Add a compiler intrinsic to back bigint_helper_methods by scottmcm · Pull Request #133663 · rust-lang/rust (original) (raw)

Also just to clarify my opinion on where this stands relative to #132195: I think that this PR just adding the intrinsic is okay, although I would like the intrinsic to also handle the signed case, even if it's only used at the moment for the unsigned case. That way, my PR can just update the library methods to use the intrinsic, without having to worry about the slow implementation.

Although the signed version of the library methods will likely return an unsigned word always for the least-significant / high-order part, it vastly simplifies the intrinsic to just have the same type for the arguments and return value, so, I would prefer that we do this specifically for the intrinsic. Since it doesn't actually affect the returned bits, I think that it's okay to just cast half of it unsigned on the library side. You can see how I did this here in a previous (closed) PR.

Otherwise, LGTM. Also, while the codegen test should suffice, if you want to copy over the library tests I added in my PR to verify that the intrinsic works correctly, feel free. I specifically added a bunch of edge cases with MAX and MIN as operands to ensure that things work correctly.