feature/spaceship: Clause 22: Containers #1046
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Thanks for this first batch of 🛸 work! I've reviewed everything and it looks solid; although some fixes are necessary, I think they should be relatively straightforward. (The biggest unknown is how to test synth-three-way and I would be comfortable marking that as a FIXME if you want.) Then it should be ready to be merged into the feature branch.
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queue<T, NonSpaceshipContainer> might be instantiated for a non-three_way_comparable NonSpaceshipContainer. In that case, it shouldn't attempt to grant friendship to the spaceship operator, because that would attempt to form compare_three_way_result_t<NonSpaceshipContainer>. As far as I can tell, there's no way in the Core Language for a queue to grant friendship to only one specialization of its non-member spaceship operator while respecting the constraint. (See WG21-N4861 [temp.friend]/9.) Instead, we need to template the friendship declaration, such that queue grants friendship to all of its spaceship operators. This is safe, just verbose.
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CaseyCarter
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Testing `_Differing_bits == 0` might seem like it's asking the compiler to do more work (always XORing). But it allows the compiler to see that the input to `_Countr_zero` is nonzero, and that allows it to optimize away the runtime ISA check, unconditionally emitting a `tzcnt` instruction. Then, at the end of the function, we know something that the compiler doesn't realize: `_Mask` selects a differing bit (the least significant one), so `(_Left & _Mask)` and `(_Right & _Mask)` are zero/nonzero or vice versa. Therefore, we only need to inspect one side! I've verified that each of these changes improves the codegen. Co-authored-by: Casey Carter <[email protected]>
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CaseyCarter
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Going to merge this into the feature branch after the tests run! 🎉 😸 🛸
I've added a note to the Spaceship Project that we need to implement spaceships for the container iterators too (which are Clause 22); that shouldn't hold up this significant PR.
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Thanks for implementing the important |
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This implements #62 for Clause 22, Containers.