Evan Jones wrote: > John Love-Jensen wrote: >> void** is not void*. > > Ah, of course. So with strict aliasing, GCC assumes that a void** > pointer only points to variables of exactly type void*? This would > explain my confusion. I was assuming that since T* is convertible to > void*, void** can point to any T*. It makes sense that this is not true. > > I wish I could get GCC to generate a "surprising" optimization for a > void**/T** type pun, but I have been unable to do so. This, of course, > doesn't mean that ignoring the warning is safe. > > It seems to me that the lesson for me is that when doing low-level > type-unsafe manipulation, void* pointers should be used. Since GCC must > assume that void* pointers can point anywhere, it will not be able to > optimize away the accesses to them, which is what I want in this case. > Please correct me if I am wrong about this. gcc will look through temporaries of type void* if the information is visible. So, trying to defeat the type system by using void* variable as temporaries doesnt work because gcc will simply delete any pointless temporaries. so, this: int *sp = &int_var; void *vp = sp; *(short*)vp = 22; is undefined behaviour, and may well not do what you want. Andrew.
