https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87744
--- Comment #11 from Jakub Jelinek <jakub at gcc dot gnu.org> --- (In reply to Jakub Jelinek from comment #10) > As discussed on IRC, probably the > if (!__builtin_mul_overflow(__l._M_lo, __x, &__lo)) > optimization isn't a good idea, because most likely the compiler will in > that case > expand that to a full 64x64->128 multiplication plus testing that the upper > 64 bits are or aren't zero, so computing everything this function needs and > doing it again if there is overflow, without possibility to query what it > computed. Though, if it is common enough, one could try to optimize the __ll[0] == 0 && __xx[0] == 0 case, one can do then either 32x32->64 or 64x64->64 multiplication and be done with it. But if it is rare in random's usage, it would just make the code larger.
