On 13/03/2008, Chris Lattner <[EMAIL PROTECTED]> wrote: > > There is no right answer, and this topic has been the subject of much > debate on the GCC list in the past. I really don't care to debate the > merits of one approach vs the other with you, I just answered your > question about what clang does.
Of course. I didn't want to get into a debate. I was just trying to figure out how clang addressed the problem of false positives (or alternatively the problem of users complaining about them). Thanks for taking the time to elaborate. > Amusingly, GCC 4.0 emitted a false positive about this example despite > using the techniques you discuss, and no GCC emits any of these > warnings at -O0 (which is a very common mode used when doing a > majority of development for some people). Oh, we certainly do worse than that. See PR 179. I hope you didn't interpret my comments as criticism of LLVM. Certainly all approaches have its drawbacks and the one in GCC have a lot of drawbacks plus deficiencies. > I have no opinion about the approach that you take in GCC. In > practice, we have been able to do this analysis very quickly and get > good results, and will continue to refine them as clang continues to > mature. Hum, that is very interesting because doing these warnings in the middle-end causes so many false positives/negatives anyway that perhaps a limited static analysis in the front-end achieves better results. And the only argument then would be the overhead of the static analysis. If that is not an issue either, then it seems a very attractive approach indeed. Do you use the static analysis for other things (other warnings, early optimizations/folding)? There are many other warnings that would be noticeably enhanced by some dataflow in the front-end. > I personally think that it is a major problem that GCC doesn't produce > these diagnostics unless optimizations are enabled, and I continue to > think that having diagnostics change depending on what optimization > level is enabled is bad. I think everybody agrees on both. Yet the alternatives to these issues right now in GCC are building SSA at -O0 and moving the warnings earlier in the pipeline, respectively. The former results in slower/bigger -O0 compiler. Both would generate much more false positives, and there have been a few patches proposing moving them even later to avoid some false positives. I guess the static analysis in the front-end was discarded because of compile-time concerns, but if you don't see that (and clang seems to be much faster than GCC), then it may be worth reconsidering it. Cheers, Manuel.
