https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66790
--- Comment #21 from Eric Botcazou <ebotcazou at gcc dot gnu.org> --- > LIVE provides live registers that MAY be initialized (are initialized on at > least one path). The comments are all wrong! > > There's no code in GCC for must-initialized. Pierre's patch gets it right > (except that I'm not sure about the comment at line 2033). The MIR name can > indeed add to the confusion about may/must. Perhaps valid registers (VR)? Thanks for the comment. Yes, I agree that the MIR name ought to be improved. > I think this issue can also be fixed easily with an approximated solution, > which is already used for example by combine's > set_nonzero_bits_and_sign_copies. If a register is undefined at the start > of the function, you punt because the register has uninitialized uses. This > is suboptimal if you have cases like: > > int y, x; > x = y; /* uninitialized */ > ... > y = 5; > ... > x = y; /* initialized */ > > but they should be pretty rare. Right, we pondered about it. The difference is that combine operates on pseudo while REE operates on hard registers, so the suboptimality could be large.
