http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59932
--- Comment #4 from Zhendong Su <su at cs dot ucdavis.edu> --- (In reply to Andrew Pinski from comment #3) > (In reply to Zhendong Su from comment #2) > > (In reply to Andrew Pinski from comment #1) > > > I don't see why you think this is not undefined behavior. > > > If p1 starts at 1, it cannot turn into 0 as p1++ overflows during the > > > 2147483646th iteration. > > > > Andrew, because "d.f1 > l" is false, so the code simply returns ("return > > b;"). > > > > I also always double-check with CompCert's reference interpreter and Frama-C > > if possible. > > I see what is happening. It is a true warning that happens due to > optimizing order differences. The place we warn does not know that f is > zero the first time through the loop. Since -Os disables copy headers, we > don't get a different copy of the header. So the code does not optimize > away the header. > > This is where I am going to say there is a false positive due to optimizing. > I want to close it as won't fix because if we change the value of l to be > 0xfe, then we always warn. Andrew, sorry, I'm baffled by your comments above. Please note: 1) The issue isn't only triggered at -Os, but also at -O2 and -O3. 2) It doesn't affect GCC 4.8. 3) I don't see how changing l to 0xfe has changed anything. 4) Also optimizations shouldn't really change the warnings issued. Perhaps I have some misunderstandings, so could you clarify? Thanks.