Chris Lattner <clattner@xxxxxxxxx> writes: > > But at my previous > > company, we got significantly better generated code in some cases by > > breaking that aliasing. In particular this aliasing forces a pointer > > to a vector of char to alias char* and therefore alias everything. > > But if the compiler breaks that aliasing, then your suggestion of > > using a cast will fail in some cases. > > The compiler shouldn't break that aliasing, and using a union will > give you poor code anyway. LLVM will optimize either idiom into > element extract/insert operations, but I don't think GCC will in an > any case. If you care about performance with GCC, you have to use > ISA-specific builtins. gcc generates efficient insert/extract with the union if the backend is written correctly. See the uses of vec_extract and vec_get in extract_bit_field and store_bit_field in expmed.c. Ian
