https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79173
--- Comment #30 from Vincent Lefèvre <vincent-gcc at vinc17 dot net> --- (In reply to Jakub Jelinek from comment #29) > (In reply to Vincent Lefèvre from comment #28) > > What do you mean by "the first additions will be less optimized"? (If you > > don't know anything about the initial carryin and the arguments, you can't > > optimize at all, AFAIK.) > > I mean that if the compiler can't see it is in [0, 1], it will need to use 2 > additions and or the 2 carry bits together. But, because the ored carry > bits are in [0, 1] range, all the higher limbs could be done using addc. If the compiler can't see that carryin is in [0, 1], then it must not "or" the carry bits; it needs to add them, as carryout may be 2. So each part of the whole chain would need 2 __builtin_add_overflow and an addition of the carry bits. However, if the compiler can detect that at some point, the arguments cannot be both 0xffffffff at the same time (while carryin is in [0, 2]), then an optimization is possible for the rest of the chain.
