Francis Moreau <francis.moro@xxxxxxxxx> writes: > I mean that I find quite dangerous that a program can subtly break (or > behave differently) depending on the optimisation level. I think it's important to say clearly that only an invalid program can subtly behave differently. A valid program will work correctly at all optimization levels. > For example a user can rely on aliasing undefined behaviour (one reason > is that is simply not aware of this fact), compile its program with no > optimisation level, do some testings and find no problem so far. Then > increase the level of optimisation and then the program start to behave > incorrectly. I guess he's going to spend some time before discovering > what's going on. > > Specially since no warnings can pop up with -O2 for example. It is unfortunately very difficult to issue correct warnings for these cases. That's why -Wstrict-aliasing is optional: it produces both false positives and false negatives. >> Would you be helped if there was an example (or two) of the basic >> problem first? > > An exhaustive list would be better I think. I don't know how to write an exhaustive list except in terms of optimizations that are not performed. Ian
