IIRC, for "normal" bit widths the codegen basically uses the next larger machine type and preserves zero bits on the high end. An i3 is an i8 with five MSB zeroes (with more custom behavior for "packed" i3 values). It's UB to fill those with non-zero values. For larger bit widths, like u729, you concatenate many large machine types, the compiler generates instructions in an unrolled loop, and the LLVM optimization pass usually doesn't clean that up (though, now that integers are apparently not using the LLVM u729 implementation, perhaps there are some more optimization opportunities).

They're situationally useful, especially when performance isn't an enormous concern. That u729 example above came from a variant sudoku solver I wrote to aid developing new puzzles (easy to check the rough magnitude of the solution space for whatever idea I was mulling over and examine how restricted the board actually was -- just an intermediate step in puzzle design). It's not optimal (hard on the icache, can be hard on registers, other issues abound), but it's dead simple to use, and the assembly isn't terrible, beating all the normal solvers I saw floating around. It's a nice point on the laziness/correctness/good-enough-perf pareto curve.

Another comment mentioned this, but they're great in packed structs for representing weird numeric entities (I think I have a logarithmic number system floating around which does that).

One thing the language does quite a lot is use them to guard against certain classes of human error at compile time. It doesn't perfectly make impossible actions unrepresentable, but shoving a full u32 into a shift argument usually doesn't make sense, so the types are constrained to be smaller.

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nvme0n1p1 3 hours ago | parent [-]

I can't imagine any situation where I'd use a u729 instead of a StaticBitSet. For size 729, it would end up backed by a bit_set.Array, not a bit_set.Integer.

https://ziglang.org/documentation/master/std/#std.bit_set.St...

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AlotOfReading 2 hours ago | parent | next [-]

I don't program zig, so it's not clear to me if you can use zig's bitsets arithmetically.

Sometimes it's just more clear to work with integers than other representations. Most situations with a state space of N bits have meaningful integer representations, where arithmetic functions on those representations are also meaningful.

For example, CRCs can be written as the remainder from long division of the message by the polynomial. Defining nontrivial cyclic permutations is also much more straightforward as functions on integers than on bitsets.

	▲	nvme0n1p1 2 hours ago \| parent [-]
		For other situations like a CRC on an arbitrarily-sized message, a big int would be better, surely? You can do long division on those. https://ziglang.org/documentation/0.16.0/std/#std.math.big.i... I was talking about GP's u729, which is 999, the state space of a sudoku board. Can you come up with a situation where dividing that number by anything is meaningful?

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hansvm 2 hours ago | parent | prev [-]

Old habits :)

If I had to steel-man the idea, I'm pretty sure the integer-based solution has better codegen with many kinds of sparse, comptime-known masks. I think you're right though, StaticBitSet looks better.

	▲	nvme0n1p1 2 hours ago \| parent [-]
		For your specific case, even a simple `[9][9]u16` might perform better (where you make use of nine bits in each u16). For each entry, the nine mask bits would be in the same bit positions, so the compiler won't have to do a bunch of shifts to extract/align the bits. CPUs love consistency. I doubt it's worth the additional codegen complexity to save 70 bytes in your data model.