https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79173
--- Comment #28 from Vincent Lefèvre <vincent-gcc at vinc17 dot net> --- (In reply to Jakub Jelinek from comment #27) > Given that the builtins exist for 10 years already, I think changing it for > them is too late, though they don't seem to take backwards compatibility as > seriously. > They don't document the [0, 1] restriction and the behavior implemented in > GCC is what I saw when trying it. Their documentation at https://clang.llvm.org/docs/LanguageExtensions.html is currently just unsigned sum = __builtin_addc(x, y, carryin, &carryout); But a carry for a 2-ary addition is always 0 or 1, so the [0, 1] restriction is implicit (by the language that is used). And in their example, the carries are always 0 or 1. > Note, in many cases it isn't that big deal, because if carry_in is in [0, 1] > range and compiler can see it from VRP, it can still optimize it. And given > that carry_out is always in [0, 1] range, for chained cases worst case the > first additions will be less optimized but the chained will be already > better. 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.)
