the stats are:
- 16.5% increase in warnings from before my patch (originally reported a 24% increase) - 47% of those new warnings being about pointer arithmetic - 6.7% of all of the warnings emitted with my patch applied are concerning pointer arithmetic. (originally reported 8.3%) - The new pointer arithmetic warnings represent a 7.7% increase in warnings from before my patch, not 10%. I'll send another email once I have a feel for how noisy the pointer arithmetic warnings are. Cheers, Kaelyn On Mon, Jul 18, 2011 at 5:54 PM, Kaelyn Uhrain <[email protected]> wrote: > Ted, > > You're welcome. I'll try to figure out what fraction of the pointer > arithmetic warnings are false positives (requires a bit of manual digging on > my part to determine if the code is indeed buggy or if it is valid / > intended). For the overall 24% increase in warnings, keep in mind that over > half of that is the existing bounds checking now being applied to cases > where it wasn't before, i.e.: > > char *foo[5]; > foo[77]; // -Warray-bounds already found > &foo[77]; // -Warray-bounds currently misses > *foo[77]; // -Warray-bounds currently misses > > The function that did the bounds checking would never catch the last two > cases because it would see a UnaryOperator (in the above cases for the '&' > and the '*') and skip the expression instead of looking inside the > UnaryOperator expression for the array subscripting. > > The new pointer arithmetic bounds checking only represents a 10% increase > in warnings--and IMHO that is the only part where the number of false > positives introduced might be an issue. > > Cheers, > Kaelyn > > > On Mon, Jul 18, 2011 at 5:36 PM, Ted Kremenek <[email protected]> wrote: > >> Hi Kaelyn, >> >> Thanks for the statistics. What would be good to know is what fraction of >> these are false positives (i.e., are these all real bugs)? A small random >> sample might be helpful. A 24% increase in warnings is fairly substantial, >> and we don't want to do that unless there is a real benefit. >> >> Ted >> >> On Jul 18, 2011, at 5:18 PM, Kaelyn Uhrain wrote: >> >> Ted, >> >> On Thu, Jul 14, 2011 at 5:04 PM, Ted Kremenek <[email protected]> wrote: >> >>> >>> The only other issue: should this be controlled under a separate warning >>> flag, at least initially so we can experiment with this new warning and see >>> how noisy it is? E.g. "-Warray-bounds-pointer-arithmetic". >>> >> >> I've tested the -Warray-bounds changes against the Google codebase and my >> patch increases the number of warnings from -Warray-bounds by 24%. Of the >> new warnings, 57.33% are for array indexes that most likely weren't picked >> up before because of unary operators like & or * (11.1% of all the warnings >> now emitted), and the remaining 42.67% are from out-of-bounds pointer >> arithmetic (8.3% of all the warnings from -Warray-bounds). >> >> Cheers, >> Kaelyn >> >> >> >
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