Ian Lance Taylor <iant@xxxxxxxxxx> writes: > "Segher Boessenkool" <segher@xxxxxxxxxxxxxxxxxxx> writes: > >>> BTW, I tried to compile the last example, and we agreed that it invokes >>> undefined behaviour. >> >> I did no such thing. With -fno-strict-aliasing, the code is well-defined. > > I think I would say this slightly differently. The code is still > undefined according to the C/C++ language standards. When you use > -fno-strict-aliasing, gcc is promising to avoid using a class of > optimizations based on those portions of the language standards. > Another way to say this is that gcc is promising to not optimize based > on the fact that memory locations are accessed using different types. > That is, the code is still incorrect; gcc is just avoiding a class of > optimizations which are known to cause trouble with real programs. > > I also am having trouble understanding what the OP is asking for here. Clarification on GCC's strict aliasing rules. But I think I got it now, although I still don't know which exactly undefined behaviours are defined by GCC's -fno-strict-aliasing. Thanks -- Francis
